Can the flu shot give you the flu?
Flu vaccines are either “dead” (shot) or “inactive” (nasal spray) viruses so they scientifically cannot replicate, infect or spread through the body. However, stories of people getting sick after getting the flu vaccine - including actual influenza infection - are not crazy talk.
So what’s the deal?
There are several plausible explanations for getting “sick” in the winter despite getting a flu shot.
Other viral illness. Other viruses can cause infections and symptoms that look much like the flu. Many people self-diagnose themselves with “flu” when it’s actually just a nasty “common cold”. The flu vaccine does not protect against these viruses that cause head colds, bronchitis, intestinal problems, sore throats, etc.
Strong immune response following vaccine. The flu vaccine causes a low-grade immune response by design; that’s how all immunizations work. The degree of the response can vary from mild (no symptoms) to more severe (resulting in flu-like symptoms for a few days following vaccination).
Immunity lag time. Your immune system will require about 2 weeks to gain full immunity. During that lag time, it is possible to get infected with a live flu virus.
Not 100% flu strain coverage. There are 3-4 main subtypes of flu but dozens of strains circle the globe each year that are constantly mutating. Each year’s vaccine is planned on these international trends. This process usually results in 90-95% match for the strains that eventually became common in the U.S., but not 100%.
Not 100% preventive against infection. Even if you have some immunity to a virus, you can still become infected. In theory, your immune response will be much quicker and stronger - lessening the severity of infection - but you may still become sick.
Poor immune response. Some people who get the flu shot do not gain significant “immunity”. This issue is complex, but most common in the very young or old, so guidelines now recommend extra vaccine (dose or quantity) in some groups.
Why should you get one at all?
For all the reasons listed above, the current flu vaccine is not 100% protective against getting sick this winter. Our public health marketing effort may have oversold the flu shot in many ways. It’s nowhere near as effective as other vaccines (measles, tetanus, etc.) at preventing illness or death. However, it’s the best available way to lower your risk of contracting the flu, becoming very ill and passing the flu to family and friends. Exercising regularly and eating a well balanced diet does not guarantee you won’t get heart disease or cancer, but I’m not afraid to recommend them either.