Doctors suggest that their patients get a flu shot every year. This suggestion is based on recommendations by the CDC, due to possible life threatening consequences of influenza in some patients. The inherent challenge with flu vaccines is that the viruses they’re fighting are constantly changing and evolving. There is also no way to know, with absolute certainty, which strain of flu will circulate in any given year. The CDC is currently recommending one of several vaccines that are designed to target the top three or four strains of flu that they expect to see this year.
Flu shots are designed to help the body build immunity to flu virus strains. The immune system needs to be exposed to a virus before it can produce signaling proteins, known as interferons, that recognize the virus as foreign and activate the body’s immune response to fight it. It is this immune response, the production of a variety of immune cells, that causes some of the symptoms of flu: fever, muscle pain, etc. This seems to be a very straightforward process, so why do flu shots fail?
Mismatch of Viral Strains
Each year, flu vaccines are produced based on which strains the CDC expects are most likely to spread during the next flu season. Various world health organizations gather year-round samples from patients suffering from the flu and the CDC expectations are based on the trends they’re seeing. Ultimately, it comes down to scientists’ best guess regarding which strains are most likely to be circulating in the coming year. Unfortunately, that guess can sometimes be wrong and a virus strain might spread that has not been included in that year’s vaccine.
Flu vaccines are produced in a laboratory, under ideal conditions. This means that the laboratory is designed to provide the flu virus with the optimum environment for growth and reproduction. But the human body is far from an ideal environment. Even among healthy adults, an individual’s immune response to a flu vaccine may not be effective in preventing later infection by the specific strain against which they have been vaccinated. Studies have shown that, even when that year’s vaccine is well matched to the circulating flu virus, the risk of illness is only reduced by 50% to 60%.
Response Differences for Flu Vaccines
Unfortunately, flu vaccines don’t work well for everyone. They tend to be most effective in healthy adults and children over the age of two. Those who are over 65, or those who suffer from chronic illness, may not respond as well. These people tend to have weaker immune systems, so their responses to vaccination may not be as effective. Likewise, children under two have immune systems that are still developing, so they may not receive full protection from flu vaccine.
Interference with Natural Immunity
According to a recent study published in the Journal of Virology, the flu immunity provided by vaccine differs from naturally acquired immunity in one key way. When someone has been infected by one flu strain, they can also acquire immunity to other, similar strains. This is referred to as heterosubtypic immunity. Unfortunately, this type of immunity does not occur with vaccination.
There are clearly several factors that can result in a failure of flu vaccines to provide immunity, even to the specific strains against which they are designed to protect. Although they can provide some immunity, total immunity is not guaranteed.
Learn more about Flu Vaccine.