Can Covid-19 become an endemic virus?

A new article by researchers from the Columbia Mailman School (United States) Jeffrey Shaman and Marta Galanti explore the potential of the Covid-19 virus to become endemic, a regular feature that produces recurrent outbreaks in humans.

In his work, published in the magazine Science, identify crucial contributing factors, including reinfection risk, vaccine availability and efficacy, as well as potential seasonality and interactions with other viral infections that may modulate virus transmission.

Shaman is a leading authority on the modeling of infectious disease outbreaks such as SARS-CoV-2 and influenza. He was one of the first to recognize the importance of asymptomatic spread and the effectiveness of containment measures, and published widely cited estimates of the hypothetical lives saved if the confinement had occurred earlier.

His new work explores a scenario in which immunity to SARS-CoV-2, through infection or vaccine, decreases in a year, like the endemic betacoronavirus

Their new work explores a potential scenario in which immunity to SARS-CoV-2, either through infection or a vaccine, declines within one year, a rate similar to that seen for the endemic betacoronavirus that causes a mild respiratory illness. The result would be annual outbreaks of Covid-19.

On the other hand, if immunity to SARS-CoV-2 were more prolonged, perhaps through the protection provided by the immune response to infection with other endemic coronaviruses, what initially appears to be an elimination of COVID- 19 followed by a revival after just a few years. Other contributing factors are the availability and efficacy of the vaccine and the innate seasonality of the virus.

“If reinfection turns out to be common, and unless a highly effective vaccine is administered to most of the world’s population, SARS-CoV-2 will likely settle into a pattern of endemicity. It remains to be understood whether reinfections will be common, how often they will occur, how contagious the reinfected people will be, and whether the risk of serious clinical outcomes changes with subsequent infection, “the authors explain.

Among those infected with COVID-19, serological studies indicate that most infections, regardless of their severity, induce the development of some specific antibodies to SARS-CoV-2. However, it remains unclear whether those antibodies are by themselves sufficient to provide long-term immunity to prevent reinfection. For many viruses, insufficient immune response, waning immunity, or mutations that allow it to ‘escape’ immune detection can undermine or circumvent immunity and allow subsequent reinfection, although a previous infection can provide partial immunity and reduce the severity of symptoms.

The immune response to SARS-CoV-2 can be affected by the fact that someone is or has recently been infected with another virus. Many pre-pandemic studies show that infection with one virus can provide short-term protection, about a week, against a second infection. Other studies confirm that simultaneous respiratory virus infections are not associated with increased disease severity. Although some SARS-CoV-2 co-infections have been documented, including co-infections with influenza virus and respiratory syncytial virus, there are insufficient data to draw conclusions. At the population level, a major outbreak of seasonal flu could overwhelm hospitals already dealing with COVID-19.

Evidence suggests that COVID-19 could be more communicable during winter. Outside the tropics, many common respiratory viruses reappear seasonally at certain times of the year. Endemic coronaviruses (OC43, HKU1, NL63, 229E) exhibit seasonality in temperate flu-like regions.

Similarly, environmental conditions can also modulate the transmissibility of SARS-CoV-2, not enough to prevent transmission during the early stages of the pandemic, when immunity is generally low, but perhaps enough to favor seasonal transmission. Recurrent during winter in temperate regions, similar to flu, once immunity increases.

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