I thought, Steve, you might comment on the risk of evolving a vaccine-resistant variant of covid-19. Back in April 2020 at least 30 variants had been identified, so it looks like the virus is fairly adaptable.   Any thoughts? Obviously, neither of us has the detailed knowledge of viruses to answer the question rigorously, but in general terms the risks and processes seem to be similar to evolving antibiotic resistant    bacteria etc.