COVID-19 transmission dynamics

COVID-19 transmission dynamics

The effects of interactions between the SARS-CoV-2 virus, humans, animals and the environment on transmission of COVID-19 respiratory illness

Public transportation

Within China COVID-19 case numbers were positively associated with the frequency of flights, trains and buses from Wuhan in January 2020. There was an inverse relationship between the numbers of COVID-19 cases in a city and its distance from Wuhan. Person-to-person transmission, transmission from the environment, fomites and potential aerosol transmission contribute to the risk for individuals spreading COVID-19 to others during the use of public transportation and to others at the travel destination. Spatial transmission is the study how disease spreads geographically. Understanding the spatial transmission of infectious disease due to the mobility of infected individuals by travel can shed light on mechanisms of transmission and inform policies on interventions that control seeding of infections.

Airline travel and spatial transmission


A New RAND Tool Helps Analyze Commercial Air Travel Involving Infected Passengers

May 2020

Risk to country or region

Airport risk of importation and exportation of the COVID-19 pandemic.

September 2020

Relative risks of importation and exportation by country, risk flow

Estimating COVID-19 outbreak risk through air travel

June 5, 2020

Risk of outbreak posed by each airport

Outbreak dynamics of COVID-19 in Europe and the effect of travel restrictions.

May 5, 2020


Average in European countries

Tracking the Spread of COVID-19 with Air Travel Data

May 2020

Not applicable

The highest risk of COVID-19 infection was found to be among Wuhan travelers between January 19-22, 2020 with international travelers from Wuhan having an approximate infection rate of up to 1.3%. Among people evacuated from Wuhan to their home country, more than 1% were infected.

Outside of China, most imported cases of COVID-19 likely originated from air travel. To control the spread of COVID-19 restrictions on airline travel and quarantine procedures for returning travellers were instituted. Severe restrictions on airline travel in Europe, reduced spread of COVID-19, which closely followed airline travel patterns. Using simulation and mathematical modelling to estimate the effectiveness of travel restrictions, Stanford University researchers found that mobility networks of air travel can predict emerging the global diffusion pattern of a pandemic and can be used to test virtual lifting of restrictions between communities. Their simulations suggest that unconstrained mobility would have accelerated COVID-19 spread significantly.

Researchers based at University of Tokyo evaluated the relative risk of importation and exportation of COVID-19 from every airport around the world using air travel flows and COVID-19 case data until March 14, 2020. China and Iran were found to have a higher risk of both importation and exportation than other airports and Italy and other European airports had a higher risk of exportation than importation. The US was found to have more airports with relative risk of importation than with relative risk of exportation. Simulation showed that in some areas the risk flow of importation and exportation remained even after airline travel was reduced, suggesting that air line travel must be reduced to more than 90% in areas with high cumulative incidence.

Research from Tel Aviv University considered a future scenario where COVID-19 cases are low and air travel returns to normal levels and estimated the risk of different locations to act as a source for future COVID-19 outbreaks elsewhere. The study showed that airports in East Asia to be highest and that travellers from these regions would be most likely to seed outbreaks in India and Brazil, which are considered potentially vulnerable regions. Outbreaks in Africa were found to be most likely to initiate from travellers outside the continent, from West Europe. For large regions such as India, Brazil, US, Europe and China have a higher risk for outbreaks to arise from infected passengers from within rather than outside. The group found that variation in flight volumes and population densities at destinations create a non-uniform distribution of risk for different airports to act as an outbreak source.

The RAND National Security Research Division (NSRD) has a COVID-19 Air Traffic Visualization (CAT-V) tool that combines case data from Johns Hopkins University with air travel data from the International Air Transport Association in order to visualize and estimate future patterns of COVID-19 transmission. The CAT-V tool simulates movement of people from one country to another and takes into account the likelihood that they are infected with SARS-CoV-2 but does not assume that the virus is transmitted on airplanes.

Canadian federal data showed that at least two flights per day into or within Canada carried a passenger with confirmed COVID-19 over a period of one week in June 2020.

Airline travel: Transmission risk for passengers


Covid-19 Risk Among Airline Passengers: Should the Middle Seat Stay Empty?

July 5, 2020

Risk of contracting from another passenger 1/4300 versus 1/7700 with middle seat empty

Detection of SARS-CoV-2 RNA in commercial passenger aircraft and cruise ship wastewater: a surveillance tool for assessing the presence of COVID-19 infected travelers.

July 4, 2020

Not applicable

Lack of COVID-19 Transmission on an International Flight

February 24, 2020

Not applicable

Potential transmission of SARS-CoV-2 on a flight from Singapore to Hangzhou, China: An epidemiological investigation.

