An adaptor for coupling with a medical container, including: a tubular body receiving a pierceable elastomeric piece, defining an inner cavity of the adaptor, the pierceable elastomeric piece being movable within the tubular body between a first position, in which a distal part of the pierceable elastomeric piece forms a seal of the cavity, and a second position, proximally spaced from the first position, in which the distal part opens the seal of the cavity, an air inlet, a filtering system for filling the inner cavity with decontaminated air, and a gripping member for securing the adaptor to the medical container. Also, an assembly comprising such an adaptor and a medical container.