Is This Year’s Flu “Immune” to the Flu Shot?

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High fever and chills, awful muscle and joint aches, unending cough and pounding headaches: The flu is a formidable adversary.

Your best protection against the flu is the flu shot. You need to get the vaccine every year because the flu virus frequently mutates. Accordingly, every year, clinical investigators sample flu virus strains around the world and design the vaccine to protect against the strains that are most likely to invade your area in the winter. It’s a huge endeavor, requiring a lot of time, manpower and expense, but it is well worth it when it works, as it usually does.

Only sometimes it doesn’t. This year, the predominant flu viral strain in North America has mutated since this year’s vaccine was concocted. Here’s a look at how mutations occur, and what it could mean for you this flu season.

How does the flu virus mutate?

There are two major types of flu virus: A and B. Type A is usually more severe and more prevalent, and such is the case so far this year. The current strain–type A H3N2–is named after the predominant antigens (proteins on the surface of the virus) against which your body (with the help of the vaccine) makes antibodies to neutralize the virus. However, mutations have occurred in these antigens in a process called antigenic drift, a rapid form of evolution that means that the current vaccine will not be as effective as it normally would be. For about half of the H3N2 viruses out there right now, the antibodies induced by the vaccine are not able to bind to the viral antigens and inactivate the virus. Thus, the vaccine is not providing the immunity we would hope for.

To put this development into perspective, know that there is another way that flu viruses can mutate, a more dramatic change in the H and N antigens called an antigenic shift. This process alters the virus so much that the new strain is given an entirely new designation (such as H1N1, aka the swine flu). When this occurs, most people have no immunity at all to the new strain, and–as in 2009–a severe pandemic can occur with a high rate of complications, including pneumonia, respiratory failure, hospitalization, and even death. Fortunately, antigenic shift has not occurred this year.

Will my flu shot work?

The antigenic drift we are seeing does not render the current vaccine useless. Getting the flu shot will still protect you against the other strains of flu virus that will circulate this year and, for many people, will still provide partial protection against the mutated H3N2 strain. However, there are likely to be more cases of flu this year in vaccinated people than we usually see. Therefore, the Centers for Disease Control (CDC) is urging health care providers to offer antiviral drugs to anyone with the flu, but particularly to those at high risk of severe flu with complications, including people with underlying lung disease (asthma or emphysema), heart disease, or diabetes. These drugs aren’t miracle workers–at most, they shorten the duration of the flu by about a day and may slightly lower the risk of life-threatening complications–but even a little benefit may make a real difference in patients who are at risk.

Should I get the flu shot if I haven’t already?

The CDC still recommends getting vaccinated because of the benefits cited above. Flu season is just beginning and it will soon get worse. If you haven’t already gotten the flu shot, get vaccinated. If, despite being vaccinated, you start to experience flu symptoms, talk to your primary care provider for guidance. He or she may recommend a short course of antiviral drugs. The most effective drugs are oseltamivir (Tamiflu), an oral pill, and zanamivir (Relenza), an inhaled medication.

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The One Medical blog is published by One Medical, an innovative primary care practice with offices in Boston, Chicago, Los Angeles, New York, Phoenix, the San Francisco Bay Area, Seattle, and Washington, DC.

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