Inside the Making of the Flu Vaccine

An in-depth look at the CDC process. (It’s basically a high-stakes game of biological matchmaking.)

Toward the end of last year’s flu season — in about the middle of winter — flu experts noticed that the number of infections was not dropping, but picking up. A new virus seemed to be emerging, and it was making many Americans sick.

The virus was a type of H3N2 strain, and it started rapidly spreading among people in the United States before spreading into Europe, Canada, and South America. This put scientists worldwide who are tasked with determining what’s in the yearly flu vaccine on high alert. Did the flu vaccine for the 2019–2020 flu season need to be updated to protect against this new virus strain? The next flu season was still many months away, but it was time to scramble.

“Almost every year there is some kind of challenge,” says Dr. David Wentworth, a leading flu expert and chief of the U.S. Centers for Disease Control and Prevention’s (CDC) Virology, Surveillance, and Diagnosis Branch. “The thing about the flu is you have to be nimble.”

One of the trickiest tasks for public health experts worldwide is creating the annual flu vaccine. The number of people who get sick from the flu each year depends largely on how well scientists…