A new article by researchers from the Mailman School of Columbia (United States) Jeffrey Shaman and Marta Galanti explores 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 the risk of reinfection, availability and efficacy of the vaccine, as well as the potential seasonality and interactions with other viral infections that can modulate the transmission of the virus.

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

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

On the other hand, if immunity to SARS-CoV-2 was longer, perhaps through the protection provided by the immune response to infection with other endemic coronaviruses, one could experience what initially appears to be a Covid-19 elimination followed by a resurgence after of a few years. Other contributing factors are the availability and efficacy of the vaccine and the innate seasonality of the virus.

“If there 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 in a endemicity pattern. It remains to be understood whether reinfections will be common, how often they will occur, how contagious the reinfected individuals 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 if those antibodies are in themselves sufficient to provide a long term immunity to prevent reinfection. For many viruses, insufficient immune response, waning immunity, or mutations that allow it to ‘escape’ immunological detection can undermine or circumvent immunity and allowing subsequent reinfection, although a previous infection may 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 been recently 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, including co-infections with influenza virus and respiratory syncytial virus, have been documented, there are insufficient data to draw conclusions. At the population level, a major outbreak of seasonal flu might overburden hospitals who are already dealing with Covid-19.

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

Similarly, environmental conditions they 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 recurrent seasonal transmission during winter in temperate, flu-like regions, once immunity increases.