Fourth COVID-19 shot could give higher immunity than a third dose

What is the research about?

While COVID-19 vaccines provide strong protection against severe disease and death, immunity wanes after a while. This is why some countries whose population has already been largely protected by the vaccine have offered booster doses.

14 days after the fourth jab, antibody levels against SARS-CoV-2 jumped 1.6-fold in those who were boosted with Pfizer/BioNTech vaccine

Many high-income countries rolled out a booster dose, but not many have yet offered a fourth dose to the general population. This research looked at whether a second booster could increase antibody levels that had eased off after the initial booster dose. Although antibodies are only part of the set of tools our immune systems use to fight off infection, they are an important part.

What did the researchers do?

Writing in the Lancet Infectious Diseases, researchers in the UK-based Cov-Boost trial looked at the antibody responses in 133 study participants who were given a second booster dose of COVID-19 vaccine seven months on average after their initial booster.

Participants had initially had either two doses of the Pfizer/BioNTech vaccine or two doses of the Oxford/AstraZeneca vaccine. In the UK, the second booster is currently only given to those who are immunosuppressed or vulnerable in some way, including those older than 75 years or people living in care homes.

Half were randomly allocated to receive a full dose of the Pfizer/BioNTech COVID-19 vaccine and the others were given half a dose of the Moderna jab.

What did they find?

The team found that 14 days after the fourth jab, antibody levels against SARS-CoV-2, the virus that causes COVID-19 jumped 1.6-fold in those who were boosted with Pfizer/BioNTech vaccine, and a more than twofold increase among those who were given the half-dose Moderna jab was observed. This is compared with 28 days after the third dose, when antibody levels were still at their peak, meaning that the second booster caused an immune response even higher than the peak response triggered by the initial booster.

In people whose antibody levels had not waned much there was a smaller boost, suggesting there is a ceiling to how much SARS-CoV-2 antibody levels can be boosted.

What does this mean?

Many countries are trying to decide whether or not to offer a second booster dose to the general population, weighing up the benefit of greater immunity against need. The authors say this research could contribute to that decision-making.