A second Covid-19 booster dose, or the fourth jab against the virus, boosts immunity more than the initial or the third dose, according to a UK trial.
The UK rolled out fourth doses to over-75s and those over the age of 12 who are immunosuppressed in April. It is also offered in Israel and Germany.
"Fourth-dose Covid-19 mRNA booster vaccines are well tolerated and boost cellular and humoral immunity," scientists wrote in the Lancet Infectious Diseases.
Even as short-term protection against infection is likely to fall away quickly, the trial showed that "peak responses after the fourth dose were similar to, and possibly better than, peak responses after the third dose".
Prof Saul N. Faust, from the University Hospital Southampton and the team explained that third-dose boosters increase humoral and cellular immunity. And they provide more short-term protection against symptomatic infection with variants of concern, including Omicron, compared with a two-dose schedule.
However, protection against symptomatic infection wanes rapidly following the second and third doses of Covid-19 vaccines.
The new trial showed that the fourth dose can rescue immune responses that have waned since a third jab.
"We've demonstrated that a fourth dose of Covid-19 vaccines can produce a substantial boost to both the antibody and cellular immunity when you give them more than six months after the third dose," Faust, who led the trial, was quoted as saying to the Guardian.
In the trial, the team analysed data from 133 participants, finding that 14 days after receiving the fourth jab, there was a 1.6-fold increase in antibodies among those who received the Pfizer/BioNTech vaccine.
More than two-fold increase was observed among those who received the half-dose Moderna jab, compared with 28 days after the third dose, when antibody levels were still at their peak.
The increase in immunity levels were seen for those over and below 70 years of age.
In addition, levels of antibodies and T-cells increased substantially between the day before the fourth vaccination and 14 days after for both types of fourth jab, the report said.
"Our results for immunogenicity are also consistent with the little observational evidence on vaccine effectiveness available from Israel, which indicates increased protection against symptomatic infection and severe illness from a fourth-dose booster," the team wrote in the paper.
Faust added that those who had little waning of their immune responses before their fourth dose gained only a limited increase in their immune responses as a result of the booster - with similar findings for others who had a recent history of a Covid infection.
"That indicates there may be a ceiling, a maximum antibody level with the T-cell response effects," he said.