Coronavirus-Why the flu “cold” never happens

Interesting theories may help explain why influenza and Covid-19 did not seize the country at the same time — so-called Twin epidemics feared by many public health professionals..

The idea is that masks, social distances, or other pandemic restrictions are the only causes of the decline of influenza and other respiratory viruses during coronavirus domination and their resurrection as they recede. It wasn’t.

Rather, exposure to one respiratory virus is very vigilant for the body’s immune defenses and can prevent other invaders from entering the respiratory tract. This biological phenomenon, called viral interference, can limit the amount of respiratory virus that circulates in the area at any given time.

“My gut sensation, and my sensation based on recent research, is that viral interference is real,” said Dr. Ellen Foxman, an immunological scientist at Yale School of Medicine. “I don’t think the flu and coronavirus peaks will occur at the same time.”

At the individual level, she said, some people could be infected with two or three viruses at the same time. But at the population level, according to this theory, one virus tends to beat the other.

Still, she warned, “As the waves of Omicron show, the health care system can be overloaded before the upper limit of circulation is reached.”

Viral interference can help explain the patterns of infection found in large populations, including those that can occur when a coronavirus becomes endemic. But research is still in its infancy, and scientists are still struggling to understand how it works.

Before the coronavirus became a global threat, influenza was one of the most common and severe respiratory infections each year.In 2018-2019 seasonFor example, it was caused by the flu 13 million medical visits, 380,000 Hospitalization and 28,000 Dead (number).

It was unclear how the two viruses would affect each other, as the 2019-2020 flu season ended before the coronavirus raged around the world.lots of Experts were afraid The virus will clash next year in a bilaterally flooded hospital.

Those worries didn’t come true.Cases remained abnormally low throughout, despite weak efforts to increase influenza vaccination 2020-2021 influenza seasonAccording to the Centers for Disease Control and Prevention, the coronavirus continued to circulate.

Only 0.2% of samples tested positive for influenza from September to May, compared to about 30% in recent seasons, and influenza hospitalizations have been incurred since authorities began collecting this data in 2005. It was the lowest ever.

Many experts believe that the flu-free season is due to masks, social distance, and movement restrictions, especially for infants and the elderly, both of which are at greatest risk of severe flu. Influenza numbers rose a year later during the 2021-202 season, when many states exempted restrictions, but the numbers were still lower than the pre-pandemic average.

So So far this yearApproximately 5 million cases, 2 million medical visits, less than 65,000 influenza-related hospitalizations and 5,800 deaths have been recorded nationwide.

Instead, the coronavirus Dominate winterInfluenza, respiratory syncytial virus, rhinovirus, much more common than the common cold virus.

Respiratory syncytial virus (RSV) usually surfaced in September and peaked in late December to February, but due to a pandemic, Seasonal pattern.. It fell throughout 2020 and peaked in the summer of 2021 — when the coronavirus plunged to its lowest level since the beginning of the pandemic.

The concept that there is a kind of interaction between viruses first Appeared in the 1960sVaccination against polio, including attenuated poliovirus, significantly reduces the number of respiratory infections. This idea created a new horizon in 2009.Europe seemed ready for a surge in swine flu cases late that summer, but when school resumed, the rhinovirus cold somehow Stop the flu epidemic..

“It encouraged many at the time to speculate on this idea of ​​viral interference,” said Dr. Foxman. Even in normal years, rhinovirus peaks in October or November at the end of the influenza season, followed by March.

Last year, a team of researchers set out to study the role of existing immune responses in the defense of the influenza virus. Since it is unethical to intentionally infect children with influenza, they vaccinated Gambian children with an attenuated viral strain.

Infection with the virus causes a complex cascade of immune responses, but the first defense comes from a set of non-specific defenses called interferons. The team found that children who already had high levels of interferon had far less influenza virus in their bodies than children who had low levels of interferon.

The findings suggest that previous viral infections stimulated the child’s immune system to fight the influenza virus. “Most of the viruses found in these children before vaccination were rhinoviruses,” said Dr. Schan de Silva, an infectious disease specialist at the University of Sheffield, England, who led the study. ..

This dynamics may partially explain why children who are more susceptible to respiratory infections than adults appear to be less likely to get the coronavirus. Influenza can also prevent adult coronavirus infections, said Dr. Guy Boavan, a virologist and infectious disease expert at Laval University in Canada.

recently the study Shows it Coinfection of influenza And coronavirus RareAnd most people with active influenza infections 60% less likely He said the test for coronavirus would be positive.

“Currently, influenza activity is increasing in Europe and North America, and it would be interesting to see if that would lead to a decrease in SARS-COV-2 circulation in the coming weeks,” he said. rice field.

Technological advances over the last decade have made it possible to provide a biological basis for this interference.Dr. Foxman’s team used a model of human respiratory tract tissue to show rhinovirus infection Stimulates interferon You can then dodge the coronavirus.

Pablo Murcia, a virologist at the MRC Virus Research Center at the University of Glasgow, said: Similar results were found..

However, Dr. Murcia also discovered a twist in the theory of viral interference. Attacks of the coronavirus did not appear to prevent infection by other viruses. He said it may have something to do with how proficient the coronavirus is in evading the initial defenses of the immune system.

“The activation of these antiviral interferons tends to be less than that of influenza,” Dr. Decilva said of the coronavirus. This finding suggests that in a particular population, which virus appears first may be important.

Dr. DeSilva and his colleagues have collected additional data from The Gambia. It had no pandemic-related restrictions that could affect the viral pattern we were observing. Rhinovirus, influenza, and coronavirus have all been shown to peak at different times from April to June 2020. 2021.

“I was a little more convinced that interference could play a role,” he said.

Nevertheless, virus behavior can be significantly affected by the rapid evolution of the virus, as well as social restrictions and vaccination patterns. Therefore, it is unlikely that the potential effects of virus interference will be revealed until the coronavirus has settled into a predictable state. Endemic patternsThe expert said.

RSV, rhinovirus, and influenza have co-existed for years, said Dr. Nacia Safdar, a medical-related infectious disease expert at the University of Wisconsin-Madison.

“Ultimately, this will happen as well. It will be one of the many viruses that circulate,” Dr. Safdar said of the coronavirus. She said that some viruses may weaken the effects of others, but the pattern may not be immediately apparent.

Looking at the cold coronavirus, some researchers have predicted that SARS-CoV-2 will be a seasonal winter infection. May match the flu.. However, the pandemic coronavirus has already been shown to be different from its cousin.

For example, although rarely seen in coinfections, one of the four common cold coronaviruses is often seen as a coinfection with the other three.

“This is an interesting example that hesitates to make a generalization that spans multiple viruses,” said Jeffrey Townsend, a biostatistician at the Graduate School of Public Health who studied the coronavirus and its seasonality. “How these things happen seems to be virus-specific.”