How will Delta evolve? A scientist predicts what the future might look like

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The COVID-19 pandemic is a dramatic demonstration of evolution in action. Evolutionary theory explains much of what has already happened, predicts what will happen in the future, and suggests which management strategies are likely to be most effective.

For example, evolution explains why the Delta variant spreads faster than the original Wuhan strain. Explain what we can see with future variants. And it suggests how we can intensify public health measures to respond.

But Delta is not the end of the history of SARS-CoV-2, the virus that causes COVID-19. This is what evolutionary theory tells us happens.

Remind me again, how do viruses evolve?

Evolution is the result of random mutations (or errors) in the viral genome when it replicates. Some of these random mutations will be good for the virus, which will bring some advantage. Copies of these advantageous genes are more likely to survive in the next generation, through the process of natural selection.

New viral strains can also develop by recombination, when viruses acquire genes from other viruses or even from their hosts.

In general terms, we can expect the evolution to favor virus strains that result in a stronger epidemic curve, producing more cases more quickly, resulting in two predictions.

First, the virus should be more transmissible. An infected person is likely to infect more people; future versions of the virus will have a higher reproductive number or R.

Second, we can also expect evolution to reduce the time it takes between someone to become infected and to infect others (a shorter “serial interval”).

Both planned changes are clearly good news for the virus, but not for its host.

Aha, that explains Delta

This theory explains why Delta is now sweeping the world and replacing the original Wuhan strain.

The original Wuhan strain had an R value of 2-3, but Delta’s R value is about 5-6 (some researchers say that figure is even higher). Therefore, it is likely that someone infected with Delta will infect at least twice as many people as the original Wuhan strain.

There is also evidence that Delta has a much shorter series interval compared to the original Wuhan strain.

This may be related to a higher viral load (more copies of the virus) in someone infected with Delta compared to previous strains. This can allow Delta to be transmitted before infection.

A higher viral load can also make Delta more easily transmitted outdoors and then “passenger contact“.

Do vaccines affect the evolution of the virus?

We know that COVID-19 vaccines designed to protect against the original Wuhan strain work against Delta, but they are less effective. Evolutionary theory predicts this; viral variants that can elude vaccines have an evolutionary advantage.

So we can expect an arms race between vaccine developers and the virus, with vaccines trying to catch up with viral evolution. That is why we are likely to see ourselves with regular booster shots, designed to overcome these new variants, as we see with flu booster shots.

COVID-19 vaccines reduce the chances of transmitting the virus to others, but do not completely block transmission. And evolutionary theory gives us a cautionary tale.

There is a trade-off between a person’s transmissibility and degree of disease (virulence) with most disease-causing microorganisms. This is because you need some viral load to be able to transmit.

If vaccines are not 100% effective at blocking transmission, we can expect a shift in compensation toward increased virulence. In other words, a side effect of the fact that the virus can be transmitted from vaccinated people is, over time, that the theory predicts that it will become more harmful to unvaccinated people.

How about future variants?

In the short term, it is very likely that evolution will continue to “fine-tune” the virus:

  • its R value will continue to increase (more people will become infected in a generation)

  • the serial interval will decrease (people will become infected sooner)

  • variants will make vaccines less effective (vaccine evasion).

But we don’t know how far these changes could go and how quickly they can happen.

Some scientists think the virus may already be approaching “maximum physical state.” Still, you may still have some tricks up your sleeve.

The UK government’s Scientific Advisory Group for Emergencies (SAGE) has recently explored scenarios for the long-term evolution of the virus.

It is said that it is almost certain that there will be “antigenic drift,” the accumulation of small mutations that will lead to current vaccines being less effective, so boosters with modified vaccines will be essential.

He then says that the most dramatic changes in the virus (“antigenic displacement”), which can occur through recombination with other human coronaviruses, are a “realistic possibility.” This would require more important reengineering of vaccines.

SAGE also thinks there is a realistic possibility of a “reverse zoonosis,” leading to a virus that may be more pathogenic (harmful) to humans or capable of evading existing vaccines. This would be a scenario in which SARS-CoV-2 infects animals before re-crossing into humans. We have already seen SARS-CoV-2 infect mink, felines and rodents.

Will the virus become more deadly?

Versions of the virus that make your host very sick (they are highly virulent) are usually selected against. This is because people would be more likely to die or be isolated, which will decrease the chance of the virus being transmitted to others.

SAGE believes that this process is unlikely to make the virus less virulent in the short term, but this is a realistic possibility in the long term. However, SAGE says there is a realistic possibility that more virulent strains may develop through recombination (something other coronaviruses are known to do).

So the answer to this critical question is that we don’t really know if the virus will become more deadly over time. But we can’t expect the virus to become magically harmless.

Will humans evolve to catch up?

Unfortunately, the answer is “no.” Humans do not reproduce fast enough and accumulate favorable mutations fast enough to keep us ahead of the virus.

The virus also does not kill most people it infects. And in countries with well-resourced health systems, it doesn’t kill many people of reproductive age. Therefore, there is no “selection pressure” for humans to mutate favorably to stay ahead of the virus.

What about future pandemics?

Finally, evolutionary theory has a warning about future pandemics.

A gene mutation that allows a virus of a relatively rare and dark species (such as a bat) to access the most common and widely distributed species of large animals on the planet, humans, will be strongly selected.

Therefore, we can expect future pandemics when animal viruses spread to humans, just as they did in the past. The conversation

Hamish McCallum, director of the Center for Planetary Health and Food Safety at Griffith University, Griffith University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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