No-one can say for sure how many times people are going to need to be infected to confer effective immunity against transmission
In October 2020, a few weeks before the experimental trial results for the BioNTech-Pfizer, Oxford/AstraZeneca Covid-19 and Moderna vaccines were released, German virologist Christian Drosten cautioned that the shots would be of limited effectiveness in preventing the spread of the disease.
Which is, of course, exactly how things played out. The vaccines have been spectacularly effective at preventing severe disease and death, much less so at preventing transmission. It is this mucosal immunity that keeps influenza from spreading most of the year, with the disease’s effective reproduction number surpassing one for only a few months in winter, when indoor crowding, drier air and other factors seem to lead to increased transmission.
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