Category: The Big Picture

Almost a year before Omicron appeared, Paul Ewald, an evolutionary biologist, predicted that the COVID virus would evolve in the direction of a highly contagious but less deadly variant.

Omicron is certainly proving more contagious than earlier variants.

Source: Bloomberg

But the hospitalization rate — for those above 30 years of age at least — is far lower than the earlier Delta variant.

Source: Bloomberg

The higher rate of hospitalizations among children under 9 may not be as concerning as it first seems:

Professor Mignon McCulloch, who is the Chairperson of the SA Paediatric Association, has urged people not to panic during her appearance on CapeTalk. Around 20% of new admissions for COVID-19 in Tshwane have been in children aged nine or under, whereas similar spikes have not been seen in any other age groups.

However, it doesn’t mean that Omicron is more dangerous, or even more deadly for children. According to McCulloch, it’s very likely that the children admitted to hospital are actually there for reasons other than the virus – and it is ‘coincidental’ that they have also tested positive for COVID-19 when checked by hospital staff. (The South African)

Evolutionary path

Earlier, researchers at the University of Exeter found that the most virulent variants of a pathogen may transmit the fastest, but tend to lose out in evolutionary terms to less virulent strains:

Pathogens have a single evolutionary goal – to produce more of themselves. “Virulence will evolve towards a level that optimizes their ability to transmit,” Dr Bonneaud said.

If the pathogen meets resistance to transmission — in the form of a recovered and immune or vaccinated host, or social distancing — then highly virulent forms die out with their host, and natural selection favors less virulent forms.

If there is no such resistance, the pathogen can kill its existing host at no evolutionary cost and will remain highly virulent…..

Paul Ewald, at the University of Louisville in Kentucky, said humanity had drawn a short straw with Sars-CoV-2 because it was both highly virulent and highly transmissible when it emerged.

Over time it is likely to reduce its virulence – in fact, that may already be happening, as reflected in falling mortality rates.

“I would expect it to evolve to a virulence that is very much like [seasonal] influenza,” Prof Ewald said in November last year.

“And containment measures, properly implemented, should accelerate that process.” (The Guardian)

Host mobility

Paul Ewald’s research focuses on the dependence of disease organisms on the mobility of their host for transmission. A patient (host) who is bed-ridden or dies is less likely to transmit the infection — unless the disease has another means of spreading other than personal contact/close proximity. A host with mild symptoms is far more likely to move around in the community and spread the disease.

If a disease organism is very dependent on healthy hosts moving around [and] contacting susceptible hosts, then we expect natural selection to favor extreme mildness in those disease organisms. If, however, the disease organism is not dependent on host mobility — for example, if the disease organism is transmitted by mosquito, or contaminated water, or because it’s durable in the external environment — then we expect that natural selection will favor high levels of harmfulness in those disease organisms…..

When we look at the population of disease organisms in any given area, we see both mild and harmful strains….. all we need to do is tip the competitive balance in favor of those mild strains.

We can look at the experience in South America and Central America as a kind of a natural experiment that allows us to evaluate these ideas. In 1991, cholera came into Peru and then quickly, within a couple of years, spread all throughout South and Central America. Some countries had clean water supplies, and other countries had contaminated water supplies. What we find is that when the organism invaded countries with clean water supplies, the organism dropped in its harmfulness.

In contrast, the organisms that invaded countries with poor water supplies — countries like Ecuador — evolved increased harmfulness over time. They’ve actually become more toxigenic. (Paul Ewald: Infectious Disease and the Evolution of Virulence)

Conclusion

We can use evolution to encourage diseases to evolve into milder forms that are not as harmful and also create resistance to more virulent strains. Use of vaccines, handwashing, masks and social distancing help to restrict more virulent forms of the virus and encourage milder versions like Omicron to take over, developing herd immunity.

Opening up populations to a pandemic — in the misguided hope of creating herd immunity — are likely to have the opposite effect. Unhindered transmission would encourage evolution of more virulent strains, with far higher hospitalization and death rates.

Acknowledgement

Hat tip to Macrobusiness for the images