Earlier studies have shown that some immune-compromised people, such as those taking medication for other chronic disorders, have carried active Covid infections for a year or even longer.
It’s critical to identify these people, the researchers stress, not only to help them get treated for Covid, but also to conduct genomic surveillance of the SARS-CoV-2 viruses that they carry.
“Rather than evolving from transmission chains of acute Covid infections in hundreds of millions of people, our results show that the variants of concern come from rare cases when someone may have an active infection for months,” said Daniel Weissman, a professor of biology and physics at Emory University in the US.
Viruses like SARS-CoV-2 continuously evolve due to occasional mutations in the genetic code that may occur when they replicate.
“When a virus copies itself, it doesn’t always make perfect copies,” said Weissman in a study published in Frontiers in Virology.
The World Health Organisation (WHO) defines a SARS-CoV-2 variant of concern as one that is more likely to cause infections even in those who are vaccinated or in those who were previously infected.
“A key take-home message is that it is important to find these individuals who are chronically infected and provide support for them to recover,” said Mahan Ghafari, first author of the paper and a post-doctoral researcher at the University of Oxford.
“In many cases they may be asymptomatic and not even realise that they are infected with Covid although they are actively shedding the virus,” he added.
The World Health Organization dubbed three variants of concern alpha, beta and gamma.
The researchers have built a mechanistic, theoretical model to study the problem.
The resulting model rules out the theory that the variants of concern emerged from sustained transmission of acute infections and fully supports the theory that each variant evolved within a single individual with a chronic infection.
Delta emerged in India in late 2020, rapidly sweeping through that country and spreading around the world.
Delta subsided after omicron, which is not a descendent of the delta variant, emerged in South Africa in late 2021.
Omicron quickly became the dominant global variant of concern.
“Ideally, we’d like to eventually be able to quantify the timing at which new variants might emerge in the future,” Weissman said. “That has huge implications from a public health perspective.”
“Who knows what variant could be boiling up next from a chronically infected individual?” Ghafari said.