A new study from the University of Oxford in the United Kingdom has provided reassurance that a second dose of the Oxford-AstraZeneca vaccine remains highly effective even after a gap of up to 45 weeks.
The researchers found that the immune response of volunteers after an extended delay was superior to the response after the recommended interval. The study, which has yet to undergo peer review, appears as a preprint.
The World Health Organization (WHO) and several Governments around the world recommend a gap of 8–12 weeks between the first and second doses of the AstraZeneca vaccine, one of the most widely distributed vaccines.
Prof Teresa Lambe, one of the authors of the study, said that the strong antibody induction with these long intervals is very encouraging for countries where there may be limited supply in the short term.
Her co-author, Prof Sir Andrew Pollard, explained that this finding is in keeping with experience from other vaccines. “This is fairly typical; it is what you expect from vaccines. If after you give the first dose, you give more time for the immune response to mature, you tend to see slightly better responses later on.”
Prof Pollard also added that there is a trade-off between the risk of infection due to steadily declining immunity after the first dose and stronger eventual immunity after a delayed second dose.
“But we don’t know at this moment with one dose how long you can safely sit there with still good levels of protection. But certainly, it’s longer than 3 months,” he said.
The Oxford-AstraZeneca vaccine is marketed as “Covishield” in India, marketed by Serum Institute of India (SII), Pune.