What’s in a flu vaccine?

Flu viruses come in 2 types and several strains

There are 2 common types of flu viruses affecting humans: A and B. The A and B viruses can also come in different strains, so there are several different strains of virus that can cause the flu every year.

The CDC decides which strains to protect against

Every year, the CDC identifies which flu strains are currently circulating around the world. Then, they forecast which strains will circulate in the upcoming flu season. Based on that forecast, the CDC selects 2 A strains and 2 B strains and gives that recommendation to flu vaccine manufacturers.

Circulating flu is unpredictable, and the circulating strain may not be the strain contained in the vaccine. Even in mismatch years, the CDC still recommends getting a vaccination.

The Advisory Committee on Immunization Practices (ACIP) recommended that FluMist Quadrivalent not be used for the 2017-18 influenza season. We encourage patients and caregivers to have conversations with their healthcare providers about this year’s ACIP recommendation as part of discussions around influenza vaccination.

Flu vaccines may protect against 3 or 4 different strains

Traditionally, all flu vaccines protected against 3 strains. These are called “trivalent” flu vaccines. Now, some flu vaccines protect against 4 strains. These are called “quadrivalent” flu vaccines.

After the CDC forecasts which flu strains are likely to circulate, flu vaccine manufacturers decide whether to design a flu vaccine that includes 3 strains (2 A and 1 B) or 4 strains (2 A and 2 B).

A quadrivalent vaccine that helps protect against 2 A strains and 2 B strains may reduce the risk of being unprotected against the flu.

To learn more, visit the CDC website at