All over-40s will be offered a second Covid vaccine within eight weeks of receiving a first jab, the UK government has claimed.
The change in tact comes after the final step of the roadmap to lift all lockdown restrictions was delayed by four weeks amid rising cases of the Delta variant first identified in India.
Prime minister Boris Johnson confirmed the delay from 21 June to 19 July for all remaining coronavirus rules to be lifted to allow for more vaccines to be administered in the UK.
It is thought around two thirds of all adults will have been offered two doses of the vaccine by mid-July, while all over-18s will be offered a first jab two weeks earlier than planned.
The Delta variant is between 40% and 80% more transmissible than the original strain of Covid but both the Pfizer and AstraZeneca vaccines have been found to be highly effective in fighting off all variants so far.
Public Health England data suggests two doses of the Pfizer-BioNTech jab is up to 96% effective in reducing hospitalisations, whereas two doses of the Oxford-AstraZeneca vaccine is 92% effective.
The plan is for all over-50s and clinically extremely vulnerable to have been offered two doses of an approved vaccine and for those second jabs to have taken effect by 19 July.
What's the gap between the first and second vaccine dose?
Following the UK's approval of the Pfizer vaccine on 2 December, the initial plan was to give priority groups a second dose three weeks after the individual received their first.
Yet increasing infection rates, hospital admissions and new variants of the virus - including a more transmissible strain from South Africa - prompted a change in thinking.
The Joint Committee on Vaccination and Immunisation (JCVI), an independent expert advisory committee, worked out that more people could get a first dose of the vaccine if the UK were to delay the administering of the second of up to 12 weeks.
It is hoped that more doses of vaccines will be available by then, while also offering more vulnerable people protection against the deadly virus.
Due to the Delta variant of Covid, over-50s and clinically vulnerable people are having their second doses brought forward to eight weeks.
Why is a second dose needed?
The second jab, which will now be administered within 12 weeks of the first, acts as a booster to strengthen the immune response and increase the length of protection.
The first Pfizer jab provides 91 percent protection against Covid, according to professor David Salisbury, who was in charge of immunisation at the Department of Health until 2013.
He said: “If you look at the New England Journal of Medicine paper about the Pfizer vaccine... you give one dose and you get 91 percent protection, you give two doses and you get 95 percent. So you are only gaining four per cent for giving the second dose.
“With the current circumstances, I would strongly urge that you should use as many first doses as you possibly can for risk groups and only after you have done all of that come back with second doses.”
The Pfizer jab provides 95 percent protection after the second dose.
The Oxford jab has proven to be 62 percent effective after two doses, with a small group of people receiving 90 percent protection when half a dose and a full dose is provided.
Why is there concern?
Volunteers in both the Oxford and Pfizer trials were given two doses each.
The impact of stretching out the doses hasn't been tested during the trials and neither has mixing up the doses, which could see one person receive a Pfizer vaccine followed by the Oxford vaccine.
The JCVI said the mixing of vaccines would only happen in "exceptional circumstances", such as supply and availability.
What has the WHO said?
Yet the World Health Organisation (WHO) has warned against delaying the second dose of the vaccine.
Alejandro Cravioto, chairman of the WHO's Strategic Advisory Group of Experts on Immunisation (Sage), said the second dose should only be delayed in “exceptional circumstances”.
He said: “We deliberated and came out with the following recommendation: two doses of this [Pfizer] vaccine within 21-28 days.
“Sage made a provision for countries in exceptional circumstances of [Pfizer] vaccine supply constraints to delay the administration of the second dose for a few weeks in order to maximise the number of individuals benefiting from a first dose.”
And added: “I think we have to be a bit open to these types of decisions which countries have to make according to their own epidemiological situations.”