What You Should Know About the Flu - The New York Times

We've had two light years in a row, which some experts worry could mean we'll be in for a rough few months.

One silver lining of the last two winters is that the flu, which typically sends hundreds of thousands of Americans to the hospital each year and causes tens of thousands of deaths, was not as severe as it has been in years past. Peter Palese, a microbiologist and flu expert at the Icahn School of Medicine at Mount Sinai in New York City, said there were 100 times fewer infections, hospitalizations and deaths from influenza last season than in average flu seasons.

But when compared with last fall, life is more back to normal — kids are in classrooms, workers are in offices and more people are planning to travel over the holidays. This is raising questions about how bad this flu season could get. To help you prepare and know what to expect, we asked two infectious disease physicians, a microbiologist and a pediatrician to weigh in.

Here's what you need to know:

• Do we know how bad this year's flu season will be?
• How well do our flu vaccines work?
• Who should get a flu vaccine?
• Does it matter where and when I get the shot?
• What's different about the flu vaccines and recommendations this year?
• What are the side effects of the flu vaccine?
• Can the flu shot prevent Covid-19?
• How can I tell if I have the flu, Covid, a cold or allergies?
• What should I do if I get the flu?
• What if I get the flu and Covid-19 at the same time?
• What else can I do to protect myself from the flu?

Do we know how bad this year's flu season will be?

"We don't have a crystal ball," said Dr. Lisa L. Maragakis, the senior director of infection prevention at the Johns Hopkins Health System. But for hints of what to expect, scientists have been watching the Southern Hemisphere, where flu season is now wrapping up. And the good news is that the flu season has been relatively mild there, Dr. Maragakis said — so ours might be, too.

Yet there are reasons to be worried as well. "When you look across the years, more serious and more severe flu seasons often follow light ones," Dr. Maragakis said. That's likely because of how the flu evolves and how susceptible people are to the strains circulating during the next season. Since we've had two light years in a row, we could be in for a bad year if the flu starts spreading, she said. (Cases are already starting to crop up in the United States, but for the most part, levels remain minimal to low — consistent with what we saw around this time last year.)

In a study published on a preprint server in August that has not yet been peer-reviewed, researchers from the University of Pittsburgh used mathematical modeling to predict how severe the upcoming flu season might be based on this increased susceptibility. They reported that if flu and flu vaccination levels are typical of prior years, 102,000 more Americans than average could be hospitalized with influenza — a 20 percent increase.

Another consideration is that Covid-19 precautions, like masking and social distancing, which also help to prevent the spread of the flu, aren't being used now as much as they were. "There's a lot more commingling, kids are back in school, mask mandates have been lifted," said Dr. Soniya Gandhi, the associate chief medical officer at Cedars-Sinai. So once the flu starts spreading, it may sweep easily through the population.

Also, ironically, the fact that the flu was so mild in the Southern Hemisphere could work against us. Scientists analyze Southern Hemisphere flu activity to determine which strains of the virus to include in flu vaccines in the United States, but when the flu is mild there, "we don't have as much opportunity to see what strains are circulating," Dr. Maragakis said. "If a different strain suddenly appears than the ones that we included in the vaccine, we could have a vaccine mismatch, and that leads to a more severe flu season."

How well do our flu vaccines work?

Because the flu virus changes so rapidly, and most flu vaccines are grown slowly and somewhat imprecisely in chicken eggs, the strains we get vaccinated against are sometimes not what our bodies end up encountering, Dr. Palese said. And because of that, flu vaccines are not as effective as some other vaccines that protect against viral illnesses. During the 2019-2020 flu season, for instance, those who received the flu vaccine were only 39 percent less likely than people who didn't receive the vaccine to develop influenza. Two doses of the Moderna and Pfizer Covid-19 vaccines, on the other hand, reduce symptomatic illness from Covid-19 by more than 90 percent, and two doses of the measles vaccine provide 97 percent protection.

That being said, Dr. Palese added, the best thing you can do to protect yourself from the flu is to get a vaccine. "That's the only thing that really makes a difference."

Who should get a flu vaccine?

With rare exceptions, the Centers for Disease Control and Prevention recommend that everyone 6 months and older get a flu vaccine. People with egg allergies can still get flu shots, although those with life-threatening allergies to other vaccine ingredients, like gelatin or antibiotics, should not.

