On average, about 200,000 people are hospitalized with the flu each year, according to data from the Centers for Disease Control and Prevention. Roberts' team calculated that the 2021-22 flu season could result in as many as 600,000 such hospitalizations.
That is a worst-case scenario, he said, if there is an extremely contagious flu strain, coupled with low vaccination rates. Fortunately, at this point, there are no new or worrisome flu variants on the horizon, according to Dr. William Schaffner, an infectious diseases expert at Vanderbilt University Medical Center.
But given that scenario, the models suggested that 75 percent of Americans would need to get the flu shot to avoid the additional hospitalizations. In general, just about half of the U.S. population gets the flu vaccination annually.
The calls for flu shots come as many hospitals are facing an unseasonal deluge of patients — especially children — sick with viruses usually only seen during the winter, such as respiratory syncytial virus, or RSV.
Those viruses, including ones that cause croup and hand, foot and mouth disease, "all came on very strong over the course of spring and summer as people loosened up in terms of masking and social distancing," said Dr. David Kimberlin, co-director of the division of pediatric infectious diseases at the University of Alabama at Birmingham. Now, he said, "it's all delta variant Covid. We are overwhelmed with Covid cases. Our hospital is full."
Adding influenza to the mix, he said, "has the potential to be catastrophic."
Schaffner, of Vanderbilt University Medical Center, agreed.
"It could very well be that we have three major viruses" circulating this winter, he said.
Beyond mathematical calculations, the usual predictor for flu seasons in the Northern Hemisphere is flu activity in the Southern Hemisphere, which is just finishing up its winter season. Flu activity there has been low, largely because many countries are still focused on mitigation, according to Song. The data show a "sharp correlation" between increased public health measures and decreased viral spread, she said.
Flu shots are key in those measures, even though they are not as effective as the Covid-19 vaccines have proven to be. During the 2018-19 season, flu shots had an overall effectiveness of just 29 percent. Kimberlin said that on average, flu shots are about 50 percent effective.
When breakthrough influenza infections occur, they are generally mild and do not require hospitalization, the American Academy of Pediatrics said Tuesday in its official recommendation that children ages 6 months and older get the flu shot.
Keeping flu patients, especially children, out of the hospital this winter is critical, given the other viruses at play.
"The flu vaccines are not outstanding, but they're good," Kimberlin said. "If I'm going into battle and someone says, 'Do you want to put armor on 50 percent of your body or wear nothing?' what do you think I'm going to do?" he said.
"Take advantage of what we have."