• Otávio Santiago

Evolving Threat - new variants have changed the face of the pandemic. What will the virus do next?

Edward Holmes does not like making predictions, but last year he hazarded a few. Again and again, people had asked Holmes, an expert on viral evolution at the University of Sydney, how he expected SARS-CoV-2 to change. In May 2020, 5 months into the pandemic, he started to include a slide with his best guesses in his talks. The virus would probably evolve to avoid at least some human immunity, he suggested. But it would likely make people less sick over time, he said, and there would be little change in its infectivity. In short, it sounded like evolution would not play a major role in the pandemic’s near future.

“A year on I’ve been proven pretty much wrong on all of it,” Holmes says.

Well, not all: SARS-CoV-2 did evolve to better avoid human antibodies. But it has also become a bit more virulent and a lot more infectious, causing more people to fall ill. That has had an enormous influence on the course of the pandemic.

The Delta strain circulating now—one of four “variants of concern” identified by the World Health Organization, along with four “variants of interest”—is so radically different from the virus that appeared in Wuhan, China, in late 2019 that many countries have been forced to change their pandemic planning. Governments are scrambling to accelerate vaccination programs while prolonging or even reintroducing mask wearing and other public health measures. As to the goal of reaching herd immunity—vaccinating so many people that the virus simply has nowhere to go—“With the emergence of Delta, I realized that it’s just impossible to reach that,” says Müge Çevik, an infectious disease specialist at the University of St. Andrews.

Yet the most tumultuous period in SARS-CoV-2’s evolution may still be ahead of us, says Aris Katzourakis, an evolutionary biologist at the University of Oxford. There’s now enough immunity in the human population to ratchet up an evolutionary competition, pressuring the virus to adapt further. At the same time, much of the world is still overwhelmed with infections, giving the virus plenty of chances to replicate and throw up new mutations.

Predicting where those worrisome factors will lead is just as tricky as it was a year and a half ago, however. “We’re much better at explaining the past than predicting the future,” says Andrew Read, an evolutionary biologist at Pennsylvania State University, University Park. Evolution, after all, is driven by random mutations, which are impossible to predict. “It’s very, very tricky to know what’s possible, until it happens,” Read says. “It’s not physics. It doesn’t happen on a billiard table.”

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