“The [Pfizer/BioNTech] vaccine applied in the third dose of vaccine showed a significant superiority in protection – both in terms of antibody level and disease status – compared to the third dose of CoronaVac vaccine,” the researchers said.

However, all of the nearly 500 participants receiving three doses of vaccine achieved protective antibody levels, “regardless of third-dose vaccine preference”, they said.

Hospital staff included in the study were vaccinated in January, as Turkey started its rollout of CoronaVac, and received their second doses in February. In July, Turkey began offering booster shots to hospital workers as concerns grew about the Delta variant and CoronaVac’s longevity and effectiveness against the highly transmissible strain.

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Between July 1 and September 24, less than 1 per cent of the 902 hospital workers who were fully vaccinated with CoronaVac and received a Pfizer/BioNTech booster were infected with Covid-19. For the 130 who had the CoronaVac booster, 3.84 per cent were infected. Results were measured 28 to 45 days after the third vaccination. None of the infected workers were admitted to hospital.

Researchers cautioned that the accuracy of the data was limited by the small number of infections and possible missed cases.

For the antibody tests, which are conducted in a laboratory using blood from vaccinated people, 95 per cent of participants who received Pfizer/BioNTech for their third dose had antibody levels above the “maximum measurable titre” in the period of 28 to 45 days after the third shot. This was the case for 8.2 per cent of those who received the third Sinovac dose in the same period.

However, all those in both booster groups had “protective antibody levels”, the researchers said. Scientists have not yet pinpointed the exact amount of antibody needed to confer protection against infection and disease.

Numbers in the antibody study were not split evenly, with 61 workers receiving a CoronaVac booster and 416 people receiving the Pfizer/BioNTech shot.

Erhan Eser, a professor of public health who was involved in the study, said a decline in antibody levels among those who had received two doses of CoronaVac, which was observed in previous months of the study, meant the same could happen with the booster shot.

“We can predict that the decrease in antibody level will continue within the next six months,” he said, noting that the nine-month mark of the study would provide a “clear answer” to that question.

Eser added that the detectable level of antibodies might not show the whole picture for how the body could mount an immune response months after vaccination.

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Sinovac has also released data on a booster regimen for its vaccine, finding that neutralising antibody titres at day 28 after a third dose were three to five times higher than those 28 days after the second shot. The boosters were given six to eight months after the original course.

Even as antibody levels declined six months after the booster dose, they remained at a “relatively high level” and above that observed after the second shot, according to Sinovac data submitted to the World Health Organization.