The researchers noted that illnesses like the common cold and the flu have become endemic in human populations, meaning everyone gets them every now and then, but for most people, they aren’t especially harmful.
To develop a better understanding of when and how COVID-19 might become endemic, researchers at the Yale School of Medicine in the US turned to rats, which, like humans, are also susceptible to coronaviruses.
By collecting data on coronavirus reinfection rates among rats, they were able to model the potential trajectory of COVID-19.
Animals like pigs and chickens live with endemic coronaviruses, too, and a key factor identified in the spread of animal and human coronaviruses alike is their tendency to evoke what is known as non-sterilising immunity, they said.
“It means that initially there is fairly good immunity, but relatively quickly that wanes,” said Caroline Zeiss, a professor at Yale School of Medicine and senior author of the study published in the journal PNAS on Tuesday.
“And so even if an animal or a person has been vaccinated or infected, they will likely become susceptible again,” said Zeiss.
Over the past two years, scientists have come to see that SARS-CoV-2 yields non-sterilising immunity. People who have been infected or vaccinated are still at risk of reinfection. So experts expect that the virus won’t go away any time soon.
Zeiss and her colleagues observed how a coronavirus similar to one that causes the common cold in humans was transmitted through rat populations.
The researchers modelled the exposure scenario to resemble human exposures in the US, where a portion of the population is vaccinated against COVID-19 and where people continue to face natural exposure to SARS-CoV-2.
They also reproduced the different types of exposure experienced by people in the US, with some animals exposed through close contact with an infected rat (high risk of infection) and others exposed by being placed in a cage once inhabited by an infected rat (low risk of infection).
Infected animals contracted an upper respiratory tract infection and then recovered. After three to four months, the rats were then re-organised and re-exposed to the virus.
The rates of reinfection showed that natural exposure yielded a mix of immunity levels, with those exposed to more virus through close contact having stronger immunity, and those placed in a contaminated cage having higher rates of reinfection.
The takeaway, Zeiss said, is that with natural infection, some individuals will develop better immunity than others.
People also need vaccination, which is offered through a set dose and generates predictable immunity.
However, the study showed, with both vaccination and natural exposure, the population accumulates broad immunity that pushes the virus towards endemic stability.
The team then used this data to inform mathematical models, finding that the median time it could take for SARS-CoV-2 to become endemic in the US is 1,437 days, or just under four years from the start of the pandemic in March 2020.
In this scenario, according to the model, 15.4 per cent of the population would be susceptible to infection at any given time after it reaches endemic phrase.
“The virus is constantly going to be circulating. So it will be important to keep more vulnerable groups in mind. We can’t assume that once we reach the endemic state that everybody is safe,” said Zeiss.
Four years is the median time predicted by the model, she said, so it could take even longer to reach the endemic stage.
This doesn’t take into account mutations that could make SARS-CoV-2 more harmful, the researchers said.
“Coronaviruses are very unpredictable, so there could be a mutation that makes it more pathogenic,” said Zeiss.
“The more likely scenario, though, is that we see an increase in transmissibility and probable decrease in pathogenicity,” she added.