July 2020

Attack rate 4.8%

Probable aircraft transmission of Covid-19 in-flight from the Central African Republic to France

May 2020

Not applicable

The risk of contracting COVID-19 from another airline passenger is 1 in 4,300 and would be reduced to 1 in 7,700 by not using the middle seat according to statistical modelling by Arnold Barnett at MIT in a paper published before peer review. The analysis by Bartnett used information from a systematic review and meta-analysis on physical distancing, face masks and eye protection for person-to-person transmission of SARS-CoV-2. Based on a 1% chance of dying from COVID-19, the COVID-19 mortality risk from traveling by air even with the middle seat not used was still estimated to be higher than mortality due to plane crash. The study noted that it is not clear that the risk of being infected by COVID-19 during a flight is higher than the risk associated with everyday activities during the pandemic in the US where late June 2020 day show that approximately 1 in 120 Americans have COVID-19. As of July 1, 2020 the US airlines American, Spirit and United Airlines were running full flights when warranted by demand and the airlines Delta, jetBLue and Soutwest Airlines planned to have the middle seat remain empty.

The lack of transmission of COVID-19 was reported on a flight with 350 passengers when one person was symptomatic and later diagnosed with COVID-19 during a 15-hour flight from Guangzhou to Toronto. It was suggested that transmission was mitigated by mild symptoms and masking during the flight and that this lack of transmission supports droplet transmission.

A flight from the UK to Vietnam was found to have one passenger who transmitted SARS-CoV-2 to up to 14 passengers and a crew member. 12 of the passengers were sitting close to the suspected first case. The International Air Transport Association from an informal survey found that 18 major airlines identified four instances in the first three months of 2020 of suspected in-flight transmission from passengers to crew.

Potential transmission of SARS-CoV-2 from one person to another was reported on a January 2020 flight from Singapore to Hangzhou which included 100 passengers that had visited Wuhan. The person suspected to have caught COVID-19 on the flight was not properly wearing a facemask. A total of 16 passengers were diagnosed with COVID-19 but in all but one of these cases transmission was thought to occur through exposure in Wuhan or to infected members in a tour group. There are other reports of patients suspected to have acquired COVID-19 on flights.

For spread of SARS, studies have found that sitting within two rows of a contagious passenger carries the greatest risk in a flight longer than 8 hours. However SARS was reported to have spread to 20 people of which less than half were within two rows of the original case on a flight from Hong Kong to Beijing.

Airbus SE and Boeing Co are reported to use HEPA filters that capable of capturing small particles such as virus. The airflow movement from ceiling to floor that is also compartmentalized into sections is expected to limit the movement of particles along the length of the plane cabin. Ventilation systems may not operate fully while the airplane is parked at the gate. In 1979 an influenza outbreak was attributed to airline passengers being kept onboard a grounded aircraft when ventilation was turned off. Although airlines tell passengers to wear face masks, it is not clear if the rule is thoroughly enforced.

SARS-CoV-2 was shown to be detectible in wastewater from aircraft and cruise ship sources and the potential for using RT-qPCR or RT-ddPCR for surveillance of wastewater was reported.

Public transit transmission


Investigation of a cluster epidemic of COVID-19 in Ningbo

May 13, 2020

Mass gathering: attack rate 33.82%, infection rate 38.24%

The risk of COVID-19 transmission in train passengers: an epidemiological and modelling study

July 2020

Attack rate in train passengers 0.32%

Next to infected passenger 3.5%

The Subway Seeded the Massive Coronavirus Epidemic in New York City

April 24, 2020

R = 3.4

New York during initial surge in March

An investigation of a COVID-19 cluster epidemic after mass gathering in Ningbo in the Zhejiang province of China related to a Buddhism rally on January 19 found rally participants that took the bus that took the bus with the initial case had a higher risk of contracting COVID-19 than those that did not take the bus with the initial case that was considered a super spreader.

The New York subway system was suggested to be a major disseminator of SARS-CoV-2 during the COVID-19 epidemic in the city in March 2020. Subway system nearly shutoff ridership in Manhatten which was down by 90 percent correlated with the substantial increase in doubling time of new cases. Subway lines with the largest decrease in ridership in the second and third wees of March subsequently had the lowest rates of infection in zip codes transversing their routes.

In Paris and Austria COVID-19 case clusters were reported to not be traceable to riding transit. Milan did not see subsequent COVID-19 infection spikes upon opening their transit system. In Japan state of emergency was lifted in May 2020 and no COVID-19 infection clusters linked to commuter trains while clusters were linked to gyms, bars, music clubs and karaoke rooms. It is suggested that this is because transit riders are normally alone, not talking to other passengers and wearing masks.

