Covid-19: Why we can't yet count on a milder, Delta-beating Omicron

The world is facing the prospect of yet another new Covid-19 threat in the freshly designated "variant of concern", Omicron.

While scientists scramble to get a better handle on the pandemic's new "variant of concern", Omicron, some commentators are already sounding optimism that it might replace Delta with a much less harmful strain. But as University of Auckland senior lecturer Dr David Welch tells science reporter Jamie Morton, we shouldn't get our hopes up just yet.

Why are we already seeing commentary that Omicron may be less virulent than Delta?

I'm not sure where this came from, as the variant has only been known for about a week so there has not been enough time to assess its effects.

Comments from Dr Angelique Coetzee, chairwoman of the South African Medical Association, that cases she has seen were mild, seem to have started this discussion – although she did indicate that she was still looking at the data and her perception may change.

The things we are interested in new variants are changes in transmissibility, virulence, or immune escape and variants that are highly transmissible tend to take over and become dominant.

When a large part of a population has immunity, immune escape of a variant becomes important to its success, as it opens up a new class of potential hosts it can infect and so become dominant.

Virulence, on the other hand, can go up or down and isn't immediately related to the success or failure of a variant to spread.

At this stage, what evidence is there to show that Omicron is not only less virulent, but may also be able to outcompete Delta? Why would this be a difficult feat for any variant?

The question of whether one variant out-competes another is context-dependent.

At the start of the epidemic when there is no population immunity, any variant that gets a foothold and spreads faster will become dominant.

Now we are reaching a situation where populations have high levels of immunity, either through previous exposure to Sars-CoV-2 or via vaccination.

The excess mortality in South Africa over the course of the pandemic suggests that about 0.4 per cent of the population has died from Covid-19, which implies that most of the population has been exposed and has some type of immunity.

For any variant to spread widely within that population, it needs to be able to infect people who have some immunity.

A woman is tested for Covid-19 in Abuja, Nigeria, as southern African nations brace for a spike in cases of the new Omicron variant of the Sars-CoV-2 virus. Photo / AP

Delta is mainly just highly transmissible - but not very good at evading immune defences.

So, if Omicron comes along and is more able to get around immunity, then it will spread faster in a population with high immunity than Delta will.

That doesn't mean that it would spread faster than Delta in a naive population.

As for virulence, this is also dependent on what immunity we have.

Breakthrough infections tend to carry a much smaller risk of severe disease or death.

So, if Omicron were spreading mainly in people who had previously been infected or had been vaccinated, we'd expect them to only have relatively mild disease.

To judge its base level of virulence, we would need to see outcomes for people with no previous immunity or do a careful comparison to account for rates of waning immunity and other population factors.

If Omicron does prove to have these traits - what might be the upside for public health?

Obviously, if you have a choice of which variant to be infected with, the less virulent one would be preferable to the more virulent one.

So if Omicron can spread more rapidly, it could displace other variants in the way that Delta has, and if it has that lower virulence, that would be an advantage.

That said, Delta causes very serious disease in people who have no immunity.

A small reduction in virulence would be welcome - but would not change much in our overall response.

And if that mild reduction in virulence came with escape from current immunity, it would certainly not be a good thing in the short to medium term as we'd see higher numbers of infections and people being infected who otherwise would not be.

There is also nothing to stop a less virulent strain mutating to be more virulent.

But so far, there is nothing that I've seen in the genomics of Omicron that would suggest it is any less virulent than other strains.

What are the prospects that Omicron may also just fizzle out, as other variants have against Delta?

Given how early things still are, there is a decent chance that Omicron will not amount to much.

It may be that its spread is fairly local and remains so, or that it simply doesn't out-compete Delta so that Delta remains dominant.

Given what we know about the nature of this pandemic, with vast numbers of people still unvaccinated and susceptible, and that viruses will always adapt to their environments, what long-term assumptions might we make at this point?

It is very hard to predict future directions of evolution.

The new variants to date have made large evolutionary jumps, probably in immune-compromised people who have harboured infections for many weeks.

We are starting to understand what various individual mutations do but how they behave in combination with others is hard to predict and we are likely to be surprised at least once or twice more with the appearance of new variants.

We need to keep up our focus on getting as much of the world's population properly vaccinated and protected against current variants so that the virus has fewer and fewer chances to evolve further.