The ineffectiveness of the flu vaccine for the 2014-2015 flu season was due to a single mutation in the H3N2 virus, according to a study published today in Cell Reports.

Officials at the U.S. Centers for Disease Control and Prevention (CDC) said this mutation meant the vaccine was less than 20 percent effective at curtailing medical visits, compared with a 60 percent effectiveness of other seasonal flu vaccines from the past 10 years.

flu virus

Seasonal flu vaccines encourage antibodies to develop in the body. These antibodies, which fully develop about two weeks after the vaccination, protect against infection against the viruses that are in the vaccine.

However, the flu virus mutates often, meaning the virus that is circulating may not be aligned with the virus present in the vaccine.

When this happens, the mismatch between virus and vaccine means that the vaccine’s antibodies are not signaled to attack the mutated flu virus. “Flu vaccines work best when they are similar to most circulating flu strains,” says senior study author Scott Hensley, Ph.D., of the Wistar Institute.

According to Hensley, the H3N2 virus that mutated this season is more likely to acquire mutations than other strains of the virus.

“This kind of vaccine mismatch is very rare,” Hensley said, “But in humans, H3N2 does tend to acquire mutations more than H1N1 [another strain of the flu virus].”

Related News: Swine Flu Epidemic in India May Be More Powerful Than 2009 Global Epidemic »

Flu Shots Still Recommended

Despite the inefficiency of the flu vaccine this year, CDC officials still recommend annual flu vaccinations for everyone 6 months and older.

“Influenza is a serious disease that can lead to hospitalization and sometimes even death,” said CDC spokesperson Ian Branam. “A flu vaccination can reduce flu illnesses, doctors’ visits, and missed work and school due to flu, as well as prevent flu-related hospitalizations and deaths.”

Hensley also agreed that the inefficiency of this year’s vaccine should not be a deterrent for future vaccinations.

“Study after study has shown that flu vaccines save lives.” Hensley said. “There’s virtually no reason not to get a flu vaccine. They’re very safe.”

Hensley also noted that a flu vaccine can do more than just protect against the influenza virus itself.

“Even if there’s a mismatch, and the vaccine doesn’t protect against influenza, getting a flu vaccine can likely stop more serious illnesses from occurring,” he said.

Healthline contacted Dr. Tom Jefferson, an epidemiologist with the Cochrane Collaboration who has been critical of flu vaccines, but he did not respond for comment.

Hensley did acknowledge that matching viruses to vaccines is a difficult task.

“It’s a tough problem,” he said, “but we do the best we can.”

Read More: Seasonal Flu Vaccine Protects Against H7N9 Bird Flu »