Dear Dr. Roach: Influenza immunity, however acquired, not always long-lasting
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Dear Dr. Roach: Influenza immunity, however acquired, not always long-lasting

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Dr. Keith Roach

Dear Dr. Roach: As I understand it, the influenza virus nomenclature is H#N#, where the numbers can be 1 through 9. I may have the letters backward but H defines how the virus enters and N defines how the virus exits the cells.

Here is the stupid question: If we get immunized for all the H1 through H9, why is it necessary to get a flu shot every year? Also, if we have had an H1 flu, aren't we immune to further H1 strains regardless how they exit? I am tired of being a pin cushion.

— R.F.

A: It's a bit more complicated. There are 18 subtypes of hemagglutinin, a viral protein that lets the influenza virus attach to target cells, and nine subtypes of neuraminidase, which in addition to opening up the infected target cells to let more flu virus out, allows the virus to move through the mucus in your respiratory tract, making it more effective at causing disease. Certain strains of neuraminidase (N1 and N2) are more virulent than others, even though N3 and N7 may also cause death.

Unfortunately, immunity, whether from natural infection (if you survive) or from the influenza vaccine, is not always long-lasting. Half of people who were infected with the 2009 H1N1 flu lost their antibodies within six months. On the other hand, survivors of the 1918 (Spanish) flu (also H1N1) continued to have protective antibodies 90 years later, and also had immunity to the 2009 strain. It's not clear why the immunity lasts so long in some strains and not others.

Even within one particular strain of hemagglutinin, the antibodies produced may or may not provide immunity to other strains. The influenza virus is highly adaptable and variable. There has, however, been promising work on a "universal" flu vaccine that may solve the problem we all have, which is that being a pin cushion year after year is the only way to ensure up-to-date immunity, and even that immunity is not perfect, as the particular strain circulating doesn't always match the strain the vaccine was made for.

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