People with weakened immune systems provide viruses like Sars-CoV-2 with a unique environment. Instead of clearing an infection quickly, an immunocompromised person might only partially wipe out an infection, leaving behind a population of genetically-hardier viruses that rebound and begin the cycle all over again. In these people, a virus can evolve at remarkable speed. ?The whole time, their immune system is effectively beating [the virus] up. So the virus has a chance to learn how to live with the human immune system,? says Emma Hodcroft, a postdoctoral research at the University of Bern in Switzerland who works on Nextstrain ? an open-source project that tracks the genetic changes of Sars-CoV-2 and other pathogens.
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But the real problem isn?t chronic infection ? it?s a situation where the pandemic is so out of control that the virus has endless opportunities to mutate into new variants. That situation is a reality in the UK and in dozens of other countries with dangerously high case rates. ?These are all edge cases, but if you have enough people infected over a long time, you run the risk of hitting those edge cases,? says Hodcroft. It is not surprising that some of the most worrying new variants ? from Brazil, South Africa and the UK ? emerged from parts of the world with relatively high levels of transmission. ?When you have millions of people infected, with millions of viruses replicating in each one, there are lots of chances for the virus to explore new mutations and combinations,? says Lauring. ?If we had control of the virus, we wouldn?t be seeing as many variants pop up, because there wouldn't be as many opportunities for evolution to happen.?