A fourth dose of the Pfizer or Moderna COVID-19 vaccine is safe and boosts antibody levels, even higher than that of a third dose, according to the latest results from a nationwide study in the UK.

Fourth doses of COVID-19 vaccines have been offered as a spring booster for those most vulnerable in the UK, the researchers said. This has been a precautionary strategy to maintain high levels of immunity prior to the study data being available, they said. The findings, published in The Lancet Infectious Disease journal, show that fourth dose mRNA booster vaccines for COVID-19 are well-tolerated in people who received Pfizer as a third dose.

They are also effective at increasing both antibody and cellular immunity up to and above baseline and peak levels observed following third dose boosters, according to the researchers.

"These 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," said Professor Saul Faust, trial lead and Director of the NIHR Southampton Clinical Research Facility.

In the study, 166 people who had received the third dose of Pfizer, following Pfizer or AstraZeneca initial doses in June 2021, were randomised to receive full dose Pfizer or half dose Moderna as a fourth dose. The fourth dose was administered approximately seven months after the third dose.

While pain at the vaccination site and fatigue were the most common side effects, there were no vaccine-related serious adverse events and fourth doses were safe and well-tolerated, the researchers said.

"We knew that it was important to offer a fourth dose to those most vulnerable earlier in the year," Professor Andrew Ustianowski, NIHR Clinical Lead for the COVID-19 Vaccination Programme, said. "These new study findings support that decision and provide the public with the confidence that fourth doses are both safe and even more effective than third doses at boosting immunity against COVID-19," Ustianowski said.