Why you can get the flu even if you've had the flu shot

Why you can get the flu even if you've had the flu shot


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ATLANTA, Georgia (CNN) — Have you noticed something missing from the news headlines this winter? Those ominous reports that the current flu season is shaping up to be a bad one?

That is because signs so far point to this flu season being a mild one. There were several hundred confirmed cases of flu per week in December, compared with a few thousand per week the previous December, according to the Centers for Disease Control and Prevention flu surveillance system. (This system only detects a fraction of all the flu cases in the country to allow comparison of trends between years.)

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The later the flu season starts, the more mild it is, usually, because there's just less time that viruses are circulating in the environment that season. But it is too soon to know what kind of flu season we are in store for, said Dr. Joseph Bresee, chief of epidemiology in the flu division at CDC.

Recent years were pretty bad, so this flu season could end up looking good by comparison, or the number of flu cases could catch up with those of previous seasons. The classic scenario is for the first flu cases to start to appear in the United States in October or November, then the pace picks up in January and peaks in February or March.

"It's clear that we are on the uptick now and I expect that will continue for the next few weeks," Bresee said.

Given the likelihood that cases of flu will continue to increase over the next several weeks, Bresee recommends that everyone who hasn't had a flu shot yet get one immediately. Ideally, people receive the vaccine when it is first available in their area, usually in September, but getting it now will still provide weeks to months of protection, he said.

So far, H1N1 and H3N2 strains have been the most commonly detected by the CDC surveillance system, and both strains match those that are in the vaccine. This year, no new mutations have been detected in the common flu viruses.

"It looks like the viruses we are seeing in the U.S. and overseas look very much like the vaccine strains," Bresee said. Although this is reassuring, mutations could still emerge, he added.

Getting the flu despite the shot

As many learn the hard way, it is possible to get the flu after you've had a flu shot.

Each February, the World Health Organization holds a meeting of flu experts to determine the strains that are most commonly infecting people, a moving target as the virus is constantly mutating and changing how efficiently it spreads. A committee determines the three or four flu strains to include in next year's flu shot based on the meeting's recommendations.

The major concern heading into last year's flu season was that mutations in an H3N2 strain of the virus were detected in circulation after the vaccine had already been made. Although H3N2 was present in the flu shot (at least a dead version of it was), the new mutations could make the virus look different enough that the immune system would not recognize it.

The concern turned out to be founded: The 2014-2015 flu shot only reduced the risk of getting the flu by about 20 percent, whereas in a good year that rate is closer to 60 percent.

Although this year's vaccine is matching the most common strains so far, there have been so few cases of flu to date it is hard to say which virus will dominate this season, said Michael L. Jackson, an associate scientific investigator at Group Health Research Institute in Seattle.

If doctors could pick which strain they want to avoid, it would be H3N2. "H3N2 viruses tend to mutate faster than the other influenza virus types [and] years where H3N2 dominate tend to have more cases of hospitalizations," said Jackson, whose institute is part of the U.S. Flu Vaccine Effectiveness Network, a CDC-funded network that measures how well the vaccine is preventing flu cases every year.

New research suggests there could be another wrinkle in identifying which flu viruses are in circulation. Researchers looked at flu spread among 84 people in Hong Kong during the 2009 flu season. They found that, although H1N1 and H3N2 were predominant, individuals were also getting infected at the same time with other, less common viral strains.

"We tend to only detect the dominant strain in flu surveillance and design the vaccine based on that," said Elodie Ghedin, a biology professor at New York University, who led the study, which was published on Monday in the journal Nature Genetics. The study suggests these minor strains can persist in the population, and may even become predominant.

Vaccine effectiveness

Even in the best-case scenario, when the commonly circulating strains match those in the vaccine, the vaccine usually only protects 50 percent or 60 percent of those immunized from getting the flu, Jackson said. The elderly and children are at highest risk of not being protected from the flu shot.

Although there are probably many still-unknown factors that determine why some people are protected and others aren't, it might be that the elderly population's immune systems do not perform as well as those of younger, healthier adults, Jackson said. Because of this, the CDC recommends that people 65 and older receive a high-dose flu shot.

Among children, other scenarios are probably at play; their immune systems are not mature enough to develop a proper immune response to flu vaccine, Bresee said. In addition, they have probably not been exposed to the flu virus, either through natural infection or immunization, and thus their immune systems have not been primed to recognize the virus. For this reason, children between 6 months and 8 years of age should receive two doses the first time they get the flu shot.

Despite the fact that the vaccine doesn't work every time, it is still the best way to protect yourself against flu, bar none.

–Dr. Joseph Bresee, CDC

"Despite the fact that the vaccine doesn't work every time, it is still the best way to protect yourself against flu bar none," Bresee said. And it is worth getting a simple vaccine to protect against a disease that is far more dangerous than the common cold, and can lead to death and hospitalization even in healthy people, he added. (There are several groups of people who should not get a flu shot, such as those with allergies to ingredients in the vaccine.)

Other ways to protect yourself

You can also protect yourself and others from the flu by doing "the things that your mom taught you — wash your hands and stay home if you are sick," Bresee said.

Avoiding the flu
  • Wash your hands often with soap and water. If not available, use an alcohol-based hand rub.
  • Avoid touching your eyes, nose and mouth.
  • Try to avoid close contact with sick people.
  • More tips at cdc.gov/flu.

He also recommends that people who think they have the flu talk with their doctor about taking antiviral medications such as Tamiflu, especially if they are elderly or have conditions such as lung or heart disease that put them at higher risk of complications from the flu.

Copyright 2016 Cable News Network. Turner Broadcasting System, Inc. All Rights Reserved.

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