Do They Really Just Guess at Which Strain to Put in the Flu Vaccine?

Every year we hear experts telling us to get vaccinated against the flu.

And more often than not, we hear critics telling us that the flu vaccine isn’t going to work that well because it isn’t a good match.

Is that because they just guess at which flu strains to put into the flu vaccine each year, as some folks say?

Do They Really Just Guess at Which Strain to Put in the Flu Vaccine?

Of course they don’t guess at which vaccine strain to put in the flu vaccine!

“Flu viruses are constantly changing, so the vaccine composition is reviewed each year and updated as needed based on which influenza viruses are making people sick, the extent to which those viruses are spreading, and how well the previous season’s vaccine protects against those viruses.”

CDC on Selecting Viruses for the Seasonal Influenza Vaccine

There are over 100 flu centers in over 100 countries that are involved in testing thousands of flu virus samples each year. Representative samples from these centers then go to the five major World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza.

The directors of these centers review these samples and other available information and make a recommendation on which vaccine strains to include in the flu vaccine for the next flu season. Each country then considers this recommendation and decides which flu strains to include in their flu vaccine.

“The influenza viruses in the seasonal flu vaccine are selected each year based on surveillance data indicating which viruses are circulating and forecasts about which viruses are the most likely to circulate during the coming season.”

CDC on Selecting Viruses for the Seasonal Influenza Vaccine

Although they don’t have a crystal ball and so can’t know exactly which flu strains will be making us sick each flu season, it is hardly a wild guess.

How often are they right?

“During seasons when most circulating influenza viruses are closely related to the viruses in the influenza vaccine, the vaccine effectiveness estimate has ranged from 50-60% among the overall population.”

WHO on Vaccine effectiveness estimates for seasonal influenza vaccines

Actually, they are right in most years! So if it is a guess, than the folks who choose which strains to include in the flu vaccine are very good guessers!

Except for a few years when their was a poor match, the flu season is typically between 37 to 60% effective.
Except for a few years when their was a poor match, the flu season is typically between 37 to 60% effective.

There have actually only been a few times in recent years when we have had mismatched flu strains. And in one of those years, they picked the right strain, but then the strain changed or drifted before the start of flu season.

“One hundred fifty-six (22%) of the 709 influenza A(H3N2) isolates were characterized as antigenically similar to A/Wyoming/3/2003, which is the A/Fujian/411/2002-like (H3N2) component of the 2004-05 influenza vaccine, and 553 (78%) were characterized as A/California/7/2004-like.”

2004-05 U.S. Influenza Season Summary

Not surprisingly, flu vaccine effectiveness goes way down during a mismatch year. During the 2004-05 flu season, for example, the overall vaccine effectiveness of the flu vaccine was just 10%.

Of course, in most years, flu vaccine is typically much more effective than that.

How effective will flu vaccines be this season?

Will this year’s flu vaccine be a good match?

We won’t know until flu season is well under way, not that you should wait for an answer to get your flu vaccine. The benefits of the flu vaccine extend beyond preventing the flu, so it is a good idea to get vaccinated even in a year when there might not be a good match.

More on Selecting Flu Vaccine Strains

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.