A study on COVID-19 spread on China’s high-speed G trains concluded that passengers on trains should be seated at least two seats apart in the same row and travel for no more than 3 hours. Transmission to nearby passengers was found to be 0% to 10% depending on how close passengers sat and for how long to infected passengers. Researchers measured the attack rate, which is the percent of individuals that test positive in a group. Out of 72,000 passengers, 234 developed COVID-19, an attack rate of 0.32%. Individuals that sat directly next to an infected person had an average attack rate of 3.5%. Those in same row showed an attack rate of 1.5% which is 10 times higher than for those sitting one or two rows away. Travel time affected risk as the attack rate increased 0.15% for every hour a person traveled with an infected passenger and increased 1.3% for every hour someone sat next to an infected person. One of the 1,342 people who sat in a seat previously occupied by an infected person after they left the train contracted COVID-19, which is an attack rate of 0.075%.

Person-to-person transmission

COVID-19 can be transmitted through respiratory droplets, which are greater than 5-10 μm in diameter and droplet nuclei, which have a diameter of less than 5 μm. SARS-CoV-2 virus is primarily transmitted between people through respiratory droplets and by contact. Droplet transmission occurs when a person is within 1 m of someone with respiratory symptoms such as coughing or sneezing and is exposed to infective respiratory droplets to the mouth, nose or eyes.

In additon to coughing and sneezing, talking and louder speech are associated with higher viral load in respiratory droplets expelled from an individual. The saliva contains high SARS-CoV-2 titers in COVID-19 patients. One key difference between SARS-CoV-2 (COVID-19) and SARS-CoV is that the former virus replicates early on in the upper respiratory tract and patients are most infectious during the initial days of infection when symptoms are mild or not present. The pre-symptomatic period for COVID-19 is from 2 to 15 days and many individuals remain asymptomatic throughout infection.

Case clusters

COVID-19 infection clusters were most common within family clusters. Other major types of clusters include community, nosocomial (hospital), gatherings, transportation, shopping malls, conference, tourists and religious organizations. The largest clusters have occurred aboard ships. A database of COVID-19 case clusters was compiled by epidemiologist Gwenan Knight and her colleagues at the London School of Hygiene and Tropical Medicine.

Indoor spaces
Shared living space


The risk of secondary attack rate for SARS-CoV-2, which means the risk that people in close contact with an infected person become infected depends on the duration and intensity of contact. Secondary attack rate among household members for SARS-CoV-2 is between 10% and 40%. In comparison airborne viral pathogens like varicella zoster, which causes chicken pox and the measles virus have a household secondary attack rate of about 85-90%. For SARS-CoV-2, less sustained contact such as sharing a meal and passing interactions among shoppers are associated with secondary attack rates of 7% and 0.6% respectively.

Contact within households is thought to be responsible for about 70% of SARS-CoV-2 transmission when community control measures are in place. In Wuhan, the use of Fangcang (field) hospitals to isolate cases outside the home reduced the reproduction number (R) from 1.18 after lockdown to 0.51. The odds that a primary case transmitted COVID-19 to others was found to be 18.7 times higher in a closed environment compared with an open-air environment.

A study in China using data from the Guangzhou Center for Disease Control and Prevention found the secondary attack rate among household contacts to be 12.4%. When those household contact were close relatives compared with contacts only living at the same residential address, secondary attack rates were 12.4% and 17.1% respectively. Risk of household infection was highest for those over 60 years old and lowest for those of less than 20 years old.

A study from researchers in Wuhan, China found the difference between secondary attack rate for children was 4% compared with 17% for adults. When index patients quarantined themselves from symptom onset, secondary attack rate was 0% compared with 17% when the index patient did not quarantine. Index cases that quarantined wore a face mask, dined separately and resided separate from others in the house. For spouses of index cases, secondary attack rate was 28%.

A research study based out of Singapore focused on transmission from adults to children. The attack rate was found to be 1.3%, 8.1% and 9.8% for children less than 5 years old, 5-9 years old and 10-16 years old respectively. The attack rate was highest when the index case was the mother at 11% compared with 6.7% or 6.3% if the index case was the father or grandparent.

Migrant worker accommodation

Close to 93% of COVID-19 cases in Singapore in the first 48 days occurred in dormitories for migrant workers.

Migrant agricultural workers who come to Canada live in crowded shared accommodation. A large proportion of COVID-19 cases are traced back to outbreaks at sites of migrant worker accommodation. In the province of Ontario, Canada, as of July, 2020 there were more than 1244 cases of COVID-19 infected workers and three deaths in the agriculture sector. The medical officer of Windsor-Essex County Health Unit in Ontario said that 90 percent or more of COVID-19 cases are temporary foreign workers. It is difficult to determine if transmission occurs in the workplace or at the accommodation. Migrant workers that live in bunkhouses were reported to account for one-fifth of total cases in the area.

