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COVID-19 and the flu share , such as fatigue, fever, and cough, so it may feel logical to think that a flu shot might provide some defense against COVID-19—or vice versa. Indeed, , some of which have , suggest that people who get their flu shots might be less likely to have a COVID-19 case. But the correlation requires further research, as it’s possible that the same people who receive their flu shots are also more likely to rigorously follow COVID-19 safety protocols.

The flu and COVID-19 are caused by very different viruses, with very different biological structures. As a result, health experts recommend that Americans receive both a flu shot for protection against the flu and a COVID-19 shot for protection against COVID-19.

Though COVID-19 and the flu require two different vaccines, Americans are able to get their shots for both diseases in one doctor’s visit or one trip to the pharmacy. Other vaccines are often administered at the same time—especially for kids getting their routine childhood immunizations—and this practice or potential side effects, the CDC says. Common pharmacies like Walgreens are making it easy for people to get , including those adults who are eligible for a Pfizer booster shot.

All of the vaccines that scientists have developed for the 2021–2022 flu season are quadrivalent, meaning they protect against at once. These include two virus strains from the “influenza A” family and two from the “influenza B” family. By incorporating multiple strains into one vaccine, scientists are providing broader protection to people who get immunized.

The CDC recommends that gets a flu shot, and considers young children (between 6 months and 4 years old) to be “high priority” in vaccine distribution. While parents of young children wait for the Food and Drug Administration to determine authorization for child-size doses of Pfizer’s COVID-19 vaccine, they can still take their kids to get flu shots.

Many health experts are specifically encouraging flu vaccination this fall. If the flu and COVID-19 spread widely at the same time, the U.S. may face a twin pandemic, or "twindemic." A "twindemic" could be because the flu and COVID-19 share some common symptoms—making them difficult to distinguish without testing—and require similar equipment for severely ill patients, such as ventilators.

Though experts worried about a “twindemic” last fall and winter as well, the 2020–2021 flu season proved to be quite mild, with . This was a “pleasant surprise,” Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases, told the Association for Health Care Journalists. Like other experts, Dr. Schaffner , social distancing, and other COVID-19 precautions, which stalled respiratory disease spread at a broader level. This winter, with restrictions lifted and many Americans vaccinated against COVID-19, the flu season may be much more intense.

The most common flu vaccine—administered as a shot in the arm—contains a that has been inactivated, meaning it won’t cause the flu. Another type of flu vaccine contains a live but weakened influenza virus. Both of these virus types differ from the COVID-19 vaccines currently on the market in the U.S., but , such as the Sinovac and Sinopharm vaccines produced in China, are inactivated virus vaccines.

Similarly to COVID-19 vaccines, the most common side effect for a flu shot is a sore arm. The flu shot shouldn’t this year, reports Julia Craven at Slate, but people may be more likely to notice that side effect because of their experiences with COVID-19 shots. Craven also wrote that the flu shots’ inclusion of four different flu virus strains does not impact side effects.

The is in September or October, and you should aim to receive that shot by the end of October, the CDC says. The agency recommends that older adults should get vaccinated in the fall, not in July or August, because the vaccine’s effectiveness may decrease over time. If you don’t get your shot by the end of October, though, you can still get vaccinated later and have protection against the flu through the remainder of the disease’s season.

If you're traveling internationally or visiting more vulnerable relatives for the holiday season, a flu shot may be an . The CDC recommends getting your vaccination at least two weeks before your trip, because—like with the COVID-19 vaccines—it takes about two weeks for your immune system to develop protection against the virus.