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.
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 in both, with an additional B strain added to the quadrivalent version.
According to the CDC, one type of flu vaccine is not better than another.
As of the 2013-2014 flu season, a majority of flu vaccines available in the United States are the traditional (trivalent) type. However, all of the nasal spray flu vaccine will be quadrivalent and some flu shots will be as well. Manufacturers estimate that around 30 million doses of the quadrivalent vaccine will be available this year out of a total of about 135-139 million total doses of flu vaccine.
Quadrivalent Flu Vaccine products and manufacturers for the 2013-2014 flu season include:
- FluMist Quadrivalent (MedImmune)
- Fluarix Quadrivalent (GlaxoSmithKline Biologicals)
- FluZone Quadrivalent (Sanofi Pasteur Inc)
- FluLaval Quadrivalent (ID BioMedical Corporation)
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.