Monday, July 27, 2009

Potential "mismatch" for the seasonal flu vaccine

Seasonal influenza vaccines are actually created to fight off several virus strains. Experts gather in late winter/early spring to select what they think will be the predominant strains next fall and winter. Then vaccine production begins.

Sometimes experts get it right, sometimes there's a "mismatch" and another virus strain begins to dominate.

Reporting from Helen Branswell and the ProMed network are raising the possibility that in addition to pandemic strain (novel H1N1), for which we will have limited vaccine supplies, there may be a mismatch for the seasonal flu vaccine as well.

"A component of the seasonal flu shot may not be well matched to the circulating viruses, potentially setting up what's known as a vaccine mismatch.

Some samples of the emerging new strain of H3N2 viruses show a substantially reduced response to antibodies generated by the corresponding virus in the seasonal vaccine, raising the possibility of significantly reduced protection in some cases.

Vaccine mismatches are bad at the best of times. More people get sick during flu seasons with mismatches. But a seasonal flu vaccine mismatch coinciding with a flu pandemic? That is no one's idea of a good time."

We don't know yet, however, how much of the seasonal flu we will see in the Northern Hemisphere or if it will be crowded out by H1N1. Also, worlwide we haven't be able to track the varriant of H3N2 because of the flood of pandemic virus reporting and analysis:

"The new variant has been seen on a number of continents, though it still remains a minority member of the H3N2 family, according to experts at the World Health Organization and the U.S. Centers for Disease Control in Atlanta.

With the demands the ongoing pandemic is placing on the WHO's laboratory network, researchers haven't yet had time to study whether the new variant is making up a growing percentage of H3N2 viruses, said Dr. Nancy Cox, director of the CDC's influenza division.

If they were, that would suggest the variant was on its way to becoming the dominant H3N2 virus and a vaccine mismatch would be on the cards.

Further clouding the issue is the fact that labs around the world haven't been submitting as many H3N2 viruses to the WHO network. There are simply fewer of them around.

"We haven't had that many H3N2 viruses to analyze because we've had such a flood of the novel H1N1 viruses because they're predominating," Cox said."

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