The head of a vaccine research group at the University of Oxford has warned that herd immunity is “not a possibility” with the current spread of the delta variant as it can still infect vaccinated individuals.

Andrew Pollard, who was involved in the development of the AstraZeneca COVID-19 vaccine, issued the warning on Tuesday during a meeting of Britain’s All-Party Parliamentary Group on COVID-19.

Pollard also projected that a variant could come along that is more prone to transmit in vaccinated populations, adding this is even more reason to not center a vaccine program on herd immunity.

Instead, Pollard said antivirus efforts should focus on improving treatment for serious COVID-19 cases. 

On booster shots, Pollard suggested they are currently not necessary. He said a decision in this regard should be based on scientific studies, and there is currently no evidence yet of an increase in severe disease or deaths among the fully vaccinated.