This year’s flu season may be particularly grim due to an unexpected shift in influenza strains, predicts the Centers for Disease Control and Prevention.
Most cases of flu this year are caused by a subtype called H3N2, which is covered in the vaccine, but about half those cases are a new strain that the shot doesn’t protect well against, said CDC officials.
This new player may make the Northern Hemisphere’s 2014-15 flu season particularly deadly — and it’s all down to “bad luck and bad timing,” said Dr. Trevor Bedford of Fred Hutchinson Cancer Research Center.
“This year may be more deadly for two reasons: more infections, and a more virulent strain,” explained Bedford, who studies virus evolution.
There are four major types of influenza virus, and myriad strains within each type, due to the many mutations each accumulates over the course of a few years. The yearly vaccine includes strains from each major type in a bid to offer protection against the most common strains circulating through the world.
But the new strain may have “drifted” enough that “our immune systems see it differently,” and cannot fight it as effectively even after vaccination, said Bedford.
On top of this, each major type differs in virulence, he noted. It’s bad luck that the new strain is an H3N2 influenza A strain, which generally produces more severe symptoms than an H1N1 strain, which was the cause of most flu cases last year. A more transmissible virus and a less effective vaccine could lead to more infections and possibly more deaths.
“We know that in seasons when H3 viruses predominate, we tend to have seasons that are the worst flu years, with more hospitalizations from flu and more deaths from the flu,” said CDC director Dr. Tom Frieden in a press briefing. “The rate of hospitalization and death can be twice as high or more than in a flu season when H3 doesn't predominate.”
It’s a case of bad timing that this year’s vaccine formulation missed the H3N2 drift. The World Health Organization makes its recommendation in February to allow enough time for vaccine production and dispersal. Unfortunately, the new H3 strain didn’t start picking up steam until March and April. Luckily for those in the Southern Hemisphere, whose flu season coincides with our summer, the strain emerged early enough to be included in their vaccine formula, Bedford noted.
It’s too early to know for sure how bad this year’s season will be, cautioned Frieden in a news release. During the 10-year span from 1997 to 2007, on average an estimated 32,000 people a year died of the flu, according to CDC data.
Even though this year’s flu vaccine doesn’t offer as much protection as hoped, people should still get it anyhow, said Bedford. “The vaccine will almost certainly provide some protection [against the new H3N2 variants], which will help. Plus you’ll get good protection against the other types of influenza. It just won’t be quite as effective as in other years.”
Also, continued viral evolution could drift the viruses back on target later in the season, upping the vaccine’s protection, noted Dr. Joseph Bresee, the CDC’s chief of the Influenza Epidemiology and Prevention Branch, during a briefing.
Keep yourself protected against the flu this year by getting vaccinated and taking precautions:
Dr. Sabrina Richards, a staff writer at Fred Hutchinson Cancer Research Center, has written about scientific research and the environment for The Scientist and OnEarth Magazine. She has a Ph.D. in immunology from the University of Washington, an M.A. in journalism and an advanced certificate from the Science, Health and Environmental Reporting Program at New York University. Reach her at firstname.lastname@example.org.