A magazine for friends of the Warren Alpert Medical School of Brown University.

Shot in the Dark


Flu vaccines are a hit-or-miss science.

If the flu shot is less effective this year, why should someone get vaccinated?

As the influenza season ramped up this winter, the CDC reported that the vaccine was significantly less effective due to a mutated strain. Penelope Dennehy, MD, professor of pediatrics, whose research includes flu vaccine effectiveness, explains why we should still get the shot.

Even in a year where the vaccine matches all the circulating flu v iruses, its effectiveness isn’t 100 percent— it’s more like 60 percent on average. Influenza viruses are RNA viruses, which can make small errors in their genome when replicating, producing a drifted strain that is not recognized by the host and can escape vaccine-induced immunity.

The available vaccines this year protect against two influenza A viruses, H3N2 and H1N1, and two influenza B viruses. During a flu season we may start off with one particular A strain, and later we may see a B strain or a different A strain. Vaccine manufacturers determine which strains to put in the vaccine months ahead of time, by looking at what happened in the Southern Hemisphere flu season, what’s happening in China, and what happened here last year. This year they did a pretty good job: three out of the four strains are right on. But unfortunately the H3N2 strain, which tends to be more severe than H1N1, drifted. Even though the vaccine is less effective, you may have milder illness, as well as protection against the pandemic H1N1 strain, which is unchanged, and the B strains.

It’s never too late to get the vaccine as long as we’re still seeing influenza. For several years we’ve been seeing it early in Rhode Island, and the flu season can drag on with a B strain appearing in March and into April. So if you’re not vaccinated, it’s not like the horse is out of the barn and galloping down the road. It’s just partially out of the barn—so you may still benefit from the flu vaccine this year.


Comments are closed.