Countries may wish to consider following the UK in delaying second Covid vaccinations in a bid to save lives, new research suggests.

Modelling by experts in the US shows that delaying the second dose so that more people can receive a first dose could reduce deaths by up to a fifth.

Published in the British Medical Journal (BMJ), one example showed that if a first dose offered 80% protection against Covid, deaths fell to 207 per 100,000 people if the second dose was delayed.

This compared with 233 deaths if people got their second dose within the usual timeframe.

The researchers used a simulation model based on a “real-world” population of 100,000 US adults, finding that delaying a second dose could prove beneficial if a single dose offers at least 80% protection and if vaccination rates stood at 0.1% to 0.3% of the population per day.

Delaying the second dose but prioritising it for people aged under 65 can result in lower mortality rates compared with the standard two-dose strategy

The experts, including from the Harvard School of Public Health in Boston and the Massachusetts Institute of Technology (MIT), calculated the strategy could prevent between 47 and 26 deaths per 100,000 people overall.

The effect was particularly strong for the under-65 age group, where a delayed second dose strategy performed consistently well under all vaccination rates tested.

The authors said: “The results suggest that, under specific conditions, a decrease in cumulative mortality, infections, and hospital admissions can be achieved when the second dose of vaccine is delayed.

“This was most significant when the second dose was delayed in people below 65 years of age, with second doses still prioritised for those over 65.

“The conditions in which these benefits were observed included the first-dose vaccine efficacy being above 70% and vaccination rates remaining below 1% of the population per day.”

They added: “Delaying the second dose but prioritising it for people aged under 65 can result in lower mortality rates compared with the standard two-dose strategy, even at vaccination rates up to 1% of the population per day.”

Dr Peter English, a retired consultant in communicable disease control, said the UK’s decision to delay the second dose until 12 weeks had “proven highly effective” and the same could apply in other countries.

He added: “There have been concerns expressed about the lack of evidence for effectiveness if the prime-boost interval is extended by delaying the second dose.

“These concerns are misplaced. Everything we already knew about vaccines also tells us that a longer prime-boost interval enhances the breadth and depth of the immune response, giving longer-lasting immunity that is likely to provide greater cross-protection against variant strains.

“There is relatively little knowledge about this specifically related to Covid-19 vaccines – although such data as we have seen is consistent with this.

“It goes beyond this paper; but it seems likely that increasing the prime-boost interval will lead to better, longer-lasting immunity, as well as protecting more people more quickly.”