What Is the Quadrivalent Flu Vaccine?

Flu shot
Quadrivalent Flu Vaccine. Peter Dazeley/Photographer's Choice/Getty Images

For years, flu vaccines have contained three strains of the influenza virus. Two strains of influenza A and one strain of influenza B.

Researchers spend months trying to figure out which strains are most likely to cause the flu the following flu season (they have to decide about 6 months prior so that the vaccine can be manufactured).

This is not an easy task as flu viruses mutate and change frequently and they essentially have to make an educated guess about which strains will be dominant in the future.

Some years they make a good "guess" and the vaccine is well matched to the viruses that are circulating and some years it is not, so the vaccine ends up providing less protection.

Typically, influenza A viruses are more dominant and tend to be more severe so including two strains of it seemed to make sense. Nothing has changed about that, but newer technology has made it possible to add a fourth strain of the virus to the vaccine.

What's New

This new quadrivalent flu vaccine contains four strains of the influenza virus instead of three. These quadrivalent versions contains two strains of influenza A and two strains of influenza B. Public health officials still decide which strains go into the vaccines and the same strains are all seasonal flu vaccines, with an additional B strain added to the quadrivalent version.

According to the CDC, one type of flu vaccine is not better than another.

All FluMist (nasal spray) flu vaccines are now quadrivalent.

There are many injected flu vaccines (flu shots) that are quadrivalent as well but you may need to ask your health care provider or pharmacist for it. 

Influenza B viruses are less common than influenza A viruses and the symptoms tend to be less severe as well, but extra coverage in the flu vaccine can only be considered a good thing.


What Isn't New

The way flu vaccines are manufactured hasn't changed much. An additional strain of influenza B is a great advancement but the flu vaccine still takes about 6 months to manufacture and public health officials still have to take their best guess as to which strains of the influenza virus will be making people sick the following flu season. It's not an exact science, but it's the best protection we have against this deadly disease. 


Quadrivalent Influenza Vaccine Seasonal Influenza (Flu) 19 Sep 13. US Centers for Disease Control and Prevention. Department of Health and Human Services. 14 Oct 13.

Influenza Virus Vaccine, Quadrivalent, Types A and Types B Vaccines, Approved Products 27 Aug 13. US Food and Drug Administration. Department of Health and Human Services. 14 Oct 13.

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