A “vaccine ceiling” may be approaching for some people who have already received three doses, according to new research, suggesting it may not be necessary to keep getting jabbed against Covid-19 on a rolling basis.
Health officials are currently assessing whether to broaden out the UK’s current booster programme beyond over-75s, care home residents and those with weakened immune systems.
But a new study led by academics at the University of Southampton suggests there is a “hint” that some people might have already reached a “ceiling” in terms of the amount of protection that they could acquire from a fourth jab.
Professor Saul Faust, the leading scientist of the CovBoost trial, said this would be dependent on the type of vaccines that an individual has already received, prior dosage volumes, and immunity levels built up through vaccination and natural infection.
His study, which assessed the protection levels of more than 150 people before and after vaccination, found that some individuals with high antibody and T-cell counts “had limited boosting from the fourth dose, indicating that there could be a vaccine-specific ceiling effect.”
Some 166 people took part in the study and offered blood samples, which were examined for antibody levels.
These were assessed at various time points, including 28 days after the third jab was given; again just before their fourth boost was administered - which took place, on average, just over 200 days later; and then 14 days after they had their fourth jab.
For most trial participants, their antibody count waned in the period between a third and fourth dose. But a fortnight after the booster jab, these levels rose to new heights, or “over and above” what had previously been recorded, said Paul Faust.
Some participants had 12 to 16 times higher levels of antibodies in the blood a fortnight after they got their fourth shot, compared with the day it was delivered.
Boosts were also seen in their T-cell levels, according to the study, which has been published in The Lancet Infectious Diseases journal.
However, the study added, “some participants … maintained high levels of humoral and cellular responses even before the fourth dose and had limited boosting from the fourth dose.”
Prof Faust said: “There’s a hint from some of the people in the trial and some of the people in the trial who had had Covid … that vaccines might reach a ceiling and that will depend on the vaccine and the host immunity and the dose, and that is true for the over-70s and the under-70s.”
A fourth dose might not “boost humoral and cellular responses if the baseline response is high,” the study said.
“These individual data are important for policy makers as the benefit of a fourth dose might be less in people who already have high levels of immune responses from recent infection or vaccination.”
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The so-called “vaccine-ceiling effect” needs to be further explored in new studies, the scientists behind CovBoost wrote.
Prof Faust said the Joint Committee on Vaccination and Immunisation (JCVI) was examining his study’s results, along with other research, as it considers whether to broaden out the booster campaign and offer a fourth dose to more of the UK population later in the year.
As part of the CovBoost trial, researchers examined data on people who had two doses of the AstraZeneca jab, followed by a Pfizer booster, who then received either a Pfizer jab or half dose of a Moderna jab 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 Moderna jab. No severe side-effects were recorded among participants, with some reporting pain or fatigue.
A number of groups have been called forward for a spring booster, including people aged 75 and over, care home residents, and those aged 12 and over who are immunocompromised.
For most this will have been a fourth vaccine dose, but for some with a weakened immune system it will have been their fifth.
Prof Faust, director of the NIHR Southampton Clinical Research Facility, said the results of his study ultimately “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.”