The study might help inform vaccination strategies against the Delta variant, which reduces the effectiveness of a first dose of COVID-19 vaccine even though two doses are still protective.

“For the longer dosing interval, neutralising antibody levels against the Delta variant were poorly induced after a single dose, and not maintained during the interval before the second dose,” the authors of the study, which is being led by the University of Oxford, said.

“Following two vaccine doses, neutralising antibody levels were twice as high after the longer dosing interval compared with the shorter dosing interval.”

The study found overall T cell levels were 1.6 times lower with a long gap compared with the short dosing schedule of 3-4 weeks, but that a higher proportion were “helper” T cells with the long gap, which support long-term immune memory.

The authors emphasised that either dosing schedule produced a strong antibody and T cell response in the study of 503 healthcare workers.

The findings, issued as a pre-print, support the view that while a second dose is needed to provide full protection against Delta, delaying that dose might provide more durable immunity, even if that’s at the cost of protection in the short-term.

Last December, Britain extended the interval between vaccine doses to 12 weeks, although Pfizer warned there was no evidence to support a move away from a three-week gap.

Britain now recommends an 8-week gap between vaccine doses to give more people high protection against Delta more quickly, while still maximising immune responses in the longer term.

“I think the 8 week is about the sweet spot,” Susanna Dunachie, joint chief investigator on the study, told reporters.

REUTERS