An eight-week gap between the two doses of the Pfizer vaccine is a "sweet spot", according to new research.
In a study funded by the Department of Health, a four-week gap between the first and second dose of the COVID-19 vaccine generates a strong immune response against the Delta variant.
But researchers found that a 10-week interval produces higher antibody levels and a higher proportion of "helper" T cells.
In December, when vaccines started to be administered to elderly people, the Joint Committee of Vaccination and Immunisation (JCVI) recommended a 12-week gap between doses.
This was shortened to eight weeks in May, as vaccine supplies increased.
Over 46.3 million people have received at least one dose of the coronavirus vaccine and over 36.4 million people are now fully inoculated.
Professor Susanna Duanchie, of the University of Oxford, who is the joint chief investigator in the study, said: "The original recommendation from JCVI was 12 weeks and this was based on a lot of knowledge from other vaccines that often having a longer interval (between doses) gives your immune system a chance to make the highest response.
"I think that eight weeks is about the sweet spot for me because people do want to get the two vaccines (doses) and there is a lot of Delta out there right now.
"Unfortunately, I can't see this virus disappearing so you want to balance that against getting the best protection that you can."
The researchers studied the immune response to the Pfizer jab of 503 healthcare workers - 44% of whom previously had coronavirus.
They found that both short and long intervals generated strong antibody and T cell responses, but a longer gap led to higher antibody levels.
The study said: "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.
"Following two vaccine doses, neutralising antibody levels were twice as high after the longer dosing interval compared with the shorter dosing interval."
The scientists added that there may be exceptions where a shortened four-week schedule may be needed, such as for those who have treatments that affect the immune system such as cancer or organ transplant.
Vaccines Minister Nadhim Zahawi said: "This latest study provides further evidence that this interval results in strong immune response and supports our decision.
"I urge every adult to get both doses of the vaccine to protect yourself and those around you and we are looking to offer millions of the most vulnerable a booster jab from September to ensure this protection is maintained."