Flu season starts this week. Getting a flu shot is linked to less severe COVID-19 symptoms.

• Flu season ramps up in October, so officials recommend getting flu shots by the end of that month.
• A study found that people who got flu shots were less likely to develop severe complications from COVID-19.
• It's safe to get the flu shot and COVID-19 vaccine or booster at the same time.

• See more stories on Insider's business page.

Our second pandemic flu season begins Friday with the arrival of October. But this time, scientists have a bit of good news: Getting a flu shot could lower your chances of serious COVID-19 symptoms.

A study published in the journal Plos One last month found that people who got flu shots during the previous six months were less likely to develop severe complications from a coronavirus infection. The conclusion came after researchers analyzed more than 74,700 medical records from people who'd tested positive for COVID-19 in the US, UK, Italy, Germany, Israel, and Singapore. They found that people who didn't get flu shots were up to 20% more likely to be admitted to the ICU than those who were vaccinated for the flu.

The unvaccinated group was also up to 58% more likely to go to the ER due to COVID-19 complications and up to 58% more likely to have a stroke. The research did not find a connection between receiving a flu shot and being less likely to die from COVID-19, however.

Previous studies have also found associations between getting the flu shot and improved COVID-19 outcomes. But the flu shot is not designed to protect against coronavirus, nor does it necessarily boost immunity.

Instead, human behavior may be a factor: It's possible that people who get flu shots are more health-conscious than those who skip the flu shot, or wealthier and more well-educated, and thus more likely to take measures to boost their personal health. Those factors, then, may also reduce their chances of severe COVID-19.

It's safe to get the flu shot and a COVID-19 shot at the same time

If you're eligible for a COVID-19 booster shot, it's fine to get your flu shot on the same day, according to the Centers for Disease Control and Prevention. That's a change from prior guidance, which had recommended waiting 14 days between the COVID-19 vaccine and any other shot.

"COVID-19 vaccines may be administered without regard to timing of other vaccines," the CDC said on September 14.

Dr. Anthony Fauci doubled down on that recommendation in an interview with CNN on Monday.

"What you should do is get it as soon as you can and in the most expeditious manner," Fauci said of the flu shot. "If that means going in and getting the flu shot in one arm, the COVID shot in the other, that's perfectly fine. There's nothing wrong with that at all. In fact, that might make it more convenient."

When's the best time to get your flu shot?

Flu activity tends to increase in October, with flu season typically peaking between December and February.

Since protection from the vaccine wanes over time, and because it takes two weeks to build antibodies against the virus after getting the shot, the optimal time to get vaccinated is September or October. The CDC recommends that everyone 6 months and older get vaccinated by the end of October, with a few rare exceptions.

The flu kills tens of thousands of Americans every year; it's especially dangerous for children 2 years old and younger, adults over 65, and people with chronic health conditions. Those groups are at increased risk of being hospitalized with or dying of the flu.

Last year's flu season was an outlier in recent history, since many people wore masks in public, practiced social distancing, and did less traveling. US labs reported just 2,136 specimens that tested positive for influenza to the CDC, according to a JAMA article. During the same period, there were 748 flu deaths, which means the country avoided the dreaded "twindemic" of flu and COVID-19. (The final flu tallies will be higher once the CDC completes its estimates, though.)

During the 2019-2020 flu season, by contrast, flu was associated with 38 million infections and 22,000 deaths in the US, according to CDC estimates.

Flu season is expected to be worse this year than last, since more Americans are traveling and attending school than they were a year ago. Plus, as of the beginning of September, 37 states had no statewide mask mandate, according to NBC News. Last fall, 37 states had statewide mandates, ABC News reported.

Although it's best to get a flu shot before the worst of the season is upon us, getting one late is better than not at all. Fauci said in 2019 that he sometimes hears people say that they no longer need to get a flu shot once December or January rolls around, since they're well into the season and haven't yet gotten sick.

"But there's flu out there," he said. "It's never too late to get vaccinated. Having said that, try to get vaccinated before October 31."