A month after the UK government decided to extend the gap between first and second jabs of the Covid-19 vaccines from three to 12 weeks to ensure maximum inoculation as soon as possible, Britain’s “first doses first” policy remains an outlier in global terms.

The UK’s decision prompted widespread concern that it might weaken the immune response to the two marketed vaccines that use groundbreaking mRNA technology made by BioNTech/Pfizer and Moderna. Most other countries are sticking with the manufacturers’ recommendations that specify no more than a three or four-week interval, though some are shifting policy toward permitting a six-week gap.

But almost every independent expert on vaccinology and virology in the UK contacted by the Financial Times has supported the 12-week interval policy formulated by the government’s medical advisers and the Joint Committee on Vaccination and Immunisation.

There is much less concern about the Oxford/AstraZeneca vaccine, which uses a different technology to the BioNTech/Pfizer and Moderna jabs. Its clinical trials also included longer intervals between doses. “I think the UK strategy is absolutely the right way to go, at least for our vaccine . . . We have data for patients who received the vaccine in one month [and] two or three month intervals,” Pascal Soriot, AstraZeneca chief executive, said this week.

The British Society for Immunology, a professional body for scientists and clinicians, said its support for an extended interval for the mRNA vaccines was based on clinical evidence of the good protection given by the first dose and expert opinion that delaying the second booster dose would not harm the long-term immune response. There was also a recognition that vaccinating the maximum number of people at top speed “has by far the largest chance of reducing the disease burden and death rate compared with other measures”.

“I feel even more sure that it is right to do ‘first doses first’ now than I did a month ago,” said Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

Given the combination of a shortage of vaccine and high levels of infection caused by the highly transmissible new variant of the virus, he said many more lives would be saved in the UK by sticking with a 12-week dosing interval.

In different circumstances — for example, in Israel, which has plentiful stocks of BioNTech/Pfizer vaccine or New Zealand, which has very few Covid-19 cases — it made sense to stick with the manufacturer’s recommendations, he added.

The strongest voice of clinical dissent in the UK is the British Medical Association, which has called on Chris Whitty, England’s chief medical officer, to review urgently the 12 week policy. It supports a six week gap between doses.

The BMA said one concern among doctors was that the unpredictability of supplies did not guarantee second doses of the Pfizer vaccine would be available in 12 weeks’ time. Supporters of the “first doses first” policy have insisted that, despite short-term fluctuations in supply, vaccines will become more plentiful over time.

Another argument of those opposing a longer delay in the second dose is that there is little data to support a 12-week interval. With antivax campaigners waiting to exploit any failures, particularly with the new mRNA technology previously untested in a mass vaccination campaign, companies and regulators are keen to stick to a regimen proven to be safe and effective in clinical trials.

France decided on Tuesday to stick to the timing that the companies had studied in their clinical trials. “We are . . . facing a period of the unknown and uncertain, and I have therefore made the decision based on the security of the validated data,” said health minister Olivier Veran. “There is an absence of scientific consensus on this question.”

A panel of outside experts advising the French government, the Haute Autorité de Santé, had earlier called six weeks “an option to consider” as it would permit “an acceleration of administering the first dose to the most vulnerable groups.”

The World Health Organisation recommended sticking to manufacturers’ three-week spacing for BioNTech/Pfizer doses when possible but added: “The interval between doses may be extended up to six weeks, on the basis of currently available clinical trial data.”

How the UK’s vaccination rate compares with other countries. Chart showing doses administered per 100 residents (top locations)*

In the US, the new Biden administration’s vaccination policy is still bedding down. The Centres for Disease Control’s updated policy is close to the WHO’s. “The second dose should be administered as close to the recommended interval as possible,” CDC says. “However, if it is not feasible to adhere to the recommended interval, the second dose . . . may be scheduled for administration up to six weeks after the first dose.”

Anthony Fauci, the White House’s chief Covid-19 adviser, was asked about the UK’s interval-extending policy at the Davos World Economic Forum conference this week. “I would be concerned about that,” he said. “I can understand why that is being done but I would be concerned because you don’t get full efficacy until you get that second dose.”

But Peter Hale, executive director of the Foundation for Vaccine Research in Washington DC, said critics sceptical of delaying the second dose overlooked the fact that the three and four week interval for the two mRNA vaccines were deliberately chosen by their manufacturers Pfizer and Moderna to optimise the chances of getting an emergency use authorisation “as rapidly as possible.”

Without the urgency of a pandemic, Mr Hale said the two drug companies would probably have opted for a longer interval. “There are members of our group who actually consider that a longer interval between doses is beneficial in terms of building a broader and longer-lasting immune response,” he said.

Additional reporting by Leila Abboud in Paris