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An intriguing theory may help explain why flu and Covid-19 have never taken over the nation at the same time. twin epidemic feared by many public health experts.

The idea was that it wasn’t just masks, social distancing or other pandemic restrictions that caused flu and other respiratory viruses to decline as the coronavirus reigned and resurfaced as it retreated.

Conversely, exposure to a respiratory virus can put the body’s immune defenses on high alert, preventing other intruders from entering the airways. This biological phenomenon, called viral interference, can limit the amount of respiratory virus circulating in an area at any given time.

An immunologist at the Yale School of Medicine, Dr. “My gut feeling, and based on our latest research, is that viral interference is real,” said Ellen Foxman. I don’t think we’ll see the flu and coronavirus peak at the same time,” she said.

On an individual level, he said, there may be some people who are infected with two or even three viruses at the same time. But at the population level, one virus tends to outperform the others, according to this theory.

Still, he warned, “As the Omicron wave shows, the health system can become overloaded long before the upper limit of circulation is reached.”

Viral interaction may help explain the patterns of infection seen in large populations, including those that may occur when the coronavirus becomes endemic. But research is in its early days and scientists are still struggling to understand how it works.

Before the coronavirus became a global threat, the flu was among the most common severe respiratory infections each year. Inside 2018-2019 seasonfor example, the flu was responsible 13 million medical visits, 380,000 hospitalizations and 28,000 deaths.

The 2019-2020 flu season was coming to an end before the coronavirus began to take the world by storm, so it was unclear how the two viruses affected each other. Many experts got scared The twin epidemics of viruses next year would collide in swamp hospitals.

These concerns did not materialize. Despite weak efforts to increase flu vaccinations, cases have remained unusually low all year. 2020-2021 flu seasonAs the coronavirus continues to circulate, according to the Centers for Disease Control and Prevention.

Only 0.2 percent of samples tested positive for influenza from September to May, with nearly 30 percent in recent seasons, and hospitalizations for flu were the lowest recorded since 2005, when the agency began collecting this data.

Many experts have attributed the flu-free season to masks, social distancing, and restricted movement, particularly of young children and older adults, both of whom are at highest risk for severe flu. Flu numbers rose a year later in the 2021-2022 season, when many states gave up restrictions, but the numbers were still below the pre-pandemic average.

Like this away this yearThe country recorded nearly five million cases, two million medical visits, and less than 65,000 hospitalizations and 5,800 flu-related deaths.

Instead, the coronavirus continued dominate the wintersmuch more common than influenza, respiratory syncytial virus, rhinovirus, and common cold viruses.

Respiratory syncytial virus, or RSV, usually emerges in September and peaks from late December to February, but the pandemic has derailed its condition. seasonal pattern. It remained low throughout 2020 and peaked in the summer of 2021, when the coronavirus hit its lowest levels since the start of the pandemic.

First the idea that there is some kind of interaction between viruses appeared in the 1960sWhen polio vaccines containing weakened polio virus significantly reduce the number of respiratory infections. The idea gained new ground in 2009: Europe seemed poised for a surge in swine flu cases late that summer, but when schools reopened, the rhinovirus fever had somehow crept up. stop the flu epidemic.

Dr. “This prompted a lot of people at the time to speculate about this idea of ​​viral intervention,” Foxman said. Even in a typical year, rhinovirus peaks at either end of the influenza season, in October or November, and then in March.

Last year, a research team set out to investigate the role of an existing immune response in fending off the flu virus. Because it would be unethical to deliberately infect children with the flu, they gave children in The Gambia a vaccine with a weakened strain of the virus.

Infection with viruses initiates a complex array of immune responses, but the first defense comes from a set of non-specific defenders called interferons. The team found that children who already have high levels of interferon end up with far fewer flu viruses in their bodies than children with lower levels of interferon.

The findings showed that previous viral infections primed the children’s immune systems to fight the flu virus. Infectious disease specialist at the University of Sheffield in England, who led the study, Dr. “Most of the viruses we saw in these children before we had the vaccine were rhinoviruses,” Itshan de Silva said.

This dynamic may partly explain why children, who tend to have more respiratory infections than adults, seem less likely to be infected with the coronavirus. At the University of Laval in Canada, virologist and infectious disease specialist Dr. Guy Boivin said the flu can also prevent coronavirus infections in adults.

End studies it showed flu co-infections and coronavirus Rareand those with active influenza infection are almost 60 percent less likely Signed up to test positive for coronavirus.

“We’re now seeing an increase in flu activity in Europe and North America, and it will be interesting to see if that leads to a reduction in SARS-COV-2 circulation over the next few weeks,” he said.

Advances in technology over the past decade have made it possible to demonstrate the biological basis of this enterprise. Foxman’s team used a human airway tissue model to demonstrate rhinovirus infection. stimulates interferons this can then fend off the coronavirus.

“Once you have an interferon response triggered by the rhinovirus, the protection is temporary for a certain period of time,” said Pablo Murcia, a virologist with the MRC Center for Virus Research at the University of Glasgow. similar results found.

However, Dr. Murcia also discovered a peculiarity in the theory of viral interference: A seizure with the coronavirus did not seem to prevent infection with other viruses. He said it may have something to do with how adept the coronavirus is at evading the immune system’s first defenses.

Dr. “Compared to the flu, these antiviral tends to activate interferons less,” de Silva said of the coronavirus. This finding suggests that it may be important which virus emerges first in a given population.

Dr. de Silva and colleagues collected additional data from the Gambia, which had no pandemic-related restrictions that may have affected the viral patterns they observed – showing that rhinovirus, flu and coronavirus peaked at different times between April 2020 and June. 2021.

These data “convinced me a little more that intervention could play a role,” he said.

Yet the behavior of viruses can be greatly influenced by their rapid evolution, societal constraints, and vaccination patterns. Therefore, the potential impact of viral interference is unlikely to occur until the coronavirus settles into a predictable state. endemic patternexperts said.

A specialist in healthcare-associated infections at the University of Wisconsin-Madison, Dr. Nasia Safdar stated that RSV, rhinovirus and flu have coexisted for years.

Dr. “It will eventually happen in this one too – one of many in circulation,” Safdar said about the coronavirus. Some viruses can mitigate the effects of others, he said, but the patterns may not be easily seen.

Looking at common cold coronaviruses, some researchers have predicted that SARS-CoV-2 will be a seasonal winter infection. may coincide with the flu. But the pandemic coronavirus has already shown that it is different from its cousins.

For example, while coinfections are rare, one in four common cold coronaviruses is frequently seen as a coinfection with the other three.

“This is an interesting example of being kind of hesitant to generalize across multiple viruses,” said Jeffrey Townsend, a biostatistician who studies the coronavirus and its seasonality at the Yale School of Public Health. “How these things happen seems somewhat virus-specific.”