Study Shows Strong Immune Response From Single Pfizer COVID-19 Shot In Previously-Infected Subjects

One dose of Pfizer COVID-19 vaccine produces an immune response in previously-infected people similar to that generated by two doses in naive individuals, and could also offer protection from variants, a British study said on Friday.

The study, led by Sheffield and Oxford Universities with support from the UK Coronavirus Immunology Consortium, and released as a pre-print on Friday, found that previously-infected healthcare workers generated T-cell responses after one dose of the Pfizer-Biontech vaccine that were 6.8-fold higher than naive individuals after one shot, and higher even than responses to two shots given three weeks apart to infection-naive people.

Responses in naive individuals were on a par with those seen after natural infection, however, and back up real-world data on protection after a single vaccine dose from a study called SIREN, the authors note.

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"SIREN is actually showing very high vaccine effectiveness against hospitalisation after a single dose, with the majority of these people having not had infection before. So what we're trying to do is look at the mechanisms for that," Susanna Dunachie of the University of Oxford's Nuffield Department of Medicine, told reporters.

"We are seeing T-cell and antibody responses after one dose in people who have not had infection before. So we find that quite reassuring."

The study is the largest real-world study on T-cell and antibody responses from Britain's vaccine rollout, and looked at healthcare workers, mainly women, after one or two doses of the Pfizer shot 23 days apart.

The researchers analysed blood samples from 237 people, of whom 113 had been infected a median of 8.9 months prior to receiving the vaccine.

Spain and Italy are among a handful of countries that have announced a dose-sparing policy of delaying a second vaccine shot in previously-infected individuals based on evidence that a single shot acts as a booster to preexisting immunity.

The UK study suggests that a single shot not only boosts, but broadens existing immunity.

The researchers also tested plasma from vaccine recipients against the B.1351 variant of concern that first emerged in South Africa. "No neutralisation of B.1.351 was seen in post-vaccine plasma in naive individuals after a single vaccine dose, whereas significantly higher titres were elicited in previously-infected (individuals)," the study team reports.

Thushan de Silva, study author from the University of Sheffield, said that boosting pre-existing antibody responses could provide protection against coronavirus variants, including the one first discovered in South Africa which has been shown to reduce the efficacy of existing vaccines.

—Reuters Staff

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