"Our aim is to try and reduce the peak [of the infections], broaden the peak, not suppress it completely," Patrick Vallance, chief scientific advisor to the U.K. government, told BBC Radio 4's Today program on Friday. "Also, because the vast majority of people get a mild illness, to build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission."
His justification for the achieving herd immunity was that most people get it mildly. Nothing to do with a vaccine.