As flu season arrives (it’s officially here now) I am urging procrastinators to get vaccinated ASAP. People often cite negative personal experiences with the flu shot as a reason for declining my recommendation. The most common concerns I hear about the flu shot are “it gave me the flu” or “doesn't work for me.” Is that possible? And why does it not work sometimes?

Influenza A Virus. by Ryan Neuhofel, DO, MPH

Let’s tackle the first issue because it’s a much simpler topic. Is it possible for the shot to give you the flu? The short answer is ‘no.' Flu vaccines are either “dead” or “inactive” viruses so they cannot replicate, infect or spread through the body. However, stories of people getting sick (including actual influenza) after getting the flu vaccine are not crazy talk. So what’s the deal? There are several plausible explanations for getting “sick” despite getting a flu shot. (in no particular order)

1) Strong immune response following vaccine. The flu vaccine does cause a low-grade immune response by design. That’s how all immunizations work. The degree of the response can vary from mild to more severe. A more severe immune (inflammatory) response can give flu-like symptoms, including fatigue, low-grade fevers and body aches last for several days (48-72 hours) following your shot.

2) Poor immune response. Some people whom get the flu shot do not gain “immunity” because of a variety of factors. This issue is most common in the very young or old, so guidelines now recommend giving young children 2 doses for their first time and possibly “high dose” vaccines to the elderly. In fact there is still plenty of legitimate debate about the vaccines effectiveness in these high-risk populations (ironically the ones whom will most benefit from the vaccine).

3) Immunity lag time. Your immune system will require approximately 2 weeks to fully gain immunity from the vaccine. So, the vaccine is not immediately protective. During that lag time, it is very possible to get infected with a live flu virus.

4) Not 100% flu strain coverage. This is probably the biggest “downfall” I hear from well-read patients, but it’s a complex and evolving issue. There are 2 main subtypes of flu but dozens of strains circle the globe each year and are constantly mutating in to new forms. Each year’s North American vaccine is based on a carefully planned study of which flu viruses are most likely to be common here based on international trends. This process has markedly improved over the years and the 2010-2011 vaccine was a 94-99% match for the strains that eventually became common in the U.S.

5) Not 100% preventive against infection. Even if you have full immunity to the circulating viruses, you can still become infected with those viruses. In theory, your immune response will be much quicker and stronger (lessening the severity and length of the flu), but you may still become sick.

6) Other viral illness. A whole host of other viruses exist that can cause flu-like illness. The flu shot does not protect against these “common cold” viruses (adeno, rhino, etc.). They are less severe than influenza, but can make you feel pretty crummy and in bed for several days.

Given all of these potential pitfalls of the flu vaccine, why should you get one at all?

For all the reasons listed above, the current flu vaccine is not close to being 100% protective. Our public health marketing effort has probably oversold the effectiveness of 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 from influenza and passing the virus to family and friends. Exercising regularly and eating a well balanced diet does not guarantee avoidance of cardiovascular disease or cancer (or contracting the flu), but I’m not afraid to recommend them either.

W. Ryan Neuhofel, DO, MPH (Dr. Neu) is the physician and owner of NeuCare Family Medicine. He is a board-certified Family Physician (American Board of Family Medicine) and Fellow-candidate in Wilderness Medicine.

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Information contained here is intended for general health education only. All personal health and medical issues should be managed by a health professional.

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