Homes for the elderly

A large proportion of COVID-19 deaths occurred in long-term care facilities or care homes for the elderly. Long-term care facilities accounted for 60% of COVID-19 deaths in Washington state, 45% in Sweden and almost one-third in the United Kingdom.

Homeless shelters

There is a high burden of COVID-19 among sheltered homeless populations. A single shelter in Boston found 36% of the 408 residents tested positive for SARS-CoV-2 with 88% of them reporting no symptoms. A microsimulation modeling study on the clinical outcomes and costs of interventions to prevent COVID-19 in adults staying in homeless shelters found that daily symptom screening of positive individuals along with temporary housing in a non-hospital care site would reduce infections. The reduction in infections was calculated to be 37%, 72% and 51% in scenarios where reproduction numbers Re were 2.6, 1.3 and 0.9 respectively.

Wastewater and plumbing

Transmission of SARS-CoV-2 has been suggested to have occurred by indirect contact in an apartment building through the transmission of viral particles on air streams within the pipe network and entry into the interior of the building from the wastewater system. The study points to feces containing virus as the source from which infectious aerosols were possibly generated by turbulent flows in wastewater plumbing. The WHO previously hypothesized that empty U-traps in a plumbing system may have created a pathway for SARS-CoV-1 virus containing droplets and aerosols to enter bathrooms and spread infections in a housing complex outbreak in Hong Kong. Air pressure surges are a common cause of empty U-traps in high-rise buildings. Mechanical bathroom extract fans and favorable outdoor air conditions were also thought to increase transmission. As of September 2020 there is limited data on the infectivity of fecal droplets and aerosols for SARS-CoV-2.


In England SARS-CoV-2 infection and outbreak rates were calculated for staff and students in early year, primary and secondary schools during June 2020, the first month after easing of the national lockdown. SARS-CoV-2 infections and outbreaks were uncommon across all educational settings. There was a strong correlation between number of outbreaks and regional incidence of COVID-19. Staff had an increased risk of infections compared to students and the majority of cases that were linked to outbreaks occurred in staff. The study demonstrated that control of community transmission is important to protect educational settings.

Contaminated surfaces

Using non dangerous tracer virus, bacteriophage MS-2, researchers found that contamination of a single doorknob or table top results in spread of viruses throughout office buildings, hotels and healthcare facilities. The tracer virus was detected on 40-60 percent of surfaces within 2 to 4 hours. Using quaternary ammonium compounds (QUATS) disinfectant containing wipes on fomites was an intervention that reduced detectible virus by 80% and the concentration of virus detected was reduced by 99%.

Ventilation to reduce airborne transmission

Ventilation is known to reduce the airborne transmission of pathogens by diluting them to lower concentrations. An increase in ventilation rate, which means an increase in the amount of clean outdoor air supplied to indoor volumes have been included in guidelines from The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), The Federation of European Heating, Ventilation and Air Conditioning associations (REHVA) and the National Health Commission of China. However many air-handling units, ducting and air supply are dimensioned to be economic and energy efficient and may not allow for large amount of outdoor air. Ventilation systems that are designed to supply large amounts of outdoor air during the pandemic may not operate economically under normal conditions and the costs may not be feasible in many cases. Air supply rate can be increased by installing portable units such as window installed fans and free-standing air handling units, filters, stand-alone air cleaners and ultraviolet germicidal irradiation units installed in rooms or in ducts.

In ventilated rooms short-distance exposure occurs when the distance between the exposed and infected person is less than 1-1.5 m and long-distance exposure is at a greater distance. Long-distance exposure mainly depends on room ventilation which includes ventilation rate and room airflow characteristics. Room airflow characteristics include air temperature, humidity, velocity and direction.

The risk of airborne cross-infection was evaluated by calculating the time-averaged intake fraction, which is the proportion of air mass exhaled by the infected person that is inhaled by the exposed person. The study found that all occupants should leave the room periodically and reduce room occupancy as much as possible. It was recommended that these strategies along with other control measures such as maximum clean air, distancing, face-to-back layout of workstations and reducing aerosol-generating activities like loud talking and singing should be applied to classrooms, offices and meeting rooms.

Complete mixing of expiratory airborne aerosols and room air was assumed and particle dynamics of airborne aerosols were not considered in the analysis. If the infected person were to stay in the room for a long time, the concentration of expired airborne aerosols would build up until reaching a steady state. When the infected person leaves the room, the concentration of aerosols begins to decay, but begin to build up again when the person re-enters the room. Intermittent source generation will result in lower time-averaged exposure to occupants compared with continuous exposure. Calculations showed that in a classroom scenario, if students left the room during breaks between lessons they reduced the risk of airborne cross-infection by 35% compared compared with staying in the room during breaks.

