Andrew Pollard, one of the creators of the Oxford AstraZeneca Covid-19 vaccine, has said that giving people booster shots twice a year is “not sustainable.”

Speaking to UK newspaper The Telegraph in an interview published Tuesday, Pollard said: “We can’t vaccinate the planet every six months.”

Pollard, who also heads up the UK’s Joint Committee on Vaccination and Immunization (JCVI), stressed the “need to target the vulnerable” going forward, rather than administering doses to all those over 12 years old.