In younger people who have been vaccinated or were infected earlier, the immune system can still potentially prevent progression to serious illness by mounting a “memory response” — essentially remembering that it encountered a similar virus before and unleashing more action. As we age, this response is less robust because our immune system becomes weaker. Even common colds can cause fatal outbreaks in nursing homes…

The most obvious and important step is to give boosters to all residents immediately. Studies have long made it clear that booster shots can improve both the quality and the quantity of the antibody response to SARS-CoV-2. It takes a few weeks for boosters to take better effect, so starting now is crucial.

“Elderly people can struggle to mount and maintain effective memory responses, making the presence of neutralizing antibodies, which function to block the virus from efficiently infecting our cells, all the more critical at the moment of exposure,” Andrew Croxford, an immunologist based in Zurich, explained to me. Croxford says that all this could compound risk in the most frail, making a third dose for these individuals a matter of utmost importance.