The flu and Covid-19 never gripped the nation at the same time, as many public health experts had feared.
The idea is that it wasn't just masks that caused flu and other respiratory viruses to fade while the coronaviruses reigned, and that it was more than that.
Exposure to a single respiratory virus may cause the body's immune defenses to be on high alert. The amount of respiratory virus circulating in a region may be capped by this biological phenomenon.
Dr. Ellen Foxman is an immunologist at the Yale School of Medicine.
She said that there may be some people who end up with two or three viruses at the same time. According to this theory, one virus tends to win at a population level.
She warned that the health care system can become overburdened before the upper limit of circulation is reached.
Patterns of infections seen in large populations may be explained by viral interference. Scientists are still trying to understand how the research works.
Influenza infections were among the most common severe respiratory infections each year. 13 million medical visits, 380,000 hospitalizations and 28,000 deaths were caused by the flu in the last year.
The flu season was winding down before the coronaviruses began to rage through the world, so it was unclear how the two viruses might be influencing each other. The experts were worried that the viruses would collide next year.
Those worries weren't realized. The Centers for Disease Control and Prevention said that the cases of the flu remained low despite a weak effort to increase vaccinations.
The number of people hospitalized for the flu was the lowest on record, and only 0.2 percent of samples tested positive for the flu, compared with 30 percent in recent seasons.
The flu-free season is attributed to masks, social distancing and restricted movement, especially of young children and older adults, who are at the highest risk for severe flu. The figures were still lower than the prepandemic average when the numbers went up a year later.
Five million cases, two million medical visits, and less than 65,000 hospitalizations and 5,800 deaths related to the flu have been recorded by the nation so far this year.
The coronaviruses is more common than the flu, respiratory syncytial virus, rhinoviruses and common cold viruses.
The seasonal pattern of the R.S.V. was distorted by the Pandemic and it peaks in late December to February. It was low through all of 2020 and peaked in the summer of 2021.
The idea of an interplay between viruses first emerged in the 1960s, when the reduction in the number of respiratory infections was due to the vaccine for polio. The idea gained new ground in 2009, when Europe was poised for a surge in swine flu cases, but when schools reopened, rhinoviruses colds seemed to interrupt the flu epidemic.
A lot of people were interested in the idea of viral interference at that time. The rhinoviruses peak in October or November and then again in March, on either end of the flu season.
One team of researchers set out to study the role of an existing immune response in fighting the flu. They gave a weakened strain of the flu vaccine to children in the country because it would be unethical to spread the flu.
Interferons are a set of nonspecific defenders that are the first defense against viruses. The team found that children with high levels of interferon had less flu virus in their bodies than children with low levels.
Most of the viruses that the children were exposed to before getting the vaccine were rhinoviruses, according to Dr. Thushan de Silva.
Children who have more respiratory infections are less likely to be exposed to the coronaviruses. The flu can prevent coronaviruses in adults, according to Dr. Guy Boivin.
Recent studies show that co-infections of flu and the coronaviruses are rare, and that people with an active flu season were less likely to test positive for the coronaviruses.
It will be interesting to see if the rise in flu activity in Europe and North America leads to a decrease in the circulation of the disease.
Over the past decade, technology has made it possible to show the biological basis of this interference. A model of human airway tissue was used by Dr. Foxman's team to show how rhinoviruses can cause inflammation in the airway.
The protection against rhinoviruses can be fleeting, according to a team from the University of Glasgow who found similar results.
A bout with the coronaviruses did not seem to prevent infections with other viruses, according to Dr. Murcia. He said that it may have something to do with the coronaviruses ability to evade the immune system's initial defenses.
The coronaviruses tend to be less active than influenza. It seems that in a given population, it may matter which virus appears first.
Dr. de Silva and his colleagues have gathered additional data from Gambia, which had no restrictions that might have affected the viral patterns they were observing, indicating that rhinoviruses, influenza and the coronaviruses all peaked at different times between April 2020 and
He said that the data made him more convinced that interference could play a role.
The behavior of viruses can be influenced by a number of factors. The impact of viral interference is not likely to become apparent until the coronaviruses settles into a pattern.
rhinoviruses and flu have been around for a long time, noted Dr. Nasia Safdar, an expert on health-care associated at the University of Wisconsin.
It will become one of many that circulate when this one does, according to Dr. Safdar. She said that the patterns may not be readily apparent, but that some viruses may reduce the effects of others.
Some researchers think that the common-cold coronaviruses will become a seasonal winter infection that will coincide with the flu. The coronaviruses have shown themselves to be different.
One of the four common-cold coronaviruses is often seen as a co-infection with the other three.
Jeffrey Townsend, a biostatistician at the Yale School of Public Health who has studied the coronaviruses, said that it was an interesting example.
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