People most vulnerable to Covid-19 could require a booster of the BioNTech/Pfizer vaccine later this year, according to scientists who said the shot elicited a lower antibody response against the Delta variant first identified in India.

A study by the researchers at the Francis Crick Institute and University College London’s biomedical research centre found that levels of neutralising antibodies in recipients of two shots of the vaccine were, on average, more than five times lower against the Delta variant than the original strain that emerged in Wuhan, China.

In a research letter published in the medical journal The Lancet, the scientists said that the results showed older people were the most affected and that their antibody levels declined over time.