The novel coronavirus has not significantly changed since it first jumped from what was likely bats to humans, researchers found in a study published in the open access PLOS Biology journal.
The researchers found that SARS-CoV-2 likely did not need to adapt much, if at all, in bats to jump to humans and evolved "relatively slowly" in the first 11 months of the pandemic from December 2019 to October 2020.
As more people have become immune and the vaccination campaign began, there have been indications of increased selective pressure in more recent samples of the virus since late 2020, the researchers found, including the British and South African variants. The variants seem to be associated with evolution in relation to host immunity and/or chronic infections in probably immunocompromised patients, instead of the slow rate of evolution that predominated until October.
The researchers added that the fact that the virus can easily transmit to other animals, including pangolins, minks and cats, among others, is "strongly indicative" that this generalist property evolved in the bat reservoir species and was not due to adaptation to human-human transmission.
It is still unclear how long it was between the first animal-to-human transmission and the first sequencing of the virus. While important changes may have occurred within that time, the researchers stated that such changes should theoretically be detected in an analysis of the Sarbecovirus in bats, which is believed to be the progenitor of SARS-CoV-2, but that they had failed to find evidence of diversifying selection on the branch leading up to the emergence of SARS-CoV-2. The researchers added that this indicates that the adaptations that allowed the virus to efficiently replicate in humans and other mammals likely did not occur in the unknown lineage of the virus.
The researchers additionally said that the apparent generalist nature of bat viruses similar to SARS-CoV-2 suggests that there are species of wild mammals infected with similar viruses that are yet to be sampled, pointing to serological studies of communities in China that have contact with bats that indicate the occurrence of incidental and dead-end spillover of SARS-like viruses to humans.
The study found that, due to the high diversity and generalist nature of Sarbecoviruses, a future jump from animals to humans, including possibly a recombination event of SARS-CoV-2 with another similar virus, is possible and could create a "SARS-CoV-3" that could evade either natural or vaccine-acquired immunity.
"What's been so surprising is just how transmissible SARS-CoV-2 has been from the outset. Usually, viruses that jump to a new host species take some time to acquire adaptations to be as capable as SARS-CoV-2 at spreading, and most never make it past that stage, resulting in dead-end spillovers or localized outbreaks," said Sergei L. Kosakovsky Pond, of the Temple University in the US, according to The Weather Channel.
The researchers stressed that surveillance of Sarbecoviruses must be dramatically increased to monitor for future SARS-CoV emergence in humans.