Influenza

Last winter was H3N2; this autumn nearly every subtyped flu A virus in the U.S. and Canada is H1N1, a close relative of this year's vaccine strain. Here is what the numbers, the lab tests and the waning evidence say about timing your shot.

For the first time in years, regulators rebuilt the flu vaccine from scratch, swapping the H1N1, H3N2, and B strains after a fast-mutating H3N2 variant cut protection in some patients to as little as 14%.