Consumer’s Guide to Flu Vaccines – Everyday Health

The flu vaccine is updated every year to match the specific strains of the influenza virus experts predict will be most prominent during flu season. To determine this, the World Health Organization Global Influenza Surveillance Response System, made up of 144 influenza centers in 114 countries, conducts year-round surveillance to determine which flu strains are actively circulating around the globe.

Twice a year, representatives from five of the top participating centers come together to review data and recommend which strains to include in the upcoming flu vaccine.

These flu experts try to anticipate what the dominant strains are going to be nine months down the road, explains William Schaffner, MD, professor of infectious disease at Vanderbilt University School of Medicine in Nashville. They're often on target, but on some occasions, the flu virus that circulates varies from the vaccine. How effective the influenza vaccine is changes from year to year too.

In other words, flu virus experts have to base their recommendations on a moving target, which means the flu vaccine cant be perfect. Even so, according to the Centers for Disease Control and Prevention (CDC), the most recent research suggests the flu shot lowers the risk of illness from 40 to 60 percent.

As for the specific influenza strains included in the flu vaccine, for the 2022-2023 season, there are four: two influenza A strains and two influenza B strains. In previous flu seasons, most vaccines protected against only three strains.

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Consumer's Guide to Flu Vaccines - Everyday Health