Can a flu shot entail a risk of secondary infection like a real flu?

I just had my flu shot today and the resulting faux-flu is hitting me hard with fever, cough, aches, etc. I was wondering if the immune response itself is what weakens me from other pathogens, or if there is something intrinsic to a true infection which creates that dilemma.

In particular, I was wondering if I need to take it easy to avoid secondary infection or if these sensations are otherwise an illusion and I am effectively as healthy as usual.

  • Few seem to be understanding my question, so I am voting it be deleted, reforming my question, and posting it again... – user515655 Oct 24 '16 at 7:23
  • Or you could just edit this question to clarify? – Lucky Oct 24 '16 at 17:47

To your questions:

Is it possible to get a secondary infection following a flu shot?

Can a flu shot entail a risk of secondary infection like a real flu?

The Centers for Disease Control and Prevention has a whole webpage on the effectiveness of flu vaccine:


Here some points which should provide some further understanding:

  • According to recent studies conducted by the CDC, flu vaccination leads to a 50% to 60% reduction in the risk of flu illness among the overall population
  • The factors that influence the effectiveness of flu vaccine: the age and associated disease (comorbidities) of the person being vaccinated and the similarity between the flu viruses the flu vaccine is designed to protect against and the prevalence of the flu viruses in your community.

Here an essential extract from the CDC website:

During years when the flu vaccine is not well matched to circulating viruses, it’s possible that no benefit from flu vaccination may be observed.

Finally, influenza vaccination does not protect against other viruses that cause flu-like symptoms.

There are many other viruses besides flu viruses that can result in flu-like illness* (also known as influenza-like illness or "ILI") that spread during the flu season. These non-flu viruses include rhinovirus (one cause of the "common cold") and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children, as well as a leading cause of death from respiratory illness in those aged 65 years and older.

Hope this helps!

  • There is certainly a lot of information on the CDC site, but none that appears to answer the question(s). – user515655 Sep 25 '16 at 10:23
  • 1
    Actually, I think, it answers your main question "Is it possible to get a secondary infection following a flu shot". – Benjamin A. Sep 25 '16 at 10:24
  • 1
    Benjamin A. Interesting answer. Thank you. However, I think the OP wants to know whether he is at higher risk of having an infection in the acute phase following the vaccination (not in the long term). @user515655 am I right? – S.Victor Sep 25 '16 at 10:31
  • Thanks @S.Victor for the input. Actually, I thought that my post answers the question in the title Is it possible to get a secondary infection following a flu shot (I suppose secondary infection means secondary flu like infection) and the second question Can a flu shot entail a risk of secondary infection like a real flu?, don't you think? But I am fully open and ready to (try) to improve the post if further clarifications can be made... TIA – Benjamin A. Sep 25 '16 at 10:41
  • @S.Victor Correct - secondary infection as in the first event having some causal connection to the probability of another infection - likely opportunist such as bacterial - as opposed to a concurrent infection, which would have no causal link to the original event. – user515655 Sep 25 '16 at 10:48

I highly recommend the Flu vaccine risks and benefits video from the Khan Academy on this subject.

The main cons of getting a flu shot are:

  • Getting the shot may be painful
  • You may get very mild flu-like symptoms (you cannot get the flu from a flu shot)

The pros are:

  • 60% - 70% efficacy against the flu
  • Herd immunity if enough people get vaccinated

I think your question would be better worded as, "can I get an infection after receiving the flu shot?" Since a secondary infection would imply that you had an infection in the first place.

Your question is not uncommon, and many people hesitate to receive it again after experiencing what you are describing.

Some patients do report very mild flu-like symptoms after receiving the flu vaccine via a shot, but the symptoms are much more common with the nasal spray. Symptoms may include feeling sleepy, light cough from throat irritation, achy, headache, tired, mild fever, joint point. However, these symptoms are not due to infection, the symptoms are from your immune system reacting to the dead virus particles (eww) to the vaccine. I'm not making this up, it's called the inflammatory response. The inflammatory response symptoms are GOOD, though, because it means your immune system is doing its job like it is supposed to.

Once your immune system has been in contact with these dead virus particles it will have learned how to combat those strains of viruses in the future. Your system begins to make what are called antibodies, which are what protect you in case you do come in contact with a live virus. In fact, these antibodies are called ig-M, the M actually stands for memory, neat huh? Your white blood cells will actually remember those strains of virus and easily destroy them on contact. So when and if you actually come in contact with the flu, your white blood cells will be ready to do their job very quickly.

It is certainly possible to get sick during any period of time before or after receiving the flu vaccine. Your body takes several weeks to build immunity to the flu virus after receiving the flu vaccine. It is still possible to contract the flu during that period. In fact, it is possible to contract the flu even after your body achieves immunity, because the flu virus changes constantly even within 1 flu season. The advantage to the vaccine is that if you do happen to get sick, the flu will not be as severe since you will have extra antibodies to begin with.

The bottom line is that it is your body and you know it best. If you have concerns I would certainly recommend seeing a physician.

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