Should You Get a Flu Shot? New Study Suggests It Might Put You at Risk

Protecting populations from the flu is proving to be more complicated than researchers previously thought. A new study published in Science Translational Medicine highlights a surprising complication of vaccine design: an increased risk of contracting the flu when vaccinated.

In 2009 the H1N1 flu strain, commonly referred to as the "Swine Flu," reached pandemic proportions. The CDC estimates that up to 18,000 people died in the U.S. from this strain during the 2009-2010 flu season. While the mortality rate is horrifying on its own, what was perhaps more troubling is that Canadian researchers found an increased risk of H1N1 infection in individuals who were vaccinated against the flu that season. 

(Interrupting PSA: Jenny McCarthy would like you to stop reading here and swear off your flu vaccination, even though reputable scientists, physicians, and health care professionals highly recommend you get your vaccination. Seriously. Get vaccinated. And finish reading this article to understand why, because science is complicated, but you're smart and can handle it.)

Researchers at the Center for Biologics Evaluation and Research (CBER), a branch of the FDA, set out to understand why people vaccinated for the 2009 flu season were more inclined to catch the H1N1 strain of the flu. Since H1N1 originated from pigs and was later transferred to humans, CBER scientists used this animal model to study the reasons why the flu vaccine backfired that year.

So, some terms need to be defined before this turns into alphabet soup and flashbacks of freshman year biology class.

All flu strains are named by two proteins on the surface of the virus: hemagglutinin and neuraminidase, which interact with the cells of your body to infect you, hence the "H" and "N." The numbers refer to the slight variations in the way the H and N proteins are shaped.

It's important for the flu virus to rapidly evolve new H and N protein shapes to escape your immune response. Once you've had one type of flu, your immune system remembers it and can quickly produce antibodies to fight it off next time. This is how vaccines work. You're given a dead flu virus so that your immune system can safely learn its "shape" and fight it off when you're exposed to the real, live deal in the form of a sniffling stranger on the bus or your adorable, germ-riddled child.

Your immune system remembers viruses very specifically and if the flu you're exposed to is slightly different than the one you are vaccinated for, you're not protected. Not being protected is one thing, but how does being protected from one virus make you more susceptible to another?

To study the relationship of flu vaccination to susceptibility, CBER scientists infected two groups of pigs with H1N1; one group had not been vaccinated, while the other had been vaccinated with H1N2. Not surprisingly, given what happened in 2009, the vaccinated pigs experienced more severe symptoms than the pigs that were not vaccinated. However, what was surprising is that the same antibodies that physically blocked the hemagglutinin of H1N2 from invading a healthy cell interacted with the hemagglutinin from H1N1 in a way that brought the virus closer to the healthy cell, making it easier for the virus to attack. This was surprising because antibodies were previously not known to help a virus infect a host's cell.

Because viruses evolve rapidly and the immune system responds so specifically, "universal flu vaccines," designed to target multiple strains of the flu, have excited the scientific community for years. However, it's clear from the CBER study that this excitement should be tempered by the desire to minimize risk.

Simply vaccinating to create an immune response, i.e. antibodies, that responds to multiple strains of the flu is not a reasonable option, given that antibody interaction with a flu virus can have vastly different outcomes, as is seen with H1N1 and H1N2. Studying the molecular mechanisms by which antibodies either neutralize or enhance viral infection will help develop and test flu vaccines that will provide protection without unintended harm.