Dr. Gandhi emphasized that those who are pregnant should get a flu shot this year, because they are more likely than others to get severely ill with the flu. "Flu shots have been given to millions of pregnant people over the years, and they are safe," Dr. Gandhi said. By getting the flu vaccine when pregnant, babies will also be protected, because flu-fighting antibodies are passed to the infant and provide protection after they are born. (The same is likely to be true for the Covid vaccines.)

Does it matter where and when I get the shot?

It's fine to get a flu shot wherever they are offered, including at flu clinics, pharmacies, health departments or doctor's offices, Dr. Palese said.

The C.D.C. usually recommends that everyone get vaccinated by the end of October at the latest. Given that population immunity against the flu is probably low, Dr. Gandhi said, it's possible that the flu will peak earlier than usual (typically between December and February). Also, it takes two weeks after vaccination for the body to develop full immunity, so if you haven't yet gotten it, "run out and get the flu vaccine immediately," she said.

Some kids — specifically children between 6 months and 8 years old who have never received a flu vaccine, as well as those who have only ever received one dose — should get two doses of the flu vaccine, at least four weeks apart, said Dr. Kelly Fradin, a pediatrician at Private Medical in New York City.

That said, even if it's late in a flu season, it still is worthwhile to get a vaccine. "The protection from the flu shot likely builds, even over years," said Dr. Fradin. If you get a vaccine after flu season has waned, it might anticipate the strains that will spread next season and provide some protection then, she said.

What's different about the flu vaccines and recommendations this year?

All of the available flu vaccines protect against the four flu strains that are most likely to circulate this year. (In past years, some only protected against three.) All approved flu vaccines — including the shots as well as the nasal spray (FluMist Quadrivalent) — are recommended by the C.D.C. Most shots are approved for babies older than 6 months, while the nasal spray is approved for healthy, nonpregnant people between 2 and 49 years old.

For children, the nasal spray may provide a bit more protection than the shots, Dr. Palese said, because it contains a live attenuated virus and might provide an additional form of immunity inside the nose. Dr. Fradin agreed, but noted that the spray is, however, more likely than the flu shot to cause congestion as a side effect.

The C.D.C. recommends that people 65 and older receive either the high-dose flu shot (Fluzone High-Dose) or the shot that contains an immune-boosting adjuvant (Fluad Quadrivalent).

Also, it is now safe and effective to get the flu shot and the Covid vaccine (including boosters) at the same time, Dr. Maragakis said.

What are the side effects of the flu vaccine?

Soreness, redness and swelling at the injection site, as well as headache, fever, nausea, muscle aches and fatigue can occur with the shot. The nasal spray vaccine in kids can cause runny nose, wheezing, headache, vomiting, muscle aches, low-grade fever and sore throat. Side effects of the nasal spray for adults may include runny nose, headache and cough.

What to Know About Covid-19 Booster Shots

The F.D.A. has authorized booster shots for millions of recipients of the Pfizer-BioNTech, Moderna and Johnson & Johnson vaccines. Pfizer and Moderna recipients who are eligible for a booster include people 65 and older, and younger adults at high risk of severe Covid-19 because of medical conditions or where they work. Eligible Pfizer and Moderna recipients can get a booster at least six months after their second dose. All Johnson & Johnson recipients will be eligible for a second shot at least two months after the first.

Yes. The F.D.A. has updated its authorizations to allow medical providers to boost people with a different vaccine than the one they initially received, a strategy known as "mix and match." Whether you received Moderna, Johnson & Johnson or Pfizer-BioNTech, you may receive a booster of any other vaccine. Regulators have not recommended any one vaccine over another as a booster. They have also remained silent on whether it is preferable to stick with the same vaccine when possible.

The C.D.C. has said the conditions that qualify a person for a booster shot include: hypertension and heart disease; diabetes or obesity; cancer or blood disorders; weakened immune system; chronic lung, kidney or liver disease; dementia and certain disabilities. Pregnant women and current and former smokers are also eligible.

The F.D.A. authorized boosters for workers whose jobs put them at high risk of exposure to potentially infectious people. The C.D.C. says that group includes: emergency medical workers; education workers; food and agriculture workers; manufacturing workers; corrections workers; U.S. Postal Service workers; public transit workers; grocery store workers.

