Key to the success of achieving herd immunity with live attenuated viruses like that found in the history-changing polio vaccine is its ability to spread the non-pathogenic virus and the immunity it confers beyond the initial recipient. A novel oral polio vaccine rolled out by the WHO two years ago specifically designed to avoid the dreaded possibility of mutations occurring upon repeated person-to-person spread that would convert the attenuated virus into a pathogenic one has now been found to have done just that with seven children recently paralyzed by this novel vaccine-derived polio strain. | Roberts, Science 2023
