Half a million people with severely weakened immune systems will be offered a third vaccine against Covid, which will increase the debate on the deployment of booster injections for the entire population.

The NHS will provide third doses to people aged 12 and over whose immune systems are severely compromised or suppressed at a time decided by their clinicians. It follows a recommendation on Wednesday from the government’s Joint Committee on Vaccination and Immunization (JCVI).

Health officials said the injections were not boosters, which are given to prolong the protection of people, but rather are part of the primary immunization schedule of 400,000 to 500,000 vulnerable UK patients. Among people with weakened immune systems, these are the 40% who cannot produce a strong response even after two shots of the vaccine, according to the recent Octave study.