Scientists debate the theory that viruses always get more infectious but less severe over time

January 19, 2022, 1:38 PM UTC

As Omicron sweeps across the U.S. and Europe, an argument is often cited along with it—that viruses tend to evolve into something more infectious but less virulent over time.

The basis of this argument is founded upon the belief that viruses “want” to ensure their survival and in order to do so, it is inefficient for them to kill their human hosts. The theory argues that it is easier for viruses to survive and propagate in a living walking person than a person who is dead.

The theory, called the virulence-transmissibility evolutionary trade-off, “is a natural consequence of virus-host coevolutionary arms race,” explained Shiu-Wan Chan, a lecturer and principal investigator in molecular virology at the University of Manchester who is an advocate of the virulence-transmissibility trade-off. 

But many scientists hold that the idea is as unproven as it is seductive in the case of COVID-19. Indeed, it is easy to make anecdotal assumptions about the patterns of the coronavirus in the face of the sudden rise and then fall in cases of the Omicron variant—which has proved to be far less deadly than the previous Delta strain—but that does not make those assumptions true.

“In general terms there used to be this idea that any pathogen would evolve so as to be less pathogenic because you don’t want to harm your host. But actually that’s a load of nonsense,” Malcolm Bennett, professor of veterinary medicine and science at the University of Nottingham, told Fortune.

Better immunity behind lower virulence

Many researchers argue that the reason Omicron is less virulent is not because of the changing nature of the virus, but rather because our immune systems are more prepared.

“These variants emerged at a time when we had a good deal of immunity to SARS-CoV-2 in certain countries,” Andrew Pekosz, a professor of microbiology at Johns Hopkins University Bloomberg School of Public Health, told ABC News

Vaccinated individuals appear in general to be less likely to have life-threatening symptoms when infected, and a South African study published in December found that Omicron was about 80% less likely than Delta to lead to hospitalization and also 30% less likely to lead to severe disease in those who have been hospitalized. 

In a statement from the Science Media Centre in December, Paul Hunter, a professor in medicine at the University of East Anglia in England, said, “Even though cases of Omicron were less likely to end up in hospital than cases of Delta, it is not possible to say whether this is due to inherent differences in virulence or whether this is due to higher population immunity in November compared to earlier in the year.” 

An evolutionary arms race

But even if the coronavirus is losing virulence as it mutates, this can change at any time, says Alfredo Corell, immunologist at the University of Valladolid in Spain. He notes that viruses like the one that causes COVID-19 can mutate spontaneously and can become more transmissible as it did with Omicron.

“Let’s not forget that a new mutation can arise that is more lethal and can occur as long as we do not have 100% of the population vaccinated,” he told the Spanish online newspaper El Español.

On whether viruses get less pathogenic over time, Lawrence Young, a virologist and professor of molecular oncology at Warwick Medical School, says we “can’t bank” on the evolution toward something more infectious but less deadly. “It’s a possibility, but I think it’s a bit naive to think that is what’s going to happen with coronavirus at this stage,” he said.

But Shiu-Wan Chan, the virulence-transmissibility trade-off advocate, notes that evolutionarily, a virus will mutate to become less virulent so the host is healthy and alive enough to move about and transmit the virus. “There is a natural selection that the virus will become less virulent to become more transmissible,” she said, noting, “A virus is a parasite. It requires a host to survive. The worst thing for the virus is to kill off its host.”

Bennett disagrees: “A general statement that pathogens evolve to be less pathogenic or more pathogenic is just silly. The true answer is, ‘It depends.’” He argues there are far too many evolutionary factors in specific communities where a mutation might propagate to make any blanket statements on the transmissibility or the virulence of a future mutation.

Will history repeat itself?

There have been instances, however, in which viruses have gotten less virulent in the past. The 1918 Spanish flu, the second-deadliest pandemic after the bubonic plague, became much less deadly and caused only ordinary seasonal flu by 1920. Similarly, with the 1889 Russian flu, the virus ran through the population and then lessened in its pathogenicity by 1890.

“It happened 100 years ago, and it lasted for two years. And they went from pandemic to endemic,” said Shiu-Wan Chan, in reference to the decline of pandemics in the past. 

But this can be put down to a number of evolutionary factors, argues Bennett: “You can look at examples from the past and you can try and work out from them why these things happen. But what you learn from that is each one of those examples has a different set of circumstances and context in which what happened, happened.”

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