A fourth Covid-19 vaccination has been found to give people protection “over and above” that experienced by people who have had three inoculations, a study suggests.
Researchers are continuing to evaluate the level of protection offered by vaccinations and how long it lasts.
Academics led by the University of Southampton have been monitoring people’s levels of antibodies and T-cells, both of which offer an accurate measure of a person’s level of protection against a virus.
The CovBoost trial also examined side-effects after a fourth shot.
About 166 people took part in the study and offered blood samples, which means that scientists could examine the concentration of antibodies in the blood.
These were examined at various time points, including 28 days after the third shot was given; again just before the fourth dose was administered, on average, just over 200 days later, and then 14 days after they the fourth vaccination.
The levels of antibodies waned in the period between third and fourth shots.
But a fortnight after the booster vaccination, antibody levels rose even higher than the levels seen after the third inoculation.
And there were significant boosts compared with the levels seen on the day they were given their fourth shot. Subjects had 12 to 16 times higher levels of antibodies in the blood a fortnight after they had their fourth vaccination, compared with the day it was delivered.
Boosts were also seen in the cellular level, according to the study, which has been published in The Lancet Infectious Diseases journal.
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Researchers examined data on people who had two doses of the AstraZeneca vaccination, followed by a Pfizer booster, who then received either a Pfizer or half dose of a Moderna inoculation for their fourth vaccine.
They also looked at people who had three doses of Pfizer, followed by a fourth Pfizer shot or a half-dose of Moderna.
No severe side-effects were recorded among participants, with some reporting pain or fatigue.
But the authors said that the study provided a “hint” that a small number of people might reach a “ceiling” in terms of the amount of protection that they could get from a fourth shot.
The authors said that some people had high levels of immune response “even before the fourth dose and had limited boosting from the fourth dose”, including people who had just been infected with the virus.
“These results underline the benefits of the most vulnerable people receiving current spring boosters and gives 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 trial leader Prof Saul Faust, director of the NIHR Southampton Clinical Research Facility.
British Health Secretary Sajid Javid said: “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.”
Groups have been called forward for a spring booster, including to those aged 75 and over, people who live in care homes for older people, or people aged 12 and over who have a weakened immune system.
For most, this will have been a fourth vaccine dose but for some with weakened immune systems it will have been their fifth.
The study supports the booster campaign and will also be used by experts who will decide on the future of the vaccination programme, including whether people with weakened immune systems should have further shots and whether the booster programme should be extended to other groups.
So far, more than three million people have received their spring booster in England alone, says the National Health Service in England.
