This is a question on everyone’s mind, as reports surface of new mutations in the UK and S. Africa that are more infectious and more virulent. I have seen experts quoted in the newspaper that it will take at least 6 years for a vaccine resistant strain to emerge, based on the rate at which other coronaviruses mutate.
But time is the wrong metric with which to measure mutation rate. The right metric is total viral load in the world – roughly, the number of infected people.
This observation is not new. In 1990, Nowak, May and Anderson proposed a model to help understand AIDS. They posited that there is a high rate of mutation of HIV within one person’s body after they get infected. Some immune cells target all the HIV strains, some are specific to each strain. (Moreover there is a complicating factor in AIDS that HIV is itself damaging the immune response system.) Their model predicted that if the number of strains of HIV within the body stayed below some threshold number, the immune system could keep the total infection low. Once that number of strains was exceeded, the immune system was overcome, and full-blown AIDS developed rapidly.
This model is incredibly important. When anti-retroviral drugs were first developed, it was debated whether they should be given to patients as soon as they were diagnosed with HIV, or whether they should wait until they developed symptoms of AIDS. The Nowak-May-Anderson model gives an answer: give the drugs right away. Why? Because mutations occur at a fairly constant rate whenever the virus replicates. Keeping the total viral load low in the body means fewer replications, therefore fewer mutations.
Case studies confirmed this, and immediate treatment is now standard practice, and has saved millions of lives.
How does this apply to SARS-CoV-2? For every person who is infected, there is some chance that a mutation will occur leading to a new strain of the disease. Once new strains emerge, evolutionary pressure will select for ones that are either more infectious, or that can infect people whom previous strains couldn’t (by virtue of having been vaccinated or caught an earlier strain).
What does this mean in practice? Anything that can keep the total number of infections down not only has the direct benefit of reducing immediate morbidity and mortality, but also decreases the chance of a new mutation. Rich countries have a moral imperative to help poor countries control COVID infection and to vaccinate their populations. They also have a selfish motivation: if they don’t, another strain will emerge that might start a new pandemic.