There are legitimate medical reasons some people should not get a flu shot, but the list is shorter than most people expect. The CDC recommends annual flu vaccination for everyone 6 months and older unless they have a specific contraindication. Understanding those contraindications, along with the real side effects and effectiveness data, can help you make a clearer decision.
Medical Reasons to Skip the Flu Shot
The most clear-cut reason not to get a flu vaccine is a history of severe allergic reaction (anaphylaxis) to a previous dose of any flu vaccine or to a component of the vaccine. If you’ve ever had a serious reaction after a flu shot, that specific vaccine type is off the table. However, multiple formulations exist. Someone who reacted to an egg-based vaccine may still be able to receive a cell-based or recombinant version, and vice versa. If you had a reaction but aren’t sure which vaccine caused it, an allergist consultation is recommended.
Infants younger than 6 months cannot receive any flu vaccine. Beyond that, the contraindications depend on which type of vaccine you’re considering.
Who Should Avoid the Nasal Spray Specifically
The nasal spray vaccine (LAIV) contains a weakened live virus, which makes it off-limits for a longer list of people than the standard shot. You should not receive the nasal spray if you:
- Are younger than 2 or older than 49
- Are pregnant
- Are immunocompromised from any cause, including HIV, sickle cell disease, or immunosuppressive medications
- Have asthma (children aged 2 through 4 with asthma or a wheezing episode in the past year are excluded; people of any age with asthma may be at increased risk for wheezing afterward)
- Care for someone who is severely immunosuppressed, since the live virus could theoretically be transmitted to that person
- Have a cochlear implant or a cerebrospinal fluid leak
- Are a child or teen taking aspirin-containing products
People with chronic heart, lung, kidney, liver, or metabolic conditions like diabetes also fall into a precautionary category for the nasal spray. The safety of LAIV in these groups hasn’t been fully established. For most of these individuals, an inactivated or recombinant flu shot remains an option.
What About Egg Allergies?
Egg allergy used to be a common reason people avoided the flu shot, but guidelines have changed significantly. Since the 2018-19 season, the CDC has recommended that people with egg allergy of any severity can receive any licensed flu vaccine, including egg-based versions. If your egg allergy has caused symptoms beyond hives (such as swelling, difficulty breathing, or anaphylaxis), you should get vaccinated in a medical setting where a provider can manage a severe reaction. Egg-free options also exist, though fewer are approved for children under 18.
Side Effects Are Common but Mild
Many people avoid the flu shot because of side effects, and it’s true that reactions are frequent. Soreness at the injection site is the most common complaint, affecting 10% to 64% of recipients depending on the formulation and age group. About 36% of people who received the high-dose version (designed for older adults) reported injection site pain, compared with 24% for the standard dose.
Systemic reactions like headache (13% to 20%), fatigue (12% to 17%), and muscle aches (up to 13%) occur in a meaningful number of people. Fever is less common in adults, typically under 1% to 5%, though children aged 1 through 5 develop post-vaccination fever about 12% of the time. The nasal spray version tends to cause runny or stuffy nose (51%), cough (34%), and fever (about 10%).
These side effects generally resolve within one to two days. They reflect your immune system responding to the vaccine, not the flu itself. The flu shot cannot give you the flu, since injected vaccines contain no live virus.
The Rare but Serious Risk: Guillain-Barré Syndrome
Guillain-Barré syndrome (GBS), a condition where the immune system attacks the nerves and causes weakness or paralysis, has been linked to flu vaccination at an estimated rate of 1 to 3 extra cases per million doses. This is genuinely rare. The risk is slightly higher in older adults because they have a higher baseline rate of GBS. If you’ve had GBS within six weeks of a previous flu vaccination, that’s generally considered a precaution (not an absolute contraindication), and the decision to vaccinate again involves weighing individual risk.
Flu Vaccine Effectiveness Varies by Year
One of the more honest criticisms of the flu shot is that it doesn’t work as reliably as many other vaccines. During the 2023-24 season, effectiveness in preventing outpatient flu illness in adults ranged from 35% to 47% depending on the study network. For preventing hospitalization in adults, it was 39% to 40%. Children fared better, with effectiveness against outpatient visits ranging from 56% to 65%, and against hospitalization from 58% to 64%.
These numbers mean the flu shot cuts your risk roughly in half in a good year for children, and by about a third to half for adults. It’s not a guarantee of protection. Effectiveness shifts each season because flu viruses mutate, and vaccine manufacturers must predict months in advance which strains will circulate. Some years the match is better than others.
That said, even moderate protection translates into millions fewer doctor visits and tens of thousands fewer hospitalizations across a population. For individuals at high risk of flu complications (older adults, people with chronic illness, young children), even a 35% to 40% reduction in hospitalization risk is meaningful.
Does Getting It Every Year Reduce Its Effect?
There’s a real scientific question about whether annual vaccination eventually blunts your immune response. Research on healthcare workers found that repeated vaccination with the same flu strain for more than three consecutive seasons did reduce the size of the antibody boost. Your body essentially reaches a plateau and responds less dramatically to each additional dose of the same thing.
But the picture is more nuanced than “annual shots stop working.” The same research showed that when the vaccine strain was updated, immune responses improved again. Repeated vaccination also made antibodies last longer, with the highest antibody durability occurring after about three vaccinations. And overall, vaccinated healthcare workers in the study still maintained protective antibody levels regardless of how many years they’d been vaccinated.
A systematic review found that people vaccinated two years in a row had somewhat lower effectiveness than first-time recipients in the current season, but higher effectiveness than people who only got vaccinated the previous year and skipped the current one. In practical terms, getting vaccinated this year still helps, even if the benefit is slightly diminished compared to someone getting their first-ever dose.
Thimerosal and Preservative Concerns
Some people avoid the flu shot over concerns about thimerosal, a mercury-based preservative used in multi-dose vials. Thimerosal contains ethyl mercury, which the body clears quickly, unlike the methyl mercury found in contaminated fish that accumulates over time. Extensive research has not found thimerosal at vaccine levels to be harmful. If it still concerns you, most single-dose vials, pre-filled syringes, and the nasal spray are thimerosal-free.