The heavily mutated Omicron variant has sparked concerns that it may evade the protection of vaccines – but there have also been suggestions the new version of the coronavirus may be less virulent.
Scientists currently have little data to go on, so hopes the new variant is not as likely to cause serious illness rests largely on reports from doctors in South Africa who treated early Omicron cases.
It will probably be weeks before researchers fully understand Omicron’s pathogenicity, transmissibility and ability to cause illness, even in vaccinated people.
But if it does represent a more mild form of the coronavirus, it could mean that the pandemic follows a pattern seen elsewhere, in which infections become endemic but less severe over time.
“If you think about the extremes, a virus that kills 90 percent of the people it infected would very quickly run out of people. It won’t have anywhere to go,” said Prof Ian Jones, a virologist at the University of Reading in the UK.
“On the other hand, a virus that you hardly notice you have, but was transmitted easily, would be transmitted over time. The tendency is for viruses to lose virulence and cause less severe disease. They become endemic.”
The idea that pathogens tend to become milder as time goes on, sometimes called the law of declining virulence, is often attributed to Theobald Smith, a celebrated American research scientist of the 19th and 20th centuries.
According to the theory, viruses spread more effectively if they replicate quickly but do not cause severe disease, at least in the early stages of infection. This is because if the hosts remain well, they can mix with others and spread the pathogen.
Several disease outbreaks appear to tie in with Mr Smith’s theory.
Among those often quoted are the “Spanish flu” pandemic of 1918, which is thought to have killed tens of millions of people around the globe. Some have suggested the virus gradually became less dangerous as the pandemic continued into 1920, when it ended.
Another example concerns a coronavirus that infects people, OC43. This may have caused the “Russian flu” pandemic of the late 19th century, which is blamed for at least one million deaths. Today, OC43 is much less deadly, as it is one of several viruses that causes the common cold.
While Mr Smith’s idea gained wide acceptance, in recent decades researchers have put forward other models that suggest a more varied range of scenarios, in which evolution towards reduced pathogenicity is not inevitable.
Among the ideas is that if pathogens are durable outside their host, they may be successful when they are both highly transmissible and virulent. Durability allows them to survive outside of their host for extended periods before finding a new host.
The myxoma virus, the cause of myxomatosis in rabbits, while often cited as a case where virulence has declined over time, may actually represent a more mixed picture.
Introduced to Australia, France and the UK in the 1950s, it initially caused devastation among rabbit populations, although as time continued, more rabbits survived infection.
This has been put down to both reduced virulence from the virus and increased resistance in the rabbit populations.
Researchers at the University of Oxford who published a study in 2019 tracing the evolution of rabbits in relation to myxomatosis noted, however, that more virulent strains of the virus have emerged in recent times. There is, they said, an ongoing “arms race” between the virus and the rabbit population.
Evidence so far with the coronavirus has suggested to some scientists that it is not becoming less harmful.
For example, as well as being highly transmissible, the Delta variant has been associated with a higher rate of hospital admissions (and transmissibility) than the Alpha variant, which emerged earlier.
Because the coronavirus spreads most readily in the early stages of disease, before severe symptoms develop, there may be little evolutionary pressure for it to become less virulent. Even if people subsequently fall severely ill and even die, the hypothesis suggests, the virus will already have spread.
According to Dr Andrew Freedman, an infectious diseases specialist at Cardiff University, in terms of evolution “it’s not inevitable” that the coronavirus will cause milder infections as time goes on.
“Mutations can make it more transmissible and virulent in the worst-case scenario,” he said.
If the coronavirus becomes endemic and milder over time, vaccination and natural infection (which confers immunity) are, he said, likely to be the key factors.
Prof Jones also emphasised the importance of widespread immunity in ending the pandemic.
“What will happen over time, the edge of the disease will fade further, as long as immunity is maintained,” he said.
In any case, Prof Jones said that, for the moment, it was unclear whether the Omicron variant causes less severe infection.
While doctors in South Africa have highlighted that cases have been mild, “exact data” has yet to be collected.
“Until we see a huge number of infections, I don’t see it’s a realistic assessment, especially in South Africa, where quite a lot of cases are not reported accurately,” he said.