Why the ‘twin epidemic’ of coronavirus may never happen

An intriguing theory may help explain why the flu and Covid-19 did not take over the nation simultaneously – the so-called double pandemic that many public health experts had feared.

The idea is that it wasn’t just masks, social distancing or other pandemic restrictions that caused the flu and other respiratory viruses to fade while the coronavirus took hold, and re-emerge as it receded.

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

“My gut feeling, and my feeling based on our recent research, is that viral interference is real,” said Dr. Elaine Foxman, an immunologist at Yale University School of Medicine. “I don’t think we’ll see a peak of influenza and coronavirus at the same time.”

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

However, she cautioned, “the health care system can become overburdened long before the upper limit of circulation is reached, as the Omicron wave has shown.”

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

Before the coronavirus became a global threat, influenza was among the most common acute respiratory infections each year. In the 2018-2019 season, for example, influenza was responsible for 13 million medical visits, 380,000 Hospitalization and 28000 death cases.

The 2019-2020 flu season was coming to an end before the coronavirus started spreading around the world, so it wasn’t clear how the two viruses could affect each other. Many experts feared that next year viruses would collide in a hospital-sinking epidemic.

Those fears were not realized. Despite weak efforts to ramp up flu vaccinations, cases remained unusually low throughout the 2020-2021 flu season, as the coronavirus continued to spread, according to the Centers for Disease Control and Prevention.

Only 0.2 percent of samples tested positive for the flu from September through May, compared to about 30 percent in recent seasons, and hospitalizations for flu were the lowest ever since the agency began collecting that data in 2005.

Many experts have attributed the flu-free season to masks, social distancing, and restriction of movement, especially for young children and the elderly, both of whom are more susceptible to severe influenza. Flu numbers spiked a year later, in the 2021-2022 season, when many states dispensed with restrictions, but the numbers were still below the pre-pandemic average.

So far this year, the country has recorded about five million cases, two million medical visits, less than 65,000 hospitalizations, and 5,800 flu-related deaths.

Instead, the coronavirus continued to dominate the winter, which is more common than influenza, respiratory syncytial virus, rhinovirus, and common cold viruses.

Respiratory syncytial virus, or RSV, usually appears in September and peaks in late December to February, but the epidemic has distorted its seasonal pattern. It remained low throughout 2020 and peaked in the summer of 2021 – when the coronavirus fell to its lowest levels since the start of the pandemic.

The idea that there was some kind of interaction between viruses first emerged in the 1960s, when polio vaccinations, which contain attenuated poliovirus, dramatically reduced 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, viral colds seemed to be able to stop the spread of the flu epidemic.

“It led a lot of people at the time to speculate about the idea of ​​a viral intervention,” Dr. Foxman said. Even in a typical year, the flu virus peaks in October or November and then again in March, at either end of flu season.

Last year, a team of researchers set out to study the role of the current immune response in staving off the influenza virus. Because it would be unethical to deliberately infect children with influenza, they gave children in The Gambia a vaccine with a weakened strain of the virus.

Infection with viruses triggers a complex series of immune responses, but the first defense comes from a group of non-specific defenders called interferons. The team found that children who already had high levels of interferon ended up with less flu viruses in their bodies compared to those with lower levels of interferon.

The results indicated that previous viral infections prepared the children’s immune systems to fight the influenza virus. “Most of the viruses we saw in these children before the vaccine was given were rhinoviruses,” said Dr. Thoushan de Silva, an infectious disease specialist at the University of Sheffield in England, who led the study.

This dynamic may partly explain why children, who tend to get respiratory infections more often than adults, appear to be less likely to contract the coronavirus. Dr. Jay Boivin, a virologist and infectious disease specialist at Laval University in Canada, said influenza may also prevent infection with MERS in adults.

He pointed out that recent studies have shown that co-infection with influenza and coronavirus is rare, and that people with active influenza infection are 60 percent less likely to test positive for coronavirus.

“We are now seeing a spike in influenza activity in Europe and North America, and it will be interesting to see if it leads to a decrease in the circulation of SARS-COV-2 in the next few weeks,” he said.

Advances in technology over the past decade have made it possible to demonstrate the biological basis for this intervention. Use the team of Dr.

“Protection is transient for a certain period of time while you have an antiviral reaction caused by rhinovirus,” said Pablo Murcia, a virologist at the University of Glasgow’s MRC Center for Virology Research, whose team found similar results.

But Dr. Murcia also discovered a flaw in the viral interference theory: a bout of coronavirus did not seem to prevent infection with other viruses. This may have something to do with how adept the coronavirus is at evading the immune system’s primary defenses, he said.

“Compared to influenza, it tends to reduce activation of these antiviral interferons,” Dr. de Silva said of the coronavirus. This finding suggests that in a particular population, it may be important to determine which virus appears first.

Dr. de Silva and colleagues collected additional data from the Gambia – which did not have pandemic-related restrictions that may have affected the viral patterns they were monitoring – indicating that rhinoviruses, influenza and the coronavirus peaked at different times between April 2020 and June. 2021.

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

However, the behavior of viruses can be greatly influenced by their rapid evolution, societal constraints, and vaccination patterns. So the potential impact of the viral intervention is unlikely to become apparent until the coronavirus has settled into a predictable endemic pattern, experts said.

Dr. Nazia Safdar, an expert in healthcare-associated infections at the University of Wisconsin-Madison, notes that respiratory syncytial virus, rhinovirus and influenza have been coexisting for years.

“Ultimately, that’s what will happen with this as well – it will become one of the many things that spread,” Dr. Safdar said of the coronavirus. She said some viruses may mitigate the effects of others, but the patterns may not be readily apparent.

Given the common cold coronaviruses, some researchers have speculated that SARS-CoV-2 will become a seasonal winter infection that may coincide with influenza. But the pandemic coronavirus has already shown itself to be different from its cousins.

For example, it is rarely seen in co-infections, while one of the four common coronaviruses is often seen as a co-infection with the other three.

“This is the kind of interesting example that makes kind of reluctance to make generalizations across multiple viruses,” said Jeffrey Townsend, a biostatistician at the Yale School of Public Health who has studied coronavirus and its seasonality. “It appears that how these things happen has to do with the virus to some extent.”

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