Yes. The C.D.C. says the Covid vaccine may be administered without regard to the timing of other vaccines, and many pharmacy sites are allowing people to schedule a flu shot at the same time as a booster dose.

Rarely, life-threatening allergic reactions can occur. Symptoms, which usually arise within a few hours after vaccination, may include breathing problems, hoarseness or wheezing, hives, paleness, weakness, rapid heartbeat or dizziness.

Can the flu shot prevent Covid-19?

It is not designed to do so, and we have no solid evidence that it does. But some recent, yet preliminary and limited, studies suggest that it could. One study published on a preprint server in September that has not yet been peer reviewed found that of nearly 18,000 employees at a Dutch hospital, those who received a previous flu vaccine were 37 to 49 percent less likely to get sick with Covid-19 during the first and second coronavirus waves, respectively, than those who didn't get the flu shot.

Researchers have been speculating that the flu vaccine might spark a broad immune response that could help prevent other infections, including Covid-19. And while there is growing interest in this theory, and other studies have found similar associations, it's hard to know whether these effects are causal or just coincidental, Dr. Fradin said.

The best way to prevent Covid-19, she added, is to get a Covid-19 vaccine.

How can I tell if I have the flu, Covid, a cold or allergies?

It can be challenging to distinguish between these conditions because they have many overlapping symptoms. For instance, both the flu and Covid-19 can cause fever, chills, cough, shortness of breath, fatigue, sore throat, runny nose, body aches, headache, vomiting and diarrhea. Therefore, "the only way to distinguish reliably between the two illnesses is to get tested," Dr. Gandhi said. (One exception is that Covid-19 can cause a loss of taste and smell, but the flu does not, she said.)

There are now combined tests available at some doctor's offices that can distinguish between flu, SARS-CoV-2 and respiratory syncytial virus (R.S.V.), a common and typically mild respiratory virus that can cause more serious symptoms in infants and older adults. Most of these tests are not covered by health insurance, though, Dr. Fradin said. They can run around $250, although costs may vary, she said.

Some cold and allergy symptoms might overlap with those from the flu or Covid-19 (like cough, runny nose and congestion), but symptoms of colds and allergies are typically more mild, and are usually limited to the nose, throat and chest. In kids, however, Covid-19 can present a lot like a cold. So it may be wise for kids with cold symptoms to get a Covid-19 test, Dr. Fradin said.

What should I do if I get the flu?

If you think you have the flu, a good first step is to call or set up a telemedicine visit with your doctor, if you can. "A virtual visit may suffice and your doctor could advise on next steps for testing," Dr. Gandhi said, to determine if you have the flu, Covid-19, both or something else.

If you do have the flu, you may be eligible to receive antiviral drugs that can reduce symptom severity, Dr. Gandhi said. Also, drink plenty of water and other clear liquids, take over-the-counter pain relievers if needed and avoid contact with others. If possible, stay home for at least 24 hours after your fever is gone.

What if I get the flu and Covid-19 at the same time?

Simultaneous infections like these have so far been rare. A spring 2020 study in New York City, for instance, found that after testing about 1,200 Covid-19 patients for co-infection with other respiratory viruses, like influenza, only 3 percent had simultaneous infections.

Kids may be more likely than adults, though, to get multiple infections at the same time. "When we look at the kids who have been hospitalized for the coronavirus, often they have more than one respiratory virus," Dr. Fradin said. Generally, people with both infections would be treated similarly to those infected with one, she added — flu patients might be given antiviral drugs, for instance.

But because so few people have had the flu and Covid-19 at the same time, we still don't know much about how sick they generally become, Dr. Palese said. "There are some cases which are really bad," he said, and some that haven't been all that serious. Still, Dr. Gandhi stressed that the possibility of simultaneous infection is yet another reason to get a flu vaccine. "Being co-infected with two very serious illnesses is never a situation you want to be in," she said.

What else can I do to protect myself from the flu?

Dr. Gandhi stressed that Covid-19 precautions — hand washing, mask wearing, social distancing and isolating yourself when sick — also help prevent the spread of flu. "I would encourage people to go into this fall and winter season trying to also be adherent to all of those preventive health measures," she said.

Melinda Wenner Moyer is a science journalist.