i'm kim vinnell.tists have long said so-called herd immunity is needed to slow down the spread of covid-19. that's when 70% or more of the population is protected from the virus, either by becoming infected, or by being vaccinated. but some experts now doubt it can be achieved. the head of the u.k.'s oxford vaccine group says that's because of the contagious delta variant. new infections and deaths are rising steadily in britain, despite three quarters of people being fully vaccinated. andrew pollard had this warning to mps. -- to mp's. >> we know very clearly with the coronavirus that this current delta variant will still infect people who have been vaccinated. and that does mean that anyone who's still unvaccinated at some point will meet the virus. i think we are in a situation here with this current variant where herd immunity is not a possibility, because it still infects vaccinated individuals, and i suspect that what the virus will throw up next is a variant which is perhaps even better at tran