Achieving herd immunity with Covid vaccines when the highly infectious delta variant is spreading is “not a possibility,” a leading epidemiologist said. Experts agree on several reasons why such a goal — where overall immunity in a population is reached and the spread of the virus is stopped — is not likely. Sir Andrew Pollard, head of the Oxford Vaccine Group, told British lawmakers Tuesday that as Covid vaccines did not stop the spread of the virus entirely — with vaccinated people still able to be infected and transmit the virus — the idea of achieving herd immunity was “mythical.”...