The flu shot was not designed to protect against COVID-19, and the strongest available evidence shows it does not reliably do so. Some earlier studies suggested a modest benefit, but more rigorous research has not confirmed a meaningful protective effect. The flu vaccine and the COVID-19 vaccine target completely different viruses, and one is not a substitute for the other.
What the Research Actually Shows
This question generated a lot of scientific interest during the pandemic, and the answers depend on which studies you look at. A large meta-analysis published in the American Journal of Preventive Medicine pooled data from multiple studies and found that people who had recently received a flu vaccine were 17% less likely to test positive for COVID-19 and 29% less likely to be hospitalized with it. Those numbers sound promising, but the same analysis found no significant reduction in ICU admission or death.
A separate systematic review looking specifically at more rigorous prospective studies (ones that follow people forward in time rather than looking backward) found no significant protective link at all. Flu vaccination showed no meaningful reduction in COVID-19 infection risk, hospitalization, or death when only these higher-quality studies were included. The summary relative risk for infection was 0.95, essentially meaning no difference between vaccinated and unvaccinated groups.
This kind of discrepancy is common in observational research. People who get flu shots tend to be more health-conscious overall. They may wash their hands more, wear masks more consistently, or have better access to healthcare. These behaviors can make it look like the flu vaccine itself is providing protection when the real explanation is healthier habits across the board. The prospective studies, which are better at controlling for these confounders, suggest the apparent benefit largely disappears once you account for them.
Why Some Scientists Thought It Might Work
The idea wasn’t entirely far-fetched. Vaccines can sometimes “train” the innate immune system, the fast-acting, nonspecific arm of your defenses that responds to threats before your body identifies the exact pathogen. A study published in PLOS Pathogens found that a standard flu vaccine reprogrammed certain white blood cells (monocytes) at the genetic level, making them more responsive to unrelated viral threats for at least six weeks after vaccination. Pathways specifically relevant to fighting COVID-19 were among those switched on.
This phenomenon, called trained immunity, is real and well-documented with other vaccines too. But there’s a gap between showing that immune cells become more active in a lab dish and proving that translates into fewer infections or milder illness in real life. The clinical data, particularly the strongest studies, suggests this trained immunity effect is not powerful enough to provide reliable protection against COVID-19 in practice.
The Flu Shot Is Not a COVID-19 Vaccine
COVID-19 vaccines work by teaching your immune system to recognize the spike protein on the surface of SARS-CoV-2. This creates targeted antibodies and immune memory cells that can respond quickly if you encounter the actual virus. The flu vaccine does nothing of the sort. It trains your body to recognize influenza strains, which are structurally and genetically unrelated to the coronavirus.
Even the most optimistic reading of the research does not support using a flu shot as a stand-in for COVID-19 vaccination. The two vaccines protect against different diseases, and both circulate during the fall and winter months.
Getting Both Vaccines at Once
You can receive a flu vaccine and a COVID-19 vaccine during the same visit. The CDC confirms there is no required waiting period between the two shots. If you’re also eligible for an RSV vaccine, all three can be given at the same appointment.
If you’d rather space them out, that’s fine too. There is no minimum interval required. Some people prefer separate visits because getting multiple vaccines at once can temporarily increase side effects like soreness, fatigue, or mild fever. These reactions are short-lived and not a safety concern, but scheduling them a week or two apart is a reasonable option if you want to minimize discomfort.
What This Means for You
The flu shot is valuable for what it’s designed to do: reduce your risk of influenza, which hospitalizes hundreds of thousands of Americans each year and can be fatal on its own. Any incidental benefit against COVID-19 appears too small and inconsistent to count on. If you want protection against both viruses heading into respiratory season, the straightforward path is getting both vaccines.