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

In March 3, 2020The head of the World Health Organization began a daily press conference urging countries around the world to do more to stop the spread of Covid.

The plea from CEO Tedros Adhanom Ghebreyesus had become a common refrain in the weeks before: 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’re not even talking about containment for seasonal flu. It’s just not possible,” she said.

Full coverage of the Covid-19 pandemic

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

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

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

Flu transmission can be stopped

The 2020-2021 flu season, the first full flu season of the Covid pandemic, challenged Tedros’ message. For the first time since 1997, when the WHO launched its global influenza tracking website FluNetcases were virtually absent that winter.

“It was shocking how the flu went 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 drop in flu that season. Mitigation measures related to the pandemic, including wearing masks, avoiding indoor travel and gatherings, and washing hands more frequently, 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 immune response that is non-specific,” explained Dr. Matthew Memoli, director of the clinical studies unit at the National Institute of Allergy Infectious Diseases Laboratory. and infections. Diseases.

Later, he said, the body develops antibodies specific to the virus, but at that initial point, the non-specific antiviral response can also reduce the risk of flu.

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

Whether it was the dominance of Covid or behavioral changes that contributed most to the non-existent 2020-2021 flu season, and many credit both, the new knowledge that flu transmission may, in fact, stop, it’s here to stay.

Work ‘Non-pharmaceutical interventions’

Before Covid, experts put a limited stock on so-called non-pharmaceutical strategies, that is, without vaccination, to prevent the transmission of the flu. While behaviors like washing hands, wearing masks and filtering the air were considered good ideas, they were not believed to significantly change the needle in stopping the spread.

“Before the pandemic, we were very focused on promoting vaccination as the primary way to decrease transmission of influenza,” said Seema Lakdawala, an associate professor of microbiology and immunology at Emory University in Atlanta. “Now we realize that, yes, vaccines are really important, but additional measures can really reduce the public health burden of influenza.”

Before 2020, he said there were a handful of studies that tried to measure how well these interventions work, but they were inconclusive. “In the aftermath of the covid-19 pandemic, we now have conclusive evidence that mitigation strategies such as wearing masks, social distancing, and staying home when sick can dramatically affect the transmission of influenza viruses,” he said. the.

The fairly consistent influenza vaccination rates further support this new appreciation of non-pharmaceutical interventions.

“I don’t think the amount of vaccine uptake has been drastically higher,” Lakdawala said of the 2020-2021 flu season. He said it was always between 40% and 60%, adding that he did not believe that the decline in flu transmission was primarily due to immunity from vaccines.

Indeed, according to Centers for Disease Control and Prevention52.1% of people in the United States had received their seasonal flu vaccinations by the end of the 2020-2021 season, only a small increase from 51.8% in the 2019-2020 season.

The flu can be spread through aerosols

In the early days of the pandemic, how covid spreads from person to person was one of the most debated topics among scientists. Initially it was thought that it was spread through respiratory droplets expelled when people coughed, sneezed or talked, but now scientists know that it can also be spread through even smaller particles called aerosols that can float in the air.

Marr said she and her colleagues don’t have a clear answer as to which mode of transmission is dominant, but their work has shown that flu, like covid, can also spread through aerosols. To be sure, many influenza researchers recognized this before the pandemic, but the evidence was mostly limited to a 1979 case study in which a plane was grounded for three hours due to engine failure. Without air filtration, 72% of passengers developed flu symptoms, and nearly everyone tested, including the original passenger who boarded the flight sick, tested positive for the flu.

“It was almost certainly airborne transmission in that case,” Marr said. Although the airline case study taught the research community about airborne transmission of flu, he said the general public’s appreciation of these risks has increased due to Covid.

Throughout the pandemic, research into effective ways to limit aerosol transmission has also strengthened support for HEPA and UV air filters and indoor humidity control. According to Marr’s research, these lessons may also apply to the flu.

The ‘long flu’ can be a risk

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

“Covid is definitely not the only one that has these long-term consequences, even after a mild infection,” he said. After the flu, it’s not uncommon to experience symptoms, especially persistent fatigue and brain fog.

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

He said that for the 2009 flu pandemic and “even the 1918 flu, there are a lot of stories about people developing psychosis or neurological diseases over a long period.” A 2015 study in the journal Vaccine showed that across Norway, people who had been infected with 2009 H1N1 flu developed chronic fatigue more than twice as often as those who had 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 after infection. Then, after longer follow-up, covid infections caused longer-term damage than 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 influenza infections may be underestimated

The Covid pandemic highlighted the extent and risk of asymptomatic infections. With much of the population swabbing their nostrils daily or weekly, and then reporting these results back to their jobs and doctors, health officials were able to collect a wealth of data on how many people tested positive for covid without symptoms and for how long. these infections persisted.

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

Because people rarely get tested for the flu unless they’re feeling sick, Lakdawala said the extent of asymptomatic flu infections is hard to measure, let alone estimate.

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

People want to try, and they’re good at it.

From Lakdawala’s point of view, the most valuable lesson of the Covid flu is the knowledge that people are not only eager to know if they have been infected with a virus, but are also capable of carrying out the protocols of test for accurate results.

“Back when widespread home testing came along, there was always the question of whether someone would know how to accurately sample or could get a sample that was good enough,” he said. “Now, we’ve shown that people are willing to rub their noses really well.”

The implications for influenza and other viruses, including respiratory syncytial virus, or RSV, are clear: Given the option of affordable home flu tests, people would readily take samples.

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

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