What Covid has taught us about the flu, symptoms and how the virus spreads

On March 3, 2020, the head of the World Health Organization launched a daily press briefing urging countries around the world to do more to stop the spread of Covid.

Chief Executive Tedros Adhanom Ghebreyesus’ plea had become a common refrain in previous weeks: If aggressive action was taken, the virus could be contained.

This possibility, he said, was one of the key differences between Covid and the flu. “We are not even talking about containment for the seasonal flu. It’s just not possible,” he said.

Full coverage of the Covid-19 pandemic

Just over a week later, the United Nations health agency declared Covid a pandemic. The new coronavirus would continue to spread in virtually every country on the planet.

But something surprising happened: the transmission of the flu stopped. It turned out that the flu virus could be contained.

This revelation would not be the only time in the last three years that Covid has helped scientists gain a new understanding of influenza. The eagle eye focus on Covid has changed the way researchers – and the public – think about seasonal flu.

Influenza 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 the WHO launched its global flu tracking website FluNet, cases were virtually absent that winter.

“It was shocking how the flu went down to zero that year,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech. “We have learned that it is possible to stop the flu.”

Researchers, to some extent, disagree on the exact reason for the unprecedented flu drop that season. Pandemic-related mitigation measures — including the use of masks, avoiding indoor travel and gatherings, and more frequent hand-washing — likely played a role.

Others attribute the fact that Covid was the dominant virus that winter.

“When you are exposed to a respiratory virus like influenza or Covid, it induces an initial non-specific immune response,” says Dr Matthew Memoli, director of the clinical studies unit of the Laboratory of Infectious Diseases at the Institute. national allergy and infectiology. Diseases.

He said that later the body develops the specific antibodies to the virus, but at this initial stage, a non-specific antiviral response can also reduce the risk of flu.

“If you’re regularly exposed to a Covid virus, that’s going to really activate your viral response…and you’re not susceptible to another respiratory virus like the flu at that time,” he said.

Whether it was Covid dominance or behavioral changes that contributed most to the non-existent 2020-2021 flu season – and many attribute both – the new knowledge that flu transmission can in fact be stopped is here to stay. .

“Non-pharmaceutical interventions” work

Before Covid, experts put a limited stock on so-called non-pharmaceutical – that is, non-vaccine – strategies to prevent the transmission of influenza. While behaviors such as hand washing, wearing masks and air filtration were considered good ideas, they weren’t meant to move the needle significantly to stop the spread.

“Before the pandemic, we were very focused on promoting vaccination as the primary way to reduce flu transmission,” said Seema Lakdawala, 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 reduce the public health burden of influenza.”

Prior to 2020, she said there had been a handful of studies attempting to measure the effectiveness of these interventions, 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 sick can have a huge impact on transmission. flu viruses,” she said.

Fairly consistent flu vaccination rates further support this new appreciation for non-pharmaceutical interventions.

“I don’t think the number of vaccines has been significantly higher,” Lakdawala said of the 2020-2021 flu season. She said it was still around 40% to 60%, adding that she did not believe the decline in flu transmission was primarily due to vaccine immunity.

Indeed, according to the Centers for Disease Control and Prevention, 52.1% of people in the United States had received their seasonal flu shots by the end of the 2020-2021 season, just a small increase from 51.8%. of the 2019-2020 season.

Influenza can be spread by aerosols

At the start of the pandemic, how Covid spreads from person to person was one of the most debated topics among scientists. Originally thought to be spread by respiratory droplets expelled when people cough, sneeze or talk, scientists now understand that it can also be spread by even smaller particles called aerosols that can drift through the air.

Marr said she and her colleagues had no clear answer as to the dominant mode of transmission, but her work has shown that influenza, like Covid, can also spread via aerosols. Granted, 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. Without air filtration, 72% of passengers developed flu symptoms, and nearly everyone who was tested – including the original passenger who boarded sick – tested positive for flu.

“It was almost certainly airborne transmission in this case,” Marr said. Although the airline’s case study taught the research community about airborne flu transmission, she said the general public’s appreciation for these risks had increased thanks to Covid.

Throughout the pandemic, the search for effective ways to limit aerosol transmission has also strengthened support for HEPA and UV air filters, and humidity control in indoor spaces. According to Marr’s research, these lessons can also be applied to the flu.

The “long flu” can be a risk

The prevalence of long Covid — that is, persistent and sometimes debilitating symptoms that persist long after initial infection — has changed the way researchers think about flu risks, said Akiko Iwasaki, professor of immunobiology. at Yale University.

“Covid is certainly not alone in having these long-term consequences, even after a mild infection,” she said. After the flu, it’s not uncommon to experience symptoms, especially persistent fatigue and brain fog.

Seasonal flu is less likely to cause long-lasting symptoms than pandemic flu strains like the 2009 H1N1 virus, Iwasaki says, but more research is needed to be sure.

She said that for the 2009 pandemic flu and “even the 1918 flu, there are a lot of stories of people developing psychosis or neurological conditions over a long period of time.” A 2015 study in the journal Vaccine showed that across Norway, people infected with the 2009 H1N1 flu developed chronic fatigue more than twice as often as those who had been vaccinated this flu season.

A recent study in the journal Cell suggested that even mild Covid could lead to chronic brain damage. In this study, the researchers compared mild Covid infections with mild flu infections in mice and humans and found that the effects on the brain were similar around seven days after infection. Then, after longer follow-up, Covid infections caused longer-term damage than flu viruses, which mostly disappeared after seven weeks. These kinds of studies, spurred by the Covid pandemic, have already begun to help explain how the flu behaves in the body, Iwasaki said.

Asymptomatic influenza infections may be underestimated

The Covid pandemic has shed light on the extent and risk of asymptomatic infections. With much of the population rubbing their nostrils daily or weekly and then reporting those results to their work and their doctors, health officials were able to collect a wealth of data on the number of people who tested positive for Covid without symptoms and the duration of these infections persisted.

“The reason we found asymptomatic cases so clearly for Covid was because everyone was repeatedly testing so they could go to work,” said Lakdawala, of Emory University. “We have repeat tests on the same population every week, but we’ve never done that for the flu.”

Because people rarely test for flu unless they feel lousy, Lakdawala said the extent of asymptomatic flu infections is difficult to measure, let alone estimate.

Scientists know that asymptomatic flu cases are possible and that there is a 24-hour period that people shed the virus before they start to feel sick. But given the value of repeat testing, both Lakdawala and Marr said widespread and inexpensive flu tests should be readily available.

People want to test – and they’re good at it

In Lakdawala’s view, the most valuable thing to take away from Covid about the flu is knowing that people are not only eager to find out if they have been infected with a virus, but are also able to carry out the protocols. test for accurate results.

“Before, when widespread home testing came up, there was always a question of whether someone would know how to dab accurately, or if they could get a good enough sample,” she said. “Now we’ve proven that people are ready to dab their noses very well.”

The implications for influenza and other viruses, including respiratory syncytial virus, or RSV, are clear: if offered the option of affordable home flu testing, people would dab easily.

“People are engaged at a level that we’ve never enjoyed,” Lakdawala said. “They want to know, so we should give them the tools to know and then collect that data.”

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