Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

It’s Possible To Get the Flu Even With a Flu Shot

Dear Doctor: I got a flu shot this fall, but I still got sick. Why did this happen?

Dear Reader: Considering how this year’s flu season is shaping up, we’re not surprised by your question. According to the Centers for Disease Control and Prevention, which tracks weekly flu statistics, the current season started early and has been unusually severe. Flu has been reported in all 50 states.

The typical flu season runs from mid-fall to early spring, usually October through April. At this point, 46 states have reported widespread flu activity. And an unusual strain of the influenza virus is hitting harder than expected, particularly among children.

Influenza types A and B, two of the four known strains of influenza virus, account for the majority of illnesses. Type C is a milder form of the virus that is not associated with epidemics; type D appears in pigs and cattle, but not in humans. Historically, influenza type A accounts for three-fourths of all confirmed cases of the flu, with type B only showing up late in the season. However, this year, type B is making a stronger showing than type A, particularly in the southeastern U.S. This is important because of how each year’s flu vaccine is developed.

Designated global partners, including the CDC, work with the World Health Organization to collect and analyze thousands of influenza virus samples. The goal is to spot trends, and, because influenza is a master at mutating, identify new strains. Based on this data, scientists develop a vaccine that targets the top three or four influenza viruses expected in the coming flu season. Sometimes the vaccine does not cover the flu strain that has made you sick. And when widespread anomalies occur, such as with this season’s robust and early appearance of influenza type B, the vaccine isn’t as widely effective as desired.

As for why you became ill despite getting a flu shot, there are several possibilities. It takes up to two weeks to build up immunity after a vaccine. If you’re exposed to the flu during that interim, it’s possible to get sick. The flu vaccine doesn’t protect against all respiratory illnesses, so if you had a cold, often caused by the rhinovirus, the vaccine won’t offer protection. And as we discussed earlier, you may have been infected with an influenza strain not covered by this year’s vaccine. Even when this does happen, it is still advised that you get the vaccine every year. If you are vaccinated but do get the flu, studies show that the illness is more likely to be both milder and shorter. That’s important because the main difference between a cold and the flu is that flu symptoms are more severe, last longer and can cause grave and even life-threatening complications.

For readers who haven’t been vaccinated yet, it’s not too late. With three or four months in an already challenging flu season, it would be wise to get your flu shot ASAP.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)