Could a flu and COVID-19 ‘twindemic’ be ‘coming for the U.S. this year’?

“Every year since the pandemic began, I’ve feared this ‘twindemic’ — and that actually hasn’t happened yet,” Dr. Katelyn Jetelina, an epidemiologist and biostatistician and author of Your Local Epidemiologist newsletter, told Yahoo News.

Jetelina isn’t alone. For the past two years, as autumn approached, health experts and media outlets alike have speculated whether fall and winter would bring about a “twindemic” — with a simultaneous onslaught of influenza and COVID-19.

This year, Jetelina says that double whammy could finally come to pass.

“Given what we saw in Australia, that they had a very bad flu season, we think it’s coming for the United States this year,” Jetelina said.

How bad will this flu season be?

A nurse prepares a flu shot
The last bad flu season in the U.S. was from 2017 to 2018. Both 2020 and 2021 saw mild flu seasons. (David Goldman/AP, File)

Since the Southern Hemisphere’s fall and winter flu season typically runs from April through September, the U.S. often looks to countries like Australia for a preview of what may be in store as its own flu season approaches. This year, the outlook isn’t great. At a media briefing Thursday with the Johns Hopkins Bloomberg School of Public Health, Dr. Andrew Pekosz, a professor of molecular microbiology and immunology, said there are "some concerns about a fall-winter surge” in the U.S. this year.

“Australia and other countries in the Southern Hemisphere had a rough influenza season,” Pekosz said. “It was Australia's worst flu season in five years, and it came earlier than any other influenza season, with the exception of the 2009 pandemic.”

Jetelina said a “bad flu season” usually means a lot of circulating flu, resulting in more cases, hospitalizations and death than in previous flu seasons. Since the last bad flu season was 2017-2018 — with very calm flu seasons in 2020 and 2021 — immunity for most individuals, especially for very small children, is likely to be weak.

“This poses a risk, especially to young children who may not have had much, if any, previous exposure to influenza viruses prior to this season, because of the pandemic precautions that were put in place — the masking, the social distancing,” Pekosz said. “The under 18 years of age population was particularly hard-hit in Australia this past year.”

While it’s by no means certain that the U.S. flu season will be an exact replica of the Southern Hemisphere’s, Pekosz said timing and the ages of individuals hospitalized with influenza are two critical things they’ll be monitoring in the coming weeks. If we start to see a significant number of cases in September or October, and if more young people who aren’t part of vulnerable populations begin needing to be hospitalized with influenza, it could be that the U.S. is in for a severe flu season, comparable to what the Southern Hemisphere experienced.

A Mississippi resident receives a Pfizer booster COVID shot.
A Jackson, Miss., resident receives a Pfizer COVID booster shot from a nurse at a vaccination site in February. (Rogelio V. Solis/AP, File)

Will there be a COVID-19 surge and ‘twindemic’?

“One of the biggest questions going into this flu season is how big is it going to get with co-circulating COVID-19 with very little restrictions, if at all,” Jetelina said, adding that this raises questions about how health systems will fare.

Jetelina points out that the past two years have been so unusual that it’s hard to predict where we could be headed this fall and winter. So far, COVID-19 forecasts have been fairly optimistic. COVID-19 scenario modeling indicates that cases and hospitalizations should continue to decline until the end of the year, thanks in part to the distribution of new bivalent boosters in September and to the immunity many Americans have accumulated from prior vaccination or infection. Although there may be a late fall surge in December, experts are hopeful that the onslaught of hospitalizations and cases in the past two winters can be avoided.

"We expect to see an increase in COVID cases this fall as people do more activities indoors, but in the absence of a new, extremely different variant emerging, we believe that surge will be moderate," Pekosz said at Thursday’s briefing.

Testing is available for both seasonal influenza and COVID-19, which will come in handy for diagnoses, given that the two viruses share so many similar symptoms, including fever, chills, fatigue, sore throat or cough, and even diarrhea and vomiting.

The Centers for Disease Control and Prevention also notes that it is possible to contract both the flu and another respiratory illness such as COVID-19 at the same time, though experts are still studying how common that is.

Still, given that the U.S. has yet to experience a bad flu season alongside COVID-19, not much is known about the potential impact of these two viruses concurrently.

One theory known as viral interference suggests that a “twindemic” may not be possible, because there’s a limit on the number of viruses that can circulate at once, with the immune system going on high alert and protecting against other viruses when exposed.

“We don't really know the interaction of two co-circulating viruses, like the flu and COVID,” Jetelina said. “They may be competing against each other, for example. We’re just not sure.”

How can you keep yourself and others safe?

The Biden administration is ramping up a fall COVID-19 bivalent booster campaign, just as the annual influenza vaccine is becoming available nationwide, and experts are encouraging individuals to get both shots at once, if possible.

“I really believe this is why God gave us two arms. One for the flu shot, and the other one for the COVID shot,” White House coronavirus response coordinator Dr. Ashish Jha joked during a press conference on Tuesday.

Ashish Jha speaks during a press briefing.
White House coronavirus response coordinator Dr. Ashish Jha addresses a press briefing in Washington, D.C. (Elizabeth Frantz/Reuters)

This will be the second year that COVID boosters and flu vaccines will be available to Americans simultaneously. In a study published in July, 8% to 11% of individuals receiving both shots at once said they experienced a slight increase in symptoms, such as muscle aches and fatigue in the week after inoculation.

Jetelina pointed out that any additional side effects of receiving both vaccines at once are usually mild and temporary, and that there are no safety concerns with knocking both out at a single session.

“It’s a lot easier to get them at the same time. You don’t have to go back to CVS two times, you don’t have to go to your physician two times, and so I certainly will be getting them at the same time,” she said.

Determining when to get your flu or COVID-19 vaccine, Jetelina said, is something that will vary by person and should be discussed with your doctor. The CDC’s flu vaccine recommendation is similar to last season’s, and stipulates that most Americans get their shot in September or October.

At Tuesday’s press conference, Jha said people can wait several months to get boosted for COVID-19 if they’ve had a recent infection or if they were recently vaccinated. The hope, Jha said, is that “we are moving to a point” where low-risk Americans will only need one annual COVID-19 booster.

While experts emphasize that vaccines will remain an important part of battling a possible “twindemic” this season, Dr. Keri Althoff, a professor of epidemiology with the Johns Hopkins Bloomberg School of Public Health, said at Thursday’s briefing that “there are so many other tools in this toolbox that we can reach for."

"There is a level of personal responsibility that's also at play here,” Althoff said. “You can wear a mask whenever and wherever you want; you can decide on the quality of your mask for both fit and the filtration level. And all of these things can come together based on your risk strategy with your family members and your household members."

Jetelina also said masks should be an important method of preventing illness this fall and winter, especially now that most COVID-19 restrictions have been lifted.

“We know that masks work — not just for COVID-19, but they work for the flu as well,” she said. “So if you want to protect yourself, wearing a mask in crowded indoor areas will help reduce your likelihood of getting sick at all, really.

“Unfortunately, we cannot rely on other people, and so the onus is placed on you.”