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Infectious Diseases

Updated 16 April 2020

Annual 'Covid-19 season' may be here to stay, scientists predict

According to a new Harvard modelling study, Covid-19 may turn out to be an ongoing fact of life, with the duration of human immunity determining exactly how often the virus returns.

Covid-19 is likely to be around for years to come, haunting humans as either a yearly flu-like illness or as a virus that occasionally resurfaces following years of dormancy, a new Harvard modelling study argues.

It's unlikely that Covid-19 will go the way of its closest cousin, SARS (severe acute respiratory syndrome), which was eradicated by an intense public health effort following a brief pandemic, the researchers said.

Instead, Covid-19 is expected to be an ongoing fact of life, with the duration of human immunity determining exactly how often the virus returns.

If immunity to the Covid-19 coronavirus is not permanent, the virus will likely enter into regular circulation – just like the influenza virus or the beta coronaviruses responsible for the common cold, the model showed.

Several years of physical distancing

"It does seem likely that, under a wide range of parameter values, SARS-CoV-2 [cause of Covid-19] will continue to circulate as a seasonal wintertime virus," said lead researcher Stephen Kissler, a research fellow of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, in Boston.

In that case, it might be necessary to have several years of intermittent physical distancing to fully introduce the entire human population to the Covid-19 virus without overwhelming the healthcare system, the researchers concluded.

That scenario would require physical distancing rules to be relaxed in the summer, when transmission of the virus would be somewhat reduced, said senior researcher Marc Lipsitch, a professor of epidemiology with Harvard T.H. Chan School of Public Health.

That way, a limited number of people could be exposed to the coronavirus and help build up herd immunity, which now is essentially non-existent because it's a novel virus, Lipsitch said.

"By permitting periods of transmission that reach higher prevalence than otherwise would be possible, they allow an accelerated acquisition of herd immunity during the 'off' periods of physical distancing," Lipsitch said.

For this study, the Harvard researchers evaluated the transmission patterns of two beta coronaviruses that are the second most common cause of colds. These two viruses are close relatives of the Covid-19 virus, said study co-author Christine Renata Tedijanto, a doctoral student of infectious disease epidemiology at the Harvard T.H. Chan School of Public Health.

Virus will wax and wane

The model then incorporated the Covid-19 coronavirus, using the other two viruses as a means of patterning the behaviour of the third.

The Harvard model found that the novel coronavirus probably will be able to proliferate at any time of the year, the researchers said. It's also very likely that the virus will wax and wane with the seasons.

"It is a consistent finding in this model that, absent control interventions, even in the summer we would expect increasing numbers of cases," Lipsitch said. "Unlike those seasonal coronaviruses, most of the population remains susceptible, and that overwhelms the modest declines in transmission in the summer."

The researchers did not rule out the possibility that coronavirus might disappear for a number of years, if human immune systems adapt both to it and to the coronaviruses that cause the common cold.

It's also conceivable that the common cold coronaviruses might provide some mild cross-immunity against Covid-19, the study team noted. In that scenario, a follow-up wave of Covid-19 cases could be delayed as much as three years, assuming that immunity against the novel coronavirus lasts two years.

Herd immunity

However, it's more likely that Covid-19 will continue to infect people year round, with the virus springing forth every time physical distancing rules are relaxed, the researchers explained.

In that case, the human immune system will determine the best strategy for adapting to the virus, Lipsitch said.

"If intermittent physical distancing is the approach that is chosen, it may be necessary to do it for several years, which is obviously a very long time," Lipsitch said.

"That could get better if, as some early indications suggest, there is more herd immunity in the population than we believe, indicating that each case we know about is actually generating more immunity through mildly ill cases or unobserved cases," he continued.

"On the other hand, there are indications coming out that not every case of Covid-19 infection, even confirmed cases, generates a robust immune response, which would mean the build-up of herd immunity is slower than is anticipated here," Lipsitch concluded. "There are many uncertainties about the scale of this."

Increased transmission during winter

A comprehensive programme of testing for both the virus and its antibodies will be needed to carefully guide the public health response to Covid-19, the researchers said. That way, enough people can be exposed to improve herd immunity without so many that hospitals are flooded with cases.

To that end, hospitals might need to beef up their emergency departments and intensive care units so they are ready to deal with cases caused by the relaxation of physical distancing in the summer, as well as increasing infections as the seasonal virus increases in transmission during the winter, the researchers added.

The new study was published online on 14 April in the journal Science.

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