Do you have a fever and headache? Is your nose running, with a sore throat and dry cough? Are you tired with muscle aches? You might have influenza or “the flu” as we commonly call it. You may have gotten vaccinated and you still got a bad case of the flu. This is one of those influenza seasons where the experts didn’t quite get it right.

So what happened?
Influenza virus is constantly changing and evolving and each year public health experts get together several months in advance (the FDA will make their recommendation for next year’s vaccine on February 21, 2008) of the flu season to choose three strains of influenza to put in the vaccine, H1N1 strain, H3N2 strain and a B virus strain. It is part science and part art. For more information on this process try your hand at predicting the flu using a program developed as part of the Disease Detective exhibition currently at the Science Museum of Minnesota.

According to Dr. Joe Bresee, the Branch Chief, in the branch of Epidemiology and Prevention of the CDC’s Influenza Division, the H1N1 strain in the vaccine this year is a good match to the circulating strain but the H3N2 type strain and the B strain are not ideal matches. So protection is probably lower than expected in a season when vaccine strains and circulating strains are well matched. Usually, the guesswork is pretty good: Bresee has said, the vaccines have been a good match in 16 of the last 19 flu seasons.

Why are people so sick this year?
It just so happens – and we don’t know why – that H3N2 strains of influenza virus are more severe. For this season the number of people infected with H3N2 influenza is on the rise. The past two years the H1N1 strain predominated and so those infected were not as sick.

What are all these H’s and N’s?
Influenza is categorized as influenza A type viruses and influenza B type viruses. The A viruses are further broken down and characterized by the proteins found on the surface of the virus. These proteins are called hemagglutinin (H) and neuraminidase (N). The proteins change as the influenza virus evolves so numbers are used to designate the different types of H and N proteins.

Remember vaccination is still your best defense against influenza. The experts have been right 16 of the past 19 seasons and as Dr. Bresee reports…

But even in those years where the vaccine matches less well against the circulating strains, we know that getting vaccinated will tend to make the illnesses milder, lessen the chances a person has a very severe outcome.

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