A study that assessed the dynamics of influenza droplet and aerosol transmission using wireless sensors to measure the location and close proximity of contacts in the population of a high school in the US. Simulations used empirical transmission levels observed in households. The study found that improvements in ventilation to recommended levels by American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) had a similar mitigating effect on viral transmission as vaccination covering 50-60 percent of the population. ASHRAE recommends 3 air changes per hour. The results were based on the assumption of substantial aerosol transmission

In a restaurant in Guangzhou where the air exchange rate was low (0.56-0.77 exchanges per hour) on January 24, 2020, 5 people sitting at adjacent tables to an infected person developed COVID-19. The restaurant had 21 people in an area of 45 m2. The volume of clean air brought in by the ventilation system per person was estimated to be 48 L/min.

Elevators have different ventilation requirements. Washington state requires ventilation at 1 cubic foot/minute per square foot (1 foot/minute). With 1.5 feet per person in an elevator cab, a person would have about 42 L/minute of clean air, whereas a general recommended air exchange rate is 480 liters per person per minute. Computer modelling suggested that elevator cabin air could remain infectious after the infector has exited. After an infected passenger exits and a second passenger enters, they could be exposed to 25% of the viral particles that the infected passenger expelled. The CDC recommended that elevator riders should wear masks and avoid speaking.

A mathematical model for COVID-19 transmission by aerosols was applied to known outbreaks and used to present quantitative guidelines for ventilation and occupancy in the workplace. The model assumes hand-hygiene protocols are followed, that surface contamination is not the dominant transmission route and that contagious individuals are wearing face coverings to catch large droplets. In this scenario aerosols are the dominant transmission mechanism. The analysis, yet to be peer reviewed as of August 2020, lead the author to propose the following guidelines: 1) To mitigate the affects of aerosol build-up in closed spaces, ventilation and short exposure times. 2) Recirculation in HVAC systems should be avoided or use high quality filters. 3) Poorly ventilated common spaces such as bathrooms, elevators and stairwells should have increased airflow or local air scrubbers. 4) Mask use can prevent direct exposure when less than 2 m space cannot be maintained but will not prevent infection in an enclosed space regardless of the distance between occupants. The author suggested that airflow in shared spaces be at 50 m3/min ∼2000 CFM per occupant beyond the first in early phased of epidemic decay.

Transmission through heating, air-conditioning and ventilation systems

A literature review evaluated the COVID-19 risk associated with air-conditioning systems using literature on outbreaks of coronaviruses SARS-CoV-1, MERS-CoV and SARS-CoV-2 in indoor environments. 14 studies were used, seven involved SARS-CoV-1, six involved SARS-CoV-2 and one involved MERS-CoV. The MERS-CoV study demonstrated that the HVAC system was contaminated by viral particles. Six of the seven SARS-CoV-1 studies suspected that air-conditioning systems played a role in infection spread. In four out of six SARS-CoV-2 studies, diffusion of viral particles through HVAC was suspected or supported through computer simulation. In two of the studies transmission of SARS-CoV-2 through HVAC was excluded based on epidemiological data.

Ventilation systems have reported to be capable of transmitting or spreading virus for measles, chickenpox, flu, smallpox and the 2009 influenza A (H1N1) pandemic. Deactivation of the air recirculation mode in indoor environments has been widely recommended for controlling SARS-CoV-2, but whether this approach is possible, technically and due to cost, in all workplaces has been questioned. It is recommended that air intake systems are designed to avoid air currents in the respiratory area of occupants of indoor spaces. Other recommendations are to increase filtering efficiency of HVAC systems using nanomaterials and estimate the probability that an infectious person is inside the building.

Humidity and temperature

Indoor temperature and humidity could be maintained at levels that are less favorable for transmission of SARS-CoV-2. Cold temperatures are postulated to increase viral half-lives and low relative humidity hinders viral inactivation through natural processes. Regulatory bodies in the USA suggest that in winter that indoor temperatures be maintained between 20-40 C and relative humidity be maintained between 20-60%. Relative humidity over 60% relative humidity significantly increases the likelihood of mold growth.

