UPDATE 1-UK vaccine roll-out speeds up but doctors want quicker second dose

* Nearly 5.9 million people have had first dose

* Another 1,348 deaths bring toll to 97,329

* Doctors urge government to cut gap between shots (Adds vaccine numbers, background)

By Sarah Young

LONDON, Jan 23 (Reuters) - Britain's COVID-19 vaccination push gathered pace on Saturday, with 5.9 million people now having had a first dose, but doctors challenged the government over its policy of delaying a second shot of the Pfizer vaccine for up to 12 weeks.

The British government is stretching out the gap between first and second shots as it seeks to ensure as many people as possible can be given some protection from an initial vaccine dose.

Prime Minister Boris Johnson warned on Friday that the new UK variant of COVID-19 may be associated with a higher level of mortality as the country's death tally from COVID-19 nears the 100,000 mark - hitting 97,329 on Saturday.

But in a letter to Chief Medical Officer (CMO) for England Chris Whitty, the British Medical Association said leaving the 12-week interval for the Pfizer vaccine went against World Health Organization guidance.

They urged the government to reduce the gap between Pfizer doses to a maximum of six weeks.

The makers of the vaccine, Pfizer and BioNTech, have warned that they have no evidence their vaccine would continue to be protective if the second dose is given more than three weeks after the first.

Leaving a 12-week gap is allowing Britain's vaccine programme to proceed quickly.

Government data published on Saturday showed 5.86 million people have now received a first dose of the vaccine, after a record 478,248 people had the jab in the last 24 hours.

Whitty said on Friday that the longer wait between doses was a "public health decision" aimed at vaccinating many more people and based on the fact that the great majority of protection comes from the first jab.

The Department of Health and Social Care said the decision on the 12-week gap was made after "a thorough review of the data" and was in line with the recommendations of the UK's four chief medical officers.

Britain is using the Pfizer vaccine and another from AstraZeneca. AstraZeneca has supported the gap between its jabs, saying data showed an 8- to 12-week gap was a "sweet spot" for efficacy.

Following Johnson's warnings about the more deadly nature of the new variant, some scientists said on Saturday it was too soon to be clear about what the evidence was showing.

"The question about whether it's more dangerous in terms of mortality I think is still open," Graham Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, told the BBC. (Editing by Jason Neely and Helen Popper)