Video report by ITV News Political Correspondent Hannah Miller
Senior doctors are calling on the government to reduce the gap between the first and second dose of the Pfizer-BioNTech coronavirus vaccine.
Currently government guidelines is there should be no longer than 12 weeks between the two jabs.
Experts are calling for this gap to be reduced to a maximum of six weeks "in exceptional circumstances", in accordance with the recommendations set out by the World Health Organisation.
Doctors warn the efficacy of the jab may be impacted if people have to wait 12 weeks between the two doses, as Pfizer said it had only tested efficacy when the two doses were given 21 days apart.
Dr Chaand Nagpaul: UK should follow 'best practice' and reduce gap between two Pfizer doses
Unpublished data suggests the Oxford-AstraZeneca vaccine is still effective with doses 12 weeks apart.
Dr Chaand Nagpaul, council chairman of the British Medical Association, the trade union which represents doctors across the UK, said he understood the "rationale" behind the decision to delay the second Pfizer dose but said the UK should follow "best practice".
He said: "No other nation in the world has taken this stance. It has been considered in Europe, it has been considered in America, and they've all felt it important to stick to recommended guidance.
"We want to be sure that if this delay is occurring, that we're not compromising the effectiveness of the vaccine and it's not going to result in people becoming infected who may have been protected by getting an earlier second dose."
He added: "We don't actually know whether the vaccine will be as effective after a first dose extending up to 12 weeks. This also comes at a time when we have a highly transmissible virus where its causing serious illness and death.
"Doctors I represent are obviously very worried because if they feel they're not adequately protected they could fall ill themselves."
"We have 50,000 NHS staff who are sick or isolating, so protecting those who are at most risk is important.
"We believe the six week delay would offer a compromise."
The BMA also said the unpredictability of vaccine supply into the UK is another reason not to have a 12 week delay.
Pfizer said it would be reducing its exports to Europe amid a ramping up of its production facilities, and there are fears this may extend the second dose if there are delays to supply chains into the UK.
However Professor Chris Whitty said the UK's decision to delay the second jab to 12 weeks would allow the UK to vaccinate more people quickly.
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And Public Health England medical director Dr Yvonne Doyle also defended the decision, saying it was taken on “public health and scientific advice”.
“The more people that are protected against this virus, the less opportunity it has to get the upper hand. Protecting more people is the right thing to do,” she told the BBC Radio 4 Today programme.
“People will get their second dose. As supplies become available more people will be vaccinated.
“It is a reasonable scientific balance on the basis of both supply and also protecting the most people.”
Shadow health secretary Jonathan Ashworth said: “While we understand this approach has been endorsed by all CMOs across the UK nation, ministers must be fully transparent and engage with all stakeholders, including the BMA, on the evidence upon which they've made their decisions.
"All of us want to see the vaccination program succeed and plans need to be in place to guarantee second doses are delivered on time. Worryingly Dominic Raab couldn’t offer that commitment earlier this week."