COVID-19 and influenza: is a double pandemic possible and simultaneously?

Why was the emergence of the coronavirus at the end of 2019 months or more than a year of absence of other respiratory viruses such as influenza or respiratory syncytial virus?

Virologist scientists are still comparing the figures of the decline in infections of other diseases while the coronavirus predominated and are looking for answers to understand the behavior of viruses, especially that of SARS-CoV-2.

An intriguing theory may help explain why influenza and COVID-19 viruses never predominated simultaneously: the so-called Twindemia in English (Twin is twin and demia comes from illness). Several specialists argue that it was not only masks, social distancing or other pandemic restrictions that caused the flu and other respiratory viruses to disappear while the coronavirus reigned and to re-emerge as it receded.

According to several experts, exposure to a respiratory virus can put the body's immune defenses on high alert, preventing other intruders from accessing the airways. This biological phenomenon, called viral interference, can limit the number of respiratory viruses circulating in a region at any given time.

“My intuition, and my feeling based on our recent research, is that viral interference is real. I don't think we're going to see the peak of flu and coronavirus at the same time,” explained Dr. Ellen Foxman, an immunologist at the Yale Medical School.

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

Still, he warned, “the health care system may become overloaded long before the upper limit of circulation is reached, as the Ómicron wave has shown.” Viral interference can help explain the patterns of infection seen in large populations, including those that may arise as coronavirus becomes endemic. But research is in its infancy and scientists are still struggling to understand how it works.

“During the first two years of the pandemic, large-scale COVID-19 prevention measures such as wearing a mask, lockdowns, distancing, ventilation, among others, helped reduce the transmission of the influenza virus and other respiratory viruses around the world. But since the end of last year, after the advance of vaccination against COVID-19, there was more intense citizen mobility and there was a return in the circulation of the influenza virus,” Leda Guzzi, of the Communication Commission of the Argentine Society of Infectology, explained to Infobae.

According to the infectiologist Pablo Bonvehí, head of Infectology at CEMIC, “the increase in influenza cases is probably explained by multiple factors. In other South American countries, such as Chile, Peru, Colombia and Brazil, cases mainly with the subtype Influenza A H3N2″ also rose.

The expert, a member of the Vaccine Commission of the Argentine Society of Infectious Diseases and the Scientific Committee of the Vaccinate Foundation, added: “This circulation of the influenza virus was not normal so early. Nor was it normal that the virus did not circulate in 2020 and 2021 when the coronavirus circulation predominated. Now the subtype Influenza A H3N2 began to circulate, which was predominant in the Northern Hemisphere. Surely this change is related to the fact that there are no more restrictions on travel abroad. Having not been in contact with the flu virus for the past two years, people are more susceptible to acquiring the infection. There are also more cases of bronchiolitis and those affected by other respiratory viruses.”

Since 2020, after the start of the coronavirus pandemic, there were fewer cases of seasonal flu than in previous years. “Since the start of the COVID-19 pandemic, in Argentina, influenza virus activity has remained low,” the official alert said. However, from the first week of last December until now, an increase in the number of cases of influenza, mainly Influenza A H3N2, has been detected.

During the first six weeks of this year (that is, January and the first half of February), 166 cases of influenza were recorded in the National Health Surveillance System of the National Health Portfolio. Within this total, virus subtypes were analyzed in 100 cases and it was identified that 99% corresponded to the subtype Influenza A H3N2 and 1% to the subtype “Influenza A H1N1″ (which was the predominant one in the 2009 pandemic).

There are at least 100 viruses that can cause the common cold, but only four that cause seasonal flu. Many people who get a cold assume they have the flu, but experts always say the same thing about how to tell the difference: “Flu makes you feel like a truck hit you.” The fever, discomfort, and headaches of a severe case of influenza are generally worse than a case of respiratory syncytial virus, rhinovirus, or any of the common cold viruses.

Many experts attributed the flu-free season to masks, social distancing, and restriction of movement, especially for young children and older adults, who are at the highest risk of getting a serious flu. Influenza figures rose a year later, in the 2021-2022 season, when many countries had dispensed with restrictions, but the numbers were still lower than the pre-pandemic average.

The notion that there is a kind of interaction between viruses first emerged in the 1960s, when poliomyelitis vaccines, containing weakened polioviruses, significantly reduced the number of respiratory infections. The idea gained ground in 2009: Europe seemed ready for a sudden surge in swine flu cases by the end of that summer, but when schools reopened, rhinovirus colds seemed to somehow disrupt the epidemic of flu.

“That led many people at the time to speculate about this idea of viral interference,” Dr. Foxman said. Even in a typical year, rhinovirus peaks in October or November and then again in March, at either end of the flu season. Last year, a team of researchers set out to study the role of an existing immune response in defending against the influenza 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. Virus infection triggers a complex cascade of immune responses, but the first defense comes from a set of non-specific defenders called interferons. Children who already had high levels of interferon ended up with much less influenza virus in their bodies than those with lower levels of interferon, the team found.

The findings suggested that previous viral infections prepared children's immune systems to fight the influenza virus. “Most of the viruses we saw in these children before they were given the vaccine were rhinoviruses,” said Dr. Thushan 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 have more respiratory infections than adults, seem less likely to be infected with the coronavirus. The flu can also prevent coronavirus infections in adults, said Dr. Guy Boivin, a virologist and infectious disease specialist at Laval University in Canada.

Recent studies have shown that coinfections with influenza and coronavirus are rare, and people with an active influenza infection were nearly 60 percent less likely to test positive for coronavirus, he added. RSV, rhinovirus and influenza have coexisted for years, noted Dr. Nasia Safdar, an expert in health-related infections at the University of Wisconsin-Madison. “Eventually, that's what will happen to this one too: it will become one of the many that will circulate,” said Dr. Safdar about the coronavirus. Some viruses can attenuate the effects of others, he said, but the patterns may not be obvious.

By looking at coronaviruses from the common cold, some researchers have predicted that SARS-CoV-2 will become a seasonal winter infection that could well coincide with the flu. But the pandemic coronavirus has already shown itself differently than its cousins. For example, it is rarely seen in co-infections, while one of the four coronaviruses of the common cold is frequently seen as a co-infection with the other three.

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