On March 3, 2020, the head of the World Health Organization began a daily press briefing exhorting countries around the world to do more to stop the spread of Covid.
Director-General Tedros Adhanom Ghebreyesus’ plea had become a common refrain in the preceding weeks: If aggressive measures were taken, the virus could be contained.
That possibility, he said, was one of the key differences between Covid and the flu. “We don’t even talk about containment for seasonal flu. It’s just not possible,” he said.
But then something surprising happened: Flu transmission ground to a halt. The influenza virus, it turned out, could be contained.
This revelation wouldn’t be the only time in the past three years that Covid helped scientists gain a new understanding about the flu. The eagle-eye focus on Covid has shifted how researchers — and the public — think about the seasonal flu.
Flu transmission can be stopped
The 2020-2021 flu season — the first full flu season of the Covid pandemic — defied Tedros’ message. For the first time since 1997, when WHO launched its global influenza tracking website FluNet, cases were virtually absent that winter.
“It was shocking how flu went to zero that year,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech. “We’ve learned that it is possible to stop the flu.”
Researchers, to some extent, disagree on the exact reason for the unprecedented drop in the flu that season. Pandemic-related mitigation measures — including the use of masks, avoiding travel and indoor gatherings, and more frequent hand-washing — likely played a role.
Others credit the fact that Covid was the dominant virus that winter.
“When you get exposed to a respiratory virus like flu or Covid, it induces an initial immune response that is nonspecific,” explained Dr. Matthew Memoli, director of the Laboratory of Infectious Diseases’ clinical studies unit at the National Institute of Allergy and Infectious Diseases.
He said that later on, the body develops the virus-specific antibodies, but at that initial point, nonspecific antiviral response can reduce flu risk, too.
“If you’re being exposed on a regular basis to a Covid virus, that’s going to have your viral response really turned on … and you’re not susceptible to another respiratory virus like flu at that moment,” he said.
Whether it was Covid’s dominance or behavioral changes that contributed most to the nonexistent 2020-2021 flu season — and many credit both — the newfound knowledge that flu transmission can in fact, be stopped, is here to stay.
‘Nonpharmaceutical interventions’ work
Before Covid, experts put limited stock in so-called nonpharmaceutical — that is, nonvaccination — strategies for preventing flu transmission. While behaviors such as hand-washing, wearing masks and air filtration were considered good ideas, they weren’t believed to move the needle significantly in stopping the spread.
“Prior to the pandemic, we were very focused on promoting vaccination as the primary way to decrease transmission of flu,” said Seema Lakdawala, an associate professor of microbiology and immunology at Emory University in Atlanta. “Now what we realize is that, yes, vaccinations are really important, but additional measures can really bring down the public health burden of influenza.”
Before 2020, she said there had been a handful of studies attempting to measure how well these interventions work, but they were inconclusive. “Coming out of the Covid-19 pandemic, we now have conclusive evidence that mitigation strategies like masking, social distancing and staying home when you are ill can drastically impact the transmission of influenza viruses,” she said.
Fairly consistent influenza vaccination rates further support this newfound appreciation for nonpharmaceutical interventions.
“I don’t think the amount of vaccine uptake was drastically higher,” Lakdawala said of the 2020-2021 flu season. She said it was always around 40% to 60%, adding that she didn’t think the drop in flu transmission was primarily driven by vaccine immunity.
Sure enough, according to the Centers for Disease Control and Prevention, 52.1% of people in the United States had received their seasonal flu vaccines by the end of the 2020-2021 season, just a small uptick from 51.8% in the 2019-2020 season.
Flu can spread via aerosols
In the early days of the pandemic, how Covid spreads from person to person was one of the most hotly debated topics among scientists. Initially thought to spread through respiratory droplets expelled when people cough, sneeze or talk, scientists now understand it can also spread through even smaller particles called aerosols that can drift through the air.
Marr said she and her colleagues don’t have a clear answer as to which mode of transmission is dominant, but her work has shown that the flu, like Covid, can also spread via aerosols. To be sure, many flu researchers recognized this before the pandemic, but the evidence was mostly limited to a 1979 case study in which an airplane was grounded for three hours due to engine failure. With no air filtration, 72 % of passengers developed flu symptoms, and nearly all who tested — including the original passenger who’d boarded the flight sick — were positive for the flu.
“It was almost surely airborne transmission in that case,” Marr said. Although the airline case study taught the research community about airborne flu transmission, she said the general public’s appreciation for these risks has increased because of Covid.
Throughout the pandemic, research into effective ways to limit aerosol transmission has also bolstered support for HEPA and UV air filters, and humidity control in indoor spaces. According to Marr’s research, these lessons can apply to the flu, too.
‘Long flu’ may be a risk
The prevalence of long Covid — that is, lingering, and sometimes debilitating, symptoms that persist long after the initial infection — has shifted how researchers think about flu risks, said Akiko Iwasaki, a professor of immunobiology at Yale University.
“Covid is definitely not alone in having these long-term consequences, even after a mild infection,” she said. After the flu, it’s not unheard of to experience symptoms, especially lingering fatigue and brain fog.
According to Iwasaki, seasonal flu is less likely to cause lasting symptoms than pandemic flu strains like the 2009 H1N1 virus, but more research is needed to say for sure.
She said that for the 2009 pandemic flu and “even the 1918 flu, there are a lot of stories about people developing psychosis or neurological diseases over a long period.” One 2015 study in the journal Vaccine showed that, across Norway, people who’d been infected with the 2009 H1N1 flu developed chronic fatigue more than twice as often as those who’d been vaccinated that flu season.
A recent study in the journal Cell suggested that even mild Covid could lead to chronic brain damage. In that study, the researchers compared mild Covid infections with mild flu infections in mice and humans and found that the brain effects were similar around seven days post-infection. Then, after a longer follow-up, the Covid infections led to longer-term damage than the flu viruses, which mostly subsided after seven weeks. These types of studies, spurred by the Covid pandemic, have already begun to help explain how the flu behaves in the body, Iwasaki said.
Asymptomatic flu infections may be underappreciated
The Covid pandemic put a spotlight on the extent and risk of asymptomatic infections. With much of the population swabbing their nostrils daily, or weekly, then reporting these results to their jobs and doctors, health officials were able to gather a wealth of data on how many people tested positive for Covid without symptoms and how long these infections persisted.
“The reason we found asymptomatic cases so clearly for Covid was that everybody was testing repeatedly to be able to go to work,” Lakdawala, of Emory University, said. “We got repeat testing on the same population every week, but we’ve never done that for influenza.”
Because people seldom test for the flu unless they feel lousy, Lakdawala said the extent of asymptomatic flu infections is difficult to measure, let alone estimate.
Scientists do know that asymptomatic flu cases are possible, and that there’s a 24-hour period where people shed virus before they start to feel sick. But given the value of repeat testing, Lakdawala and Marr both said that widespread, cheap flu tests should be readily available.
People want to test — and they’re good at it
In Lakdawala’s view, the most valuable flu takeaway from Covid is the knowledge that people are not only eager to know if they’ve been infected with a virus, but are also capable of carrying out the test protocols for accurate results.
“Before, when widespread at-home testing would come up, there was always a question of whether someone would know how to swab themselves accurately, or if they could get a sample that was good enough,” she said. “Now, we’ve proven that people are willing to swab their noses really well.”
The implications for the flu, and for other viruses including respiratory syncytial virus, or RSV, are clear: if given the option of affordable, at-home flu tests, people would readily swab themselves.
“People are engaged at a level that we never appreciated,” Lakdawala said. “They want to know, so we should give them the tools to know, and then collect that data.”