Prepare for another complicated flu season, with chance of a ‘twindemic’

3D print of influenza virus by NIH Image Gallery via Flickr.

As flu season nears, infectious disease and public health experts are even more worried than they were in 2020 about the possibility of a “twindemic” of both the delta variant of the SARS-CoV-2 virus and an influenza virus.

As of early September, hospitals in many states were already overwhelmed with patients sick from COVID-19, while the behavioral restrictions that prevented a severe flu season (i.e., masking, and social distancing) had been lifted in large swaths of the country.

“We have taken off our masks, people are going out and traveling and they are going out to restaurants and entertainment venues and most importantly, children are back to school,” said Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases and professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine. “This is particularly ominous because at the moment institutions are extraordinarily stretched and the prospect of having additional nasty respiratory virus out there causing serious illness, is making everyone in health care shudder.”

As with COVID-19, there is a vaccine to prevent the flu, and the CDC as well as infectious disease physicians have begun urging the public to get their flu vaccine. But adding to the challenge this year is the likelihood that people will need to receive a COVID-19 booster shot at the same time as the flu vaccine.

Journalists will play a role in helping the public sort out questions about the need for two simultaneous vaccines in the 2021-2022 flu season, Schaffner said.

“I’m already getting questions about whether a COVID-19 shot will prevent the flu, or if a flu shot will prevent COVID-19,” he said. “There is a lot of confusion out there.”

The viruses that cause COVID-19 and influenza are different, so a vaccine for one does not confer protection against the other disease, Schaffner stressed. He added that people can receive a COVID-19 and flu vaccines at the same time, but not in the same arm.

“I would caution you not to plan anything elaborate the next day [after you get your shots] because you may feel, in particular with the COVID vaccine, some side effects,” he said. “Some have some pain and headaches and fatigue.”

At the moment, there is no combination of flu and COVID-19 vaccine, however Moderna announced on Sept. 9 it would begin testing one in the coming months.

In the fall of 2020, infectious disease and public health experts also expressed concern that there could be a twindemic, but it did not materialize. In fact, the flu was almost non-existent. Researchers say the primary reason was social distancing measures in place during last year’s flu season.

“It was a pleasant surprise that we had a low influenza season,” Schaffner said. “It was impressive how behavioral interventions inhibited the spread of the influenza virus… but everything we did last year has been reversed” because of the lifting of behavioral interventions.

Schaffner said children are generally a primary vector of influenza virus to others, particularly grandparents and those who are immune compromised. In the winter of last year, most children were not in school and were mostly home, limiting the potential they could spread the flu. With kids in school this year, public health experts are on even higher alert for a potentially tougher flu season.

There has already been a delayed surge in respiratory syncytial virus (RSV) cases this summer among children because social distancing measures were lifted, potentially signaling what could happen with the flu this coming winter.

“We almost have to reintroduce everyone to the flu and remind everyone that the flu isn’t trivial, and in this environment of real vaccine fatigue, there is going to be all kinds of confusion about this,” Schaffner said.

Before COVID-19, the flu was among the deadliest respiratory viruses in the U.S., killing in the range of 24,000 to 62,000 people in the 2019-2020 flu season, and making 410,000 to 740,000 people ill enough to send them to the hospital.

See more AHCJ flu coverage background here, including sources for reporters to call.

The National Foundation for Infectious Diseases and the CDC will hold their annual flu briefing for the media on Oct. 7. Journalists can register here.

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