Relative humidity can cause fewer droplets to be inhaled due to these conditions causing a slower evaporation from large droplets to small droplets. In the host, too low or too high relative humidity affects the nasal mucosa, mucous viscosity and mucociliary activity. Extremely low humidity has been associated with enabling virus settlement in human hosts and may allow greater penetration of foreign particles. To avoid dry eyes and dry nasal passages relative humidity is recommended to be greater than 30 % and 10% respectively. Viral stability is also effected by humidity. Whereas relative humidity measures how close the air is to saturation, absolute humidity is a fixed measure of water vapor content in the air. Survival and transmission potentials of influenza viruses in winter are inversely associated with absolute humidity rather than relative humidity. Under conditions of high humidity, low temperature can stabilize and protect viruses by stabilizing the lipid-containing envelope. Cold temperatures and low relative humidity have been associated with survival and transmission of some influenza viruses and an increased occurrence of respiratory tract infections.

An Italian study found that optimum thermodynamic conditions for moist air to reduce exposure risk of COVID-19 in indoor spaces would be typical summertime conditions at 15-26 C and 50% relative humidity. For the winter researchers advise that absolute humidity of the supply air should be kept higher than usual, which would imply a larger humidification in winter and a smaller dehumidification in summer. It was also recommended that indoor temperatures should be kept higher than usual. A relationship between the survival of coronaviruses and thermodynamic potential specific enthalpy of moist air was inferred. The use of a the parameter of specific enthalpy 55 kJ/kg-dry air could be used to satisfy requirements for SARS-CoV-2 inactivation and the hygrothermal comfort of people using the space.

Determining degree of risk

The application of control banding, which is a qualitative method for determining the degree of risk for occupations and job tasks, has been proposed for aerosol-transmissible diseases including COVID-19. SARS-CoV-2 would be categorized as a Risk Group 3 organism as has been determined by the NIH Office of Science Policy. An employee’s exposure depends on the two variables which are air concentration of virus and length of time in contact with that concentration. The authors assume that a short exposure to a high aerosol concentration will be as likely as a longer exposure to a lower concentration. Level of exposure is determined by the likelihood that the employee will have person-to-person interactions (unlikely, possible or likely) with potentially infected people and the duration of time they are exposed (0-3 hours, 3-6 hours, >6 hours). Exposure level E1, E2 or E3 is combined with the organism risk ranking, which is R3 for COVID-19. Control band levels A, B and C are then determined. Control band B would be jobs where exposures unlikely but risk is severe or where exposures are possible or likely and the risk is moderate. Using a hierarchy where first source of infection is controlled, then pathway, then the receptor (wearing personal protective equipment), a scenario is proposed where most workers will not be required to wear personal protective equipment. Source controls are those that limit the number of sources of infectious aerosol, like conducting business by phone or employee health screening. Pathway controls include barriers, diluting the air and distancing.

Pedestrian traffic

Mathematical modelling was used to investigate virus exposure levels associated with one-way and two-way pedestrian traffic patterns within academic buildings. Two assumptions used were that risk of infection is a product of exposure and time an that exposure rate decreases with distance. That a small exposure to a large number of people is similar to a large exposure to a few people is an underlying assumption used to minimize exposure risk. The study highlighted that restricting foot traffic to one direction reduces exposure per unit time, it can increase the total exposure time in the hallway.

Airborne and aerosol transmission

Airborne transmission refers to the presence of the virus within particles less than 5 μm, referred to as droplet nuclei or aerosols, that can remain in the air for long periods of time and transmitted over distances greater than 1 m. Aerosols evaporate in the air and leave behind droplet nuclei that are similar to pollen in that they are so small and light that they remain suspended in the air for hours.

Airborne transmission of COVID-19 may be possible during procedures or support treatments that generate aerosols such as endotracheal intubation, bronchoscopy, open suctioning, administration of nebulized treatment, manual ventilation before intubation, turning the patient to a prone position, disconnecting the patient from the ventilator, non-invasive positive-pressure ventilation, tracheostomy, and cardiopulmonary resuscitation.

Outside of of hospital procedures, data suggest that aerosol transmission of SARS-CoV-2 is possible but is probably not the dominant form of transmission. Airborne transmission was not reported in an analysis of 75,465 COVID-19 cases in China.SARS-CoV-1 was detected in hospital air samples and the animal coronavirus PEDV was found to travel 16.1 km from an infected farm. SARS-CoV-2 RNA was detected in air samples in a hospital in the U.K. but no virus was culturable. Speaking and coughing has been shown to produce a mixture of both droplets and aerosols in a range of sizes that can travel up to 27 feet. Poor ventilation has been shown to prolong the amount of time that aerosols remain airborne.

In a study where aerosols of SARS-CoV-2 were generated using a three-jet Collison nebulizer, the virus remained viable for 3 hours. A similar study, that as of July 2020 was not yet peer reviewed, found SARS-CoV-2 to maintain infectivity when suspended in aerosols for up to 16 hours.

