Since the press release was originally issued on Thursday by now surely most of you have seen the news stories that popped up beginning yesterday morning with headlines like “CDC Warning: Flu Viruses Mutate and Evade Current Vaccine“, or “Flu vaccine protects against wrong strain, US health officials warn“, or “Flu shots may not be good match for 2014-15 virus, CDC says“, or “Health Officials Warn This Year’s Flu Vaccine Won’t Prevent New H3N2 Strain Of Influenza“. You get the idea. This year, apparently, the flu vaccine isn’t as effective as health officials and physicians would like. How could this have happened?
Those of you who are knowledgeable about the flu vaccine know that, as useful as it is, it’s not one of the greatest vaccines as far as effectiveness. Actually, that’s not true. Its effectiveness can and does vary considerably from year to year. The reason is simple. There are many strains of influenza, and the vaccine as currently formulated generally only covers a handful of strains. Basically, every year the World Health Organization, in collaboration with the CDC and other health organizations throughout the world, has to make an educated guess which strains of influenza will be circulating the following winter. Many months’ lead time is required because vaccine manufacturers require it to develop and test the new formulations and then to ramp up their manufacturing capabilities and distribute the vaccine. Generally, the WHO chooses the three strains it deems most likely to cause significant human suffering and death in the coming flu season. Specifically, the chosen strains are the H1N1, H3N2, and Type-B, although, starting with the 2012–2013 Northern Hemisphere influenza season, the WHO has also recommended a second B-strain for use in quadrivalent (four strain) vaccines. Basically, the WHO coordinates the contents of the vaccine each year to contain the most likely strains of the virus to attack the next year. Wikipedia has a helpful article that lists the formulations of all the flu vaccines recommended for the Northern and Southern Hemispheres dating back to 1998, to give you an idea what’s been recommended in the past. Also, there are exceptions. In the 2009-2010 season, for example, the H1N1 pandemic was occurring, and it was recommended that everyone be vaccinated against H1N1 in addition to the normal flu vaccine.