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Health officials are urging people to get their flu shots now, in an attempt to prevent further strain on hospitals already overwhelmed by Covid-19 and other viruses. The push to get flu shots as soon as possible comes as two studies warn that this flu season could be a miserable one.

“There are some factors that we cannot control as far as how bad the flu season is going to be,” said Xiaoyan Song, chief infection control officer at Children’s National Hospital in Washington, D.C., “but there are some that we absolutely have control over.”

“Get vaccinated,” she said.

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It is notoriously difficult to predict what will happen in any flu season. But a combination of factors could make this winter particularly tough, experts said.

Children are back in school, often in communities that have eased up on mask mandates and physical distancing measures. And since flu was minimal to nonexistent last year, people were not exposed to the virus, potentially undermining the protection they’d normally have.

“Much of the immunity that we have as a population occurs because people in the population had influenza last year, and if they get a similar strain circulating, they won’t get influenza the second year,” Dr. Mark Roberts, director of the Public Health Dynamics Laboratory at the University of Pittsburgh Graduate School of Public Health, said during a media briefing recently.

In other words, he said, the dramatic decrease in flu cases last year has the potential to dramatically increase cases this year.

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Using mathematical modeling, Roberts and his team conducted two studies aimed at predicting hospitalizations during this flu season. Both studies were posted on a preprint server called medRxiv, and have not been peer-reviewed.

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.”

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