A fourth dose of a Covid mRNA vaccine – Pfizer or Moderna – can increase immune defences beyond the peak immunity seen following a third dose, new research suggests.

The fourth dose of a Covid vaccine – sometimes known as the second booster – is currently available in the UK to those aged 75 or over, people living in care homes and those over the age of 12 who are immunosuppressed.

According to new research published in the Lancet yesterday (9 May), the fourth dose of Covid-19 mRNA vaccines is ‘well tolerated and can provide a substantial boost to both humoral and cellular immunity’.

The study, from the COV-BOOST trial, which measured immune responses in 166 participants aged 50-80, found those who had received their fourth vaccine of Pfizer or Moderna had twice as many more antibodies as those who had received just their third vaccine.

Participants who had previously tested positive for Covid had high levels of immunity before the fourth vaccine, which was not boosted by the fourth vaccine.

Professor Saul Faust, director of the NIHR Southampton Clinical Research Facility and the trial lead, said the results ‘underline the benefits of the most vulnerable people receiving current spring boosters and give confidence for any prospective autumn booster programme in the UK, if the Joint Committee on Vaccination and Immunisation considers it needed at that time’.

Sajid Javid, the health secretary, added: ‘This is further evidence underlining the importance of people coming forward for their booster as soon as they are eligible.

‘We’re able to live with Covid thanks to the protection provided by our phenomenal vaccine programme and a booster dose will top up your immunity to continue to keep you and your loved ones safe.’

Almost three million people have had their spring booster jab in the six weeks since the latest phase of the NHS Covid vaccination programme started.

This means more than half - 2.8 million - of the 5.5 million people eligible for the jab by the end of June have so far been protected, according to figures from Thursday (5 May).

The NHS is inviting hundreds of thousands of people for their spring booster weekly, as more people become eligible, and is urging people to come forward as soon as they are eligible.