Prior to the COVID-19 pandemic, public health experts didn't believe the flu could be contained. But, when flu transmission came to a halt during the pandemic, scientists were able to gain a new understanding of the virus.
In the 2020-2021 flu season, the WHO found that cases were virtually absent that winter.
"It was shocking how flu went to zero that year," Linsey Marr, PhD, a professor of civil and environmental engineering at Virginia Tech, told NBC News. "We've learned that it is possible to stop the flu."
Researchers, however, disagree to some extent about why flu cases dropped so drastically. Some credit pandemic-related mitigation measures, including the use of masks, more frequent hand washing and avoiding indoor gatherings, while others credit the fact that COVID-19 was the dominant virus during the winter.
Prior to 2020, greater emphasis was placed on vaccinations than preventative measures such as hand-washing, wearing masks and air filtration for the flu.
"Now what we realize is that, yes, vaccinations are really important, but additional measures can really bring down the public health burden of influenza," Seema Lakdawala, PhD, associate professor of microbiology and immunology at Emory University in Atlanta, told NBC News.
Additionally, the COVID-19 pandemic made scientists more conscious of how flu can spread via aerosols. Though this was known prior to the pandemic, research into effective ways to limit aerosol transmission has increased support for HEPA and UV air filters as well as humidity control in indoor spaces.
According to Dr. Marr's research, these lessons can be applied to the flu as well.
The prevalence of long COVID-19 has also brought attention to similar conditions and could suggest that more research needs to be done on potential "long flu" symptoms.
While seasonal flu is less likely to cause lasting symptoms than pandemic flu strains, Akiko Iwasaki, PhD, a professor of immunobiology at Yale University, told NBC News that more research on similar conditions since the onset of COVID-19 has already begun to better explain how flu behaves in the body.
Lastly, the pandemic also placed more attention on the possibility of asymptomatic cases.
"The reason we found asymptomatic cases so clearly for Covid was that everybody was testing repeatedly to be able to go to work," Dr. Lakdawala said. "We got repeat testing on the same population every week, but we've never done that for influenza."
But, Dr. Lakdawala added, COVID-19 showed that people were willing to test at home to confirm they weren't infected with a virus.
"People are engaged at a level that we never appreciated," Dr. Lakdawala said. "They want to know, so we should give them the tools to know, and then collect the data."