Fears of a 'twindemic' are over: The US has seen a 98% drop in flu hospitalizations, likely due to COVID-19 measures

• Experts worried the winter would bring a "twindemic" of flu and the coronavirus.
• But the US and UK, along with most of the globe, are experiencing historically mild flu seasons.
• COVID-19 public-health measures blunted the flu's spread, experts say.

Public-health experts prepared for a "twindemic" as fall approached last year: a double threat of the coronavirus and the seasonal flu.

But even as cold, dry weather descended on the Northern Hemisphere and COVID-19 cases surged, the US and UK have experienced historically mild flu seasons.

Between October 1 and January 30, just 155 Americans were hospitalized with the flu, compared to 8,633 during roughly the same time frame a year ago. That's a 98% decrease. Labs in the US have collected and tested more than half a million samples for the flu since late September, but just 0.2% of those samples tested positive (1,300 in total), according to the Centers for Disease Control and Prevention.

Every US state is experiencing "minimal" flu activity, the agency reported. That's in contrast from last season, when 22,000 Americans died of the flu.

A likely driver of the unusually low infection and hospitalization rates is COVID-19. Measures meant to slow or prevent the coronavirus' spread have also stopped other pathogens like influenza, according to Sonja Olsen, a CDC epidemiologist at the National Center for Immunization and Respiratory Diseases.

"Measures including extensive reductions in global travel, teleworking, school closures, social distancing, and face mask use may have played a role," she told Insider. Olsen noted, however, that it's challenging to tease out precisely which of those measures mattered most for flu prevention.

'I don't think anybody anticipated a season where there was no flu'

Across the globe, influenza activity is at lower levels than expected for this time of the year, according to the World Health Organization. Since the start of the pandemic, the Southern Hemisphere has had "virtually no influenza circulation," the CDC reported.

That's despite increased testing for flu in some countries, according to Olsen.

"There is some influenza circulating in tropical countries, but in these countries it appears that the season is blunted compared to other years," she said.

David Battinelli, chief medical officer at Northwell Health and a professor of medicine at Hofstra University, told Insider that experts had always "hoped that social distancing, masking, and more hand hygiene would mitigate the flu."

"But I don't think anybody anticipated a season where there was no flu," he said, adding, "nobody's seen it on the whole planet."

Flu season usually peaks in February, so it's likely the US is in the clear, but Battinelli warned it's too soon for a sigh of relief.

"Flu seasons can extend certainly into March," he said. In the last 40 years, six flu seasons peaked in March, according to the CDC.

Why coronavirus restrictions worked so well against the flu

Influenza doesn't spread as well as the coronavirus. The average number of people one person with the flu infects - a measure known as the reproductive number - is 1.28. Typically, someone sick with the coronavirus passes it onto between 2 and 2.5 people. In part, this difference is because the coronavirus can be airborne, remaining suspended in the air for hours. That's not true of the flu, though viruses can jump from person to person via respiratory droplets and contaminated surfaces.

The coronavirus' higher reproductive number "means it is more challenging to prevent transmission through non-pharmaceutical interventions than it is for influenza," Olsen said.

What's more, existing immunity in the population - whether from previous infections or vaccinations - can also boost the effects of public-health measures like masking, resulting in a more dramatic reduction in transmission, Olsen said.

The flu vaccine has been around for more than 75 years. Vaccine manufacturers projected that they will supply the US with as many as 194 to 198 million doses of influenza vaccine for the 2020-2021 season, according to Olsen.

"Contrast this against a novel coronavirus, to which nearly the entire population is susceptible," she said.

Research also shows that superspreader events - a circumstance in which one person infects a disproportionately large number of others -are the primary way the coronavirus spreads, unlike the flu.

Decreased travel played a larger preventative role than flu shots

CDC data suggests US pharmacies and physicians' offices administered more flu vaccines to Americans this season than the one prior, when Americans got 174.5 million doses. That increase came after public-health experts pushed people to get flu shots; some research suggested the shot could reduce the risk of getting COVID-19.

But Olsen said it's unlikely the extra vaccinations were the reason for low flu case counts globally.

"While some places in the world are using more vaccine, this is not universally true," Olsen said. "We have seen less influenza circulation even in places that do not use, or use very little, vaccine."

Both Olsen and Battinelli think a drop in travel did play a significant role, however.

"The traditional movement of flu around the globe hasn't occurred," Battinelli said.

Typically, the flu gets ferried from the Southern Hemisphere, which has its flu season between April and September, to the North Hemisphere in the fall. But travel bans and limited air travel mostly stopped that spread.

'We would expect influenza to return at some point'

Experts don't think a non-existent flu season will become a regular occurrence, and a study from Cambodia suggests that influenza will start circulating if pandemic-related restrictions are lifted.

"We would expect influenza to return at some point," Olsen said.

Still, the emergence of more transmissible coronavirus variants and the slow pace of global vaccinations may mean the pandemic will extend into 2022, necessitating another year of rigorous masking and social distancing. If that's the case, Battinelli thinks next year's flu season will be similarly mild.

To him, the takeaway from this unprecedented season is that Americans have the ability to actively limit flu transmission - and prevent tens of thousands of flu deaths - by masking up, social distancing, and practicing good hand hygiene.

"This is a wake-up call that we shouldn't be tolerating that many deaths," Battinelli said.