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A ‘remarkable’ flu season: It nearly disappeared

Several factors may have caused dramatic drop in rates of infection
Magaly Torres, a medical assistant at Mercy Family Medicine Horse Gulch, holds a package of 2020-21 influenza vaccine on Tuesday at the clinic. Flu rates dropped to virtually nonexistent levels in Colorado and nationwide during the 2020-21 season compared with previous years.

To the relief of medical and public health professionals, there has been a key absence in the health scene this winter: the seasonal flu.

Heading into Colorado’s 2020-21 flu season, health professionals were preparing for a possible “twindemic” of the coronavirus and the flu. Instead, the flu rates have dropped to virtually nonexistent levels in Colorado and nationwide compared with previous years.

“It’s been a remarkable season. It’s different than any I’ve seen in the 30 years I’ve lived in Colorado,” said Eric France, chief medical officer for the Colorado Department of Public Health and Environment.

Statewide, there were 29 hospitalizations as of March 13 based on the most recent state data from 64 counties, according to CDPHE. The season begins in September and ends in May.

There have been zero outbreaks in long-term care facilities and zero pediatric influenza deaths.

In comparison, the 2019-20 influenza season resulted in 3,546 hospitalizations in 64 counties. The 2018-19 season was more severe with 3,832 hospitalizations in 59 counties, according to CDPHE.

“It’s really incredible,” France said.

Flu was also nearly nonexistent in La Plata and Archuleta counties, according to San Juan Basin Public Health and Mercy Regional Medical Center.

“Positive flu tests are rare, and we have yet to have anyone hospitalized with the flu, which is a blessing considering some anticipated a double punch of both flu and COVID-19,” said Chris Hudson, Mercy’s chief medical officer.

Why so low?

Health professionals are still studying why flu rates dropped so significantly.

One likely reason: Case numbers dropped because people are practicing good public health measures to prevent the spread of COVID-19, such as staying 6 feet apart, wearing masks, washing hands and staying home when sick, France said.

During remote schooling caused by the pandemic, children were at home instead of school, where they often act as a source of spread for respiratory illnesses. That might have offered some protection, France said.

“All respiratory infections will be lower this year because we’re doing all the right behaviors and activities that reduce respiratory spread,” France said.

That’s true for another wintertime respiratory virus, respiratory syncytial virus, commonly known as RSV. This virus is a strong driver of hospitalizations and clinical visits in young children, ages 2 to 5, France said.

“That also has been nonexistent this year,” he said.

Concerned about a “twindemic” of the coronavirus and influenza, health officials highly encouraged vaccination against the flu this year.

The public health community also ramped up public vaccination outreach efforts, which could have contributed to higher vaccination rates and in turn lower case rates, said France and Liane Jollon, SJBPH executive director.

There are other factors to consider, Jollon said. For example, the flu virus travels between the southern and northern hemispheres through international travel, which was significantly reduced during the pandemic.

Viral competition or viral interference could also play a role. That theory suggests that if one virus is already circulating in a population, it’s harder for another virus to take hold.

“These are all things people are studying right now,” Jollon said.

Why wasn’t COVID-19 defeated?

While other respiratory viruses all but stopped in their tracks, the novel coronavirus – also a respiratory virus – is still spreading even with public health measures in place.

One explanation is that the virus causing COVID-19 is more contagious, France said.

Scientists measure the contagiousness of a disease with a figure called R0 (pronounced r-naught). This number indicates the average number of people infected by one person who has a virus.

For the flu, about 1.6 people will be infected by someone who has the flu. For the coronavirus, about 4 to 5 people will contract the virus from one person who has the coronavirus, he said.

“It’s higher, so coronavirus spreads more than flu does,” France said.

Jollon said the novel coronavirus, SARS-CoV-2, appears to be more contagious for several reasons.

People can spread it while asymptomatic. Flu symptoms appear sooner, which offers a smaller window of time for asymptomatic spread.

Most people have been exposed to flu strains. But with the novel coronavirus, the majority of the population is still “immune naive,” because their bodies have never been exposed to it, Jollon said.

The coronavirus also appears to have a “super spreader” component to its contagiousness, when one person spreads it to a higher-than-average number of people. That doesn’t appear with the flu, she said.

What to expect next season

As medical professionals look to the 2021-22 flu season, they aren’t sure what impacts such a low flu season this year could have.

It could impact vaccine development. Scientists look at flu strains in January and February to determine which strains are going to be dominant in the coming season. They then create vaccines based on those strains.

“That may be easier to do when there’s tens of thousands of cases of flu. ... That’s harder when there hasn’t been a dominant flu season,” France said. “I imagine they’ll make their best estimate based on the cases they were seeing in the Southern Hemisphere.”

One big unknown: how well the vaccine will match the dominant strains.

“If it’s a good match, then we should expect 50% effectiveness. If it’s a bad match, it might be only 30%,” he said.

Some flu strains might have died off during the low flu season. That could make it a more manageable disease in the future – or leave room for less prevalent strains to take hold and make the flu more severe, Jollon said.

“While we had a low flu season, we will be having this discussion next fall,” Jollon said. “My guess is, we’ll be working every bit as hard as we did this fall to ensure that flu vaccines are available ... as we go into another unknown flu season.”


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