Why does the Delta variant spread faster than the original Wuhan strain?This is what theory teaches us

For example, evolution explains why delta variants spread faster than the original Wuhan strain. It explains what we may see in future variants. And it suggests how we can strengthen public health measures to respond.

However, Delta is not the end of the story of SARS-CoV-2, the virus that causes COVID-19. This is what evolution tells us to happen next.

Evolution is the result of random mutations (or errors) in the viral genome during replication. Some of these random mutations are good for the virus and offer some benefits. Copies of these lucrative genes are more likely to survive the next generation through the process of natural selection.

When a virus acquires genes from other viruses, or even from their hosts, new virus strains can also develop via recombination.

Generally speaking, evolution supports virus strains that result in a steeper epidemic curve, generates more cases more quickly, and can be expected to lead to two predictions.

First, the virus should be more contagious. One infected person can infect more people. Future versions of the virus will have higher reproductive or R numbers.

Second, evolution can also be expected to reduce the time between someone getting infected and another getting infected (shortening the “serial interval”).

Both of these predicted changes are clearly good news for the virus, but not for its host.

Oh, this theory it explains Delta explains why Delta is now sweeping the world and replacing the original Wuhan stock.

The original Wuhan strain had an R-value of 2-3, but the Delta has an R-value of about 5-6 (some researchers say this is even higher). Therefore, a person infected with Delta can infect at least twice as many people as the original Wuhan strain.

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

This may be related to the higher viral load (more copies of the virus) in people infected with Delta compared to previous strains. This allows Delta to become infected soon after infection.

High viral load can make Delta more susceptible to infection outdoors and after “contact with the fleet.”

Do vaccines affect the evolution of the virus? We know that the COVID-19 vaccine designed to protect against the original Wuhan strain works against Delta, but it is less effective. Evolution predicts this. Viral variants that can evade vaccines have evolutionary benefits.

Therefore, an arms race between vaccine developers and viruses is expected, and vaccines will try to keep up with the evolution of the virus. That’s why you’re likely to see regular booster shots designed to overcome these new variants, as well as flu booster shots.

The COVID-19 vaccine reduces the chances of the virus infecting others, but it does not completely stop the infection. And evolution gives us a tale.

There is a trade-off between infectivity and how humans get sick (pathogenic) with the microorganisms that cause most diseases. This is because a certain viral load is required to be able to send.

If the vaccine is not 100% effective in stopping the infection, a trade-off shift to higher toxicity can be expected. In other words, the theory predicts that the side effects of the virus, which can be transmitted from vaccinated people, will become more harmful to unvaccinated people over time.

What about future variants? In the short term, evolution is likely to continue to “fine-tune” the virus. Its R value continues to increase (more people are infected in one generation), the serial interval is reduced (people are infected faster), and variants reduce the effectiveness of the vaccine (vaccine avoidance).

But I don’t know how far these changes will go and how fast this will happen.

Some scientists believe the virus may already be approaching “peak fitness,” but there are still some tricks.

The UK Government’s Emergency Science Advisory Group (SAGE) recently investigated scenarios of long-term evolution of the virus.

It is almost certain that “antigenic drift” will occur, and it is said that booster immunization with a modified vaccine is essential because the accumulation of small mutations reduces the effectiveness of current vaccines.

He then states that the more dramatic changes in the virus (“antigenic shift”) that can occur due to recombination with other human coronaviruses are “realistic possibilities”. This will require a more substantive redesign of the vaccine.

SAGE also believes that there is a real potential for “zoonoses” and that the virus may be more pathogenic (harmful) to humans or evade existing vaccines. I have. This is a scenario in which SARS-CoV-2 infects animals. Before returning to humans, I have seen SARS-CoV-2 infect mink, cats and rodents.

Will the virus be more deadly? The version of the virus that makes the host very ill (very virulent) is generally chosen. This is because people are more likely to die or become isolated, and the virus is less likely to infect others.

SAGE believes that this process is unlikely to reduce virus toxicity in the short term, but this is a realistic possibility in the long term. Nevertheless, SAGE states that recombination can result in more virulent strains (known to be done by other coronaviruses).

Therefore, the answer to this important question is really uncertain whether the virus will become more deadly over time. But you can’t expect the virus to be magically harmless.

Will humans evolve to catch up? Sadly, the answer is no. Humans do not reproduce fast enough and accumulate enough favorable mutations fast enough to stay ahead of the virus.

The virus also does not kill most infected people. And in a country with a well-resourced health system, it does not kill many people of reproductive age. Therefore, there is no “selective pressure” for humans to mutate favorably to stay ahead of the virus.

What about the future pandemic? Finally, evolution has a warning about future pandemics.

Genetic mutations that allow viruses of ambiguous and relatively rare species (such as bats) to access humans, the most common and widely distributed large animal species on the planet, are strongly selected.

Therefore, as in the past, if an animal virus spreads to humans, a future pandemic is expected.

By Hamish McCallum at Griffith University, Queensland.