Last fall, as coronavirus cases climbed and the world hoped for vaccines, health experts feared influenza and COVID-19 would combine for a devastating “twindemic.”

While pandemic measures appeared to keep the flu at bay, this year experts are again concerned, especially as some countries and state authorities roll back lockdown rules. Many officials and experts are urging the public: Do not dismiss the danger of the flu, and seek a flu vaccine.

“This year we are guaranteed to have the flu, and we are going to have some version of a twindemic,” said Dr. William Schaffner, medical director for the National Foundation for Infectious Diseases. “It could really further strain an already extraordinarily stretched, strained, tired-to-the-bone health care system.”

The United States is grappling with an average of more than 160,000 new coronavirus cases a day. Hospitals and intensive care units are filling up with COVID-19 patients. At the same time, mask mandates and social distancing have been relaxed in some places, meaning contagious respiratory illnesses can spread more easily than they did last year.

Schaffner warned that medical providers now had to remind people about influenza: “We are going to have to say, ‘There is another nasty respiratory virus, and don’t blow it off.’”

This flu season unlike any other.

In the U.S., flu activity was significantly lower during the 2020-21 season than during any previous flu season since at least 1997, the first for which data is publicly available.

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Scientists said pandemic precautions most likely played a role, as many people adopted masking, social-distancing and hand-washing habits.

“The lack of influenza last year was truly remarkable,” said Dr. Patrick Jackson, an infectious disease expert at the University of Virginia. “It may be that people’s willingness to wear masks and wash hands regularly and be aware of symptoms may help us moving forward. And I really hope that turns out to be the story.”

Experts hope those behaviors will carry over into the coming flu season, especially as more people return to public transit, restaurants, schools and offices.

But the U.S. and a number of other countries are not unified in how to confront the pandemic, and some people have stopped taking those precautions.

That could mean additional burdens for hospitals already treating large numbers of COVID-19 patients.

“Given our politics, a COVID/flu surge will be unevenly distributed to health care systems,” Jackson said.

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Who should get the flu vaccine and when?

The relative lack of flu cases over the past 18 months could also mean that population-level immunity to the flu is lower this season, said Lynnette Brammer, leader of the Centers for Disease Control and Prevention’s domestic influenza surveillance team.

And while it is still uncertain how the season will play out, she added, relaxed pandemic measures in some places will “likely result in the resumption of seasonal flu virus circulation.”

“This all could set us up for a potentially severe flu season,” she added.

In the Northern Hemisphere, the season starts in October and can last through May. In the Southern Hemisphere, it typically occurs from April through September.

The CDC advises everyone 6 months and older to get the shot, with a few exceptions. Vaccination is especially important for some groups of people, including those with underlying health conditions, older adults and young children, Brammer said.

It takes about two weeks for protection to develop after vaccination, so experts recommend getting vaccinated from September through the end of October, but even after that, it is better to get the vaccine than not.

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Because viruses mutate, requiring a new vaccine every year, even people who received a flu shot last year need another this year, they said. For any given season, vaccines are tailored to the types of influenza viruses — there are many — that are circulating or most likely to be circulating.

If I have been vaccinated for COVID-19, do I still need the flu vaccine?

Yes, one does not protect against the other. Vaccines are intended to target specific viruses: Just as the COVID-19 vaccines are manufactured for the coronavirus, flu vaccines target influenza viruses.

That means flu shots this year will be doled out as some people are getting initial and booster shots for the coronavirus.

“There is vaccine fatigue out there,” Schaffner said. “But we are going to have to tell people, ‘Oh, no, you need a different one,’ right at the time we are boosting.”

Can I get the flu vaccine and the COVID-19 vaccine in the same visit?

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Yes. The CDC says the vaccine may be administered without regard to the timing of other vaccines.

Side effects are generally similar when the vaccines are given simultaneously as when they are administered alone, the CDC says.

Dr. Kevin Ban, chief medical officer at Walgreens, said, “Not only is it possible, but we highly encourage people to be vaccinated for both flu and COVID.”

Vaccines received at the same time do not cause cumulative reactions, Dr. L.J. Tan, Immunization Action Coalition’s chief policy and partnerships officer, said in an interview. “It is not like you are adding it on.”

Common reactions to the flu vaccine can be a sore arm, and some people might get a little tired, he said.

If you do get the flu shot and the COVID-19 vaccine at the same time, experts advise using different arms to avoid soreness or at least spacing the injection site for each shot by at least 1 inch.

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Can I catch the flu and COVID-19 at the same time?

Yes. The flu, COVID-19 and other respiratory illnesses have similar symptoms, including cough, fever and body aches. Only testing can identify which virus a patient has or if a patient has both.

Different tests are used for each, and identifying the difference is essential to treatment. Antiviral treatments exist for influenza, and some patients with COVID-19 receive monoclonal antibody treatment.

“Doctors are going to have to do a whole lot more testing this year than in the past,” Schaffner said. “It is important to know who has got what.”

What about children and pregnant women?

The CDC estimates that the flu has killed 12,000 to 61,000 people a year since 2010. (More than 650,000 deaths have been linked to the coronavirus pandemic in the U.S.)

About 80% of children who have died of influenza in past years had not been vaccinated, according to research cited this week by the American Academy of Pediatrics.

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The flu has the capacity to take a “perfectly normal child and put them in the emergency room in 48 hours,” Schaffner said. Children have died from the flu when they became septic or got pneumonia, he said.

A big factor for the low flu numbers last season was that children were attending school remotely and avoiding groups, he said.

“Children are really the great disseminators of the influenza virus,” Schaffner said. “They shed more virus than do adults, and they shed the virus for a longer period of time. They spread it among themselves and then bring it home.”

This academic year, many children are going back to in-person learning, and some are not required to wear masks.

If children who have not gotten a flu shot get the flu, they should still be vaccinated when they recover. Experts added that children sick with COVID-19 should not get the flu vaccine until they have recovered.

They also recommend pregnant women get the flu vaccine, as its protection passes through the placenta.

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How is flu treated?

Antibiotics are not effective against the flu, but antiviral drugs can ease symptoms. They should ideally be given within 48 hours of the onset of symptoms. People with the flu should rest and drink plenty of water, and most will recover within a week.

Where can I get the vaccine?

Pharmacies including Walgreens and CVS, and at Walmart offer free flu shots. There are mobile apps to make appointments and online vaccination finders.

Public health officials and experts said that vaccination was so important because it reduces the chances of severe, sometimes fatal illness.

“We can’t shut it off like a light switch,” Schaffner said. “But we can dim it.”