One of the major strains of influenza virus circulating among people this flu season only partly matches what’s in this year’s vaccine, the Centers for Disease Control and Prevention (CDC) announced earlier this month.
But before you buy into the headlines and cancel your plans to get your flu shot, consider this:
“There is no real correlation between percentage match and how well the vaccine works in given a season,” says Craig Roberts, physician assistant and epidemiologist at University Health Services at the University of Wisconsin-Madison. “People shouldn’t jump to conclusions about their risk of getting sick.”
In other words, just because one of the strains in the vaccine isn’t a perfect match, it doesn’t mean the flu vaccine doesn’t work, and it doesn’t mean you’ll still get sick if you get vaccinated.
This is especially true in Wisconsin. According to Dr. Jon Temte, a family medicine physician with UW Health, the flu vaccine in north-central states, including the Badger State, is a better match to the viruses circulating here.
When you get a flu vaccine, you get versions of the virus that can’t make you sick, yet, it tells your immune system what defenses it needs to build. That way, if you pick up a natural virus from say, the sneezing person sitting next to you on the bus, your body already has the arsenal to fight it off.
Each year, the flu shot contains a cocktail of three or four viruses predicted to be the predominant strains for a given season (the nasal vaccine contains four): one or two type B influenza viruses and two type A influenza viruses, which are named for two proteins on their outer coats (the ‘H’ and the ‘N’ in flu viruses like H1N1 and H3N2). Health officials work hard to include the most likely strains in the vaccine, but because they must choose before the flu season starts, its an imperfect process, especially because the influenza virus is an elusive enemy.
Though the vaccine protects against several different types of flu viruses each season, influenza viruses are prone to tiny genetic changes that allow them to morph, ever so slightly, into different subtypes that can evade your body’s defenses. This is called antigenic drift.
Sometimes, the changes are minor enough not to cause significant differences and the vaccine and virus then match. Other times, as happened this year, changes alter the parts of the virus that your body is taught by the vaccine or prior illness to recognize and fight off, and the vaccine and the virus are mismatched. (Yet other times, the virus changes so much as to look like a completely different animal. This is, for instance, what happened with H1N1 in 2009.)
This year, the H3N2 (influenza A) type virus is circulating as two subtypes, one that matches the vaccine and one that does not, and it’s also the dominant virus strain making people sick so far, accounting for nearly 95 percent of the illnesses reported to the CDC through the first week of December.
Nationally, roughly 30% of the circulating H3N2 strains match the flu vaccine, while close to 70% of the strains are a different H3N2 subtype.
This is yet another reason to get the flu vaccine, Roberts says, since 30% of the time, the virus and the vaccine will be a match. Historically, the H3N2 virus tends to cause more illness than other flu types, with hospitalization and death rates that can be twice as high as in seasons where H3N2 does not predominate, CDC Director, Tom Frieden, said in a recent briefing.
Of course, the dominant strain could also change over the course of the season, Roberts points out, and you’ll want to be protected against H1N1 and type B influenza, which usually becomes more dominant later in the season.
Children and the elderly are more likely to become sick with the flu and pregnant women, especially, want to get the flu shot, says Dr. Sarah Bradley, an obstetrician/gynecologist and clinical assistant professor at the University of Wisconsin School of Medicine and Public Health.
“During the 2009 H1N1 epidemic, pregnant women represented 5% of all deaths that occurred from the flu even though pregnant women make up only 1% of the population,” she says. “Pregnant women are at increased risk of these complications because of the physiologic changes that happen to women’s bodies in pregnancy, including changes in lung function and in how the immune system works.”
Pregnant women who catch the flu may be at risk for other complications such as pre-term labor and low birth weight, Bradley says, while the vaccine will cause no harm to the fetus or the mother.
At UW-Madison, Roberts says University Health Services has already confirmed 22 influenza cases in students so far this semester and all of them had one of the H3N2 subtypes. The majority of them were not vaccinated against the flu this year.
Susan Lampert Smith and Toni Morrisey contributed to this post.