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

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Link
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Date

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

Web

May 2020

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

Web

September 2020

Estimating COVID-19 outbreak risk through air travel

Web

June 5, 2020

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

Web

May 5, 2020

Tracking the Spread of COVID-19 with Air Travel Data

Web

May 2020

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 passenger person-to-person transmission

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

Web

July 5, 2020

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.

Web

July 4, 2020

Lack of COVID-19 Transmission on an International Flight

Web

February 24, 2020

Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis.

Web

June 2020

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

Web

July 2020

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

Web

May 2020

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 hither 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

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Investigation of a cluster epidemic of COVID-19 in Ningbo

Web

May 13, 2020

The Subway Seeded the Massive Coronavirus Epidemic in New York City

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April 24, 2020

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.

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.

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 is between 10% and 40%. 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.

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 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 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

Fomites

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.

Material
Survival time
Reference

Banknote

2 days

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

Cardboard

1 day

N Engl J Med 2020; 382:1564-1567

Cloth

1 day

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

Copper

4 hours

N Engl J Med 2020; 382:1564-1567

Glass

2 days

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

Paper

30 minutes

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

Plastic

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

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 taking into consideration contribution of social distancing, population density and seasonal weather changes which decreased Rt for SARS-CoV-2.

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.

Timeline

People

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Further reading

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Author
Link
Type
Date

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

Web

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

Web

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

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

Journal

March 2020

Documentaries, videos and podcasts

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Companies

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Products/Services

References

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