Three really could prove to be the magic dosing number — if the coronavirus remains relatively stable. Protecting against Hepatitis B, for example, requires three vaccine doses for lasting immunity…

And there is a chance that Covid vaccines are going to become an annual tradition. If the virus mutates enough, people would require follow-up doses of a differently formulated vaccine.

This is what happens with the flu. More than 100 years after the 1918 pandemic, the flu is still with us, with a different strain dominating each winter. The flu shot you get (or should get) every fall isn’t a booster. It’s a different vaccine formula that’s designed to protect against whatever new strain pops up.