Who needs COVID boosters? Experts must decide with incomplete data

COVID-19 vaccine holdouts have no monopoly on doubt. Even scientists who fervently believe the shots are the way out of the pandemic often wish they had a bit more proof.

Proof of when the vaccines start to protect, and of how long they do so. Proof that an individual would be better off with a booster dose. Proof, even, that one vaccine might work better than another.

But all of those things require something that has not yet been established: a common currency by which peoples' levels of protection can be reliably measured, valued and compared. If it rises, that means a person's defenses are stronger; if it falls, they've become more vulnerable.