The reproduction number (R0) for COVID-19, the average number of people each person with COVID-19 infected was about 2.5 before measures were taken to mitigate spread. This number is similar to the reproduction for influenza and different from measles which has a reproduction number (R0) around 18 known to spread by aerosol. It possible that a much larger amount of SARS-CoV-2 is needed to cause infection than for measles or that the aerosol route is not the dominant mode of transmission for COVID-19.

A study that compared trends and mitigation measures during COVID-19 outbreaks in Italy, New York City and China suggests that airborne transmission contributed dominantly to the linear increase in infection prior to the onset of mandated face covering.

Environmental transmission

SARS-CoV-2 may be transmitted on fomites, which are objects or materials such as clothing, utensils and furniture, in the immediate environment or used by an infected person. Coronaviruses can be transmitted from dry surfaces by self-inoculation of mucous membranes of the nose, eyes or mouth.

Survival and transmission on surfaces

Generally coronaviruses are infective for shorter periods on copper, copper nickel and brass than stainless steel and zinc surfaces but each coronavirus can differ. SARS-CoV-2 survived longer on surfaces with higher porosity than those with lower porosity, where the difference was between hours and days.

Survival of SARS-CoV-2 on surfaces

Survival time


2 days

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3


1 day

N Engl J Med 2020; 382:1564-1567


1 day

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3


4 hours

N Engl J Med 2020; 382:1564-1567


2 days

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3


30 minutes

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3


4 days

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3

Stainless steel

3 days

N Engl J Med 2020; 382:1564-1567

Stainless steel

4 days

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3

Surgical facemasks (inner layer)

4 days

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3

Surgical facemasks (outer layer)

7 days

The Lancet Microbe doi: 10.1016/S2666-5247(20)30003-3

Waterborne and fecal-oral transmission

As of July 2020, fecal-oral transmission has not been demonstrated nor has the occurrence of infectious SARS-CoV-2 in water environments been proven. COVID-19 shows evidence for intestinal infection and presence in stool which gives it the potential to be a waterborne disease where water sanitation is not safely managed. According to the Environmental Protection Agency standard treatment and disinfectant processes for wastewater treatment are expected to be effective. Wastewater treatment workers could potentially have a route of respiratory exposure during the pumping of wastewater through sewerage systems the transport of coronaviruses in water where there is the potential for the virus to become aerosolised.

There is evidence that the SARS-CoV-2 virus that causes COVID-19 may infect the intestine and be present in feces. Intestinal cells, enterocytes express high levels of ACE2 which could support infection with SARS-CoV-2. Fecal excretion of PCR detectible SARS-CoV-2 virus was found to persist 1 to 11 days after sputum excretion of the virus. Infectious SARS-CoV-2 virus particles were isolated from feces in studies from China. . A study from the US found no infectious SARS-CoV-2 virus in ten stool samples using a cell-based assay.

A Spanish study reported that SARS-CoV-2 RNA was detected in frozen sewage samples from March 12, 2019, 41 days before the first COVID-19 case was reported in Spain. The RT-PCR technique used in the study cannot distinguish between infectious virus and inactive viral fragments so did not prove oral-fecal transmission but showed the potential for wastewater surveillance. Wastewater surveillance for SARS-CoV-2 is under investigation or implemented in the US, Netherlands, Finland and Germany as a method to monitor communities for COVID-19 resurgence.

Survival and transmission in water

Data suggest that coronaviruses are sensitive to oxidants like chlorine and that in water they are inactivated faster than non-enveloped human enteric viruses that are know to have waterborne transmission. The titer of infectious virus decrease more rapidly at 23°C–25 °C than at 4 °C.

Research from University of Glasgow that as of July 2020 was not yet peer reviewed, investigated the risk associated with sewage spill dilution within rivers and suggested that countries with the lowest relative risk have high domestic water usage and high dilution and including Canada, Norway and Venezuela. Countries with highest relative risk have low to medium domestic water usage and low dilution such as in Morocco, Spain and Germany. In addition the study pointed out the potential for the water environment to serve as a reservoir for SARS-CoV-2 and that bioaccumulation is possible in aquatic organisms that may produce a circular viral transmission from land to sea and back to land. Another possible route of transmission was suggested to be foodborne transmission on produce washed with contaminated water. The study predicted that the virus can remain stable for up to 25 days in water.

Transmissibility and non-pharmaceutical intervention

The transmissibility of SARS-CoV-2 and other viral pathogens is estimated using the reproduction number (R) which is the number of people that an infected individual will infect. When the R value exceeds 1, the numbers of incident cases will increase and below 1 the transmission of the pathogen will eventually cease. R is often assumed to be constant, although when it is considered to vary with time and location it is referred to as instantaneous R (Rt). Rt values were estimated in 211 counties across the US and it was concluded that social distancing, lower population density and temperate weather decreased Rt for SARS-CoV-2.

