Your example proves exactly the opposite of the point you think it does. Every year the flu virus comes back slightly mutated, and the immunity process starts a

Your example proves exactly the opposite of the point you think it does. Every year the flu virus comes back slightly mutated, and the immunity process starts all over again. In the summer the southern hemisphere acts as of reservoir of the virus and vice versa in the winter. This exact process could happen with COVID too. You are right that it does mutate more slowly, but there are upwards of 4 billion people in these countries. A very small number times 1 billion is still a much larger number.