Non-pharmaceutical interventions (NPIs) aim to reduce the effective reproduction number (Rt) of the infection, which represents the average number of infections generated at time t by each infected case over the course of infection. The effects of major NPIs across 11 European countries from the beginning of COVID-19 epidemics until May 4, 2020 when lockdowns began to be lifted were studied by calculating backwards from observed deaths to estimate transmission that occurred previously. The time-varying reproduction number (Rt) was estimated to be driven below 1 with current interventions from initial reproduction numbers around 3.8. Lockdown had a large impact on transmission and was estimated to impact transmission by 81%.

Masks and face coverings

Community-wide mask wearing is thought to control the spread of COVID-19 by reducing the emission of saliva and respiratory droplets from COVID-19 patients that may have subclinical or mild symptoms. In the Hong Kong Special Administrative Region (HKSAR) compliance for mask wearing in the general public was 96.6% from December 31, 2019 to April 8, 2019. HKSAR had a much lower COVID-19 incidence over that time (129.0 per million population) compared with other countries such as Spain (2983.2) and the US (1102.8) that did not have have high compliance for wearing masks.

Although masks are most important for protecting others when the wearer is infected, a report in the Journal of Hospital Infection studied protection to the wearer with a variety of mask materials using mask data, modeling and simulation. The study found that N99 (FFP3) masks can reduce average risk by 94 and 99% for 20-minute and 30-second exposures to airborne particles. N99 masks are not easily available. Vacuum cleaner filters inserted into the filter pockets of cloth masks reduced infection risk by 83% for 30-second exposure and 58% for a 20-minute exposure. Scarves offered the lowest protection and reduced infection risk by 44% for 30 second exposure and 24% for a 20 minute exposure. The study did not consider potential viral transfer from hands to masks and assumed all masks were work in the same way. The fit of the mask is variable in homemade masks.

Face shields are clear plastic barriers that cover the face. Face shields are easily cleaned with soap and water or disinfectants, can prevent the user from inoculating themselves through touching the face and allow visibility of facial expressions and lip movements. Face shields can provide protection for the wearer against droplets expelled from another individual but they can still be inhaled in the space around the shield. Face shields may provide less protection to others compared with masks because respiratory droplets expelled by the wearer can escape around the sides. Healthcare workers often wear both face masks and face shields. Qatar Airways requires passengers and crew to wear both face masks and face shields on flights.

Testing influenza exposure with cough simulator and model of a shield-wearing person, face shields were found to reduce inhaled viral exposure by 96% when worn by a health care worker at a distance of 18 inches. After 30 minutes the protective effect of face shields were shown to be 80% and 68% for small particle aerosols. At a distance of 6 feet face shields were similar to distancing alone in reducing exposure to inhaled virus by 92%. The study found that for longer durations (30 minutes) a face shield may have a less protective effect against smaller particles with a volume median diameter (VMD) of 3.4 μm because they stay airborne longer and flow around the face shield.

NPI effects on COVID-19 transmission


Estimating the effects of non-pharmaceutical interventions on COVID-19 in Europe

June 2020

Rt below 1 (May 2020), initial R=3.8, lockdown reduced transmission by 81%

Estimation of Effective Reproduction Number for COVID-19 in Bangladesh and its districts

August 2020

Bangladesh Rt 1.07 (July 2020), reduced from Rt of 3, Rt continued to fall after lockdown measures were eased

Lockdown Strategies, Mobility Patterns and COVID-19

May 2020

Physical distancing, face masks, and eye protection to prevent person-to-person

June 2020

Eye protection relative risk RR 0.34 of infection

Face masks RR 0.34 of infection

Quantifying the impact of physical distance measures on transmission of covid-19 in the UK

May 2020

UK lockdown R0 from 2.6 down to 0.62

Domestic cats

Human-to-feline transmission of SARS-CoV-2 and airborne transmission among cats has been reported, suggesting the potential of a human-cat-human chain of transmission. Cats may not show appreciable symptoms and may be silent intermediate hosts of SARS-CoV-2. While cats and ferrets were shown to be permissive to SARS-CoV-2 infection the virus replicated poorly in dogs, pigs, chickens and ducks.




Further reading


Airborne Spread of SARS-CoV-2 and a Potential Role for Air Disinfection


Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough


Persistence of coronaviruses on inanimate surfaces and their inactivation with biocidal agents

G. Kampf, D. Todt, S. Pfaender, E. Steinmann


March 2020

Theory versus Data: How to Calculate R0?

Romulus Breban, Raffaele Vardavas, Sally Blower


March 14, 2007

Documentaries, videos and podcasts





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