The Army will be sent to hotspots worst-hit by the Indian variant of coronavirus under a "surge vaccination" plan to protect the vulnerable, the Prime Minister has announced. Boris Johnson has also declared that second doses of the coronavirus vaccine for the over-50s will be accelerated across the country. Speaking at a televised Downing Street press conference on Friday evening, he said that the gap between vaccines will be reduced to 8 weeks to provide additional protection to the most vulnerable as rapidly as possible. The Joint Committee on Vaccination and Immunisation (JCVI) is thought to have made the recommendation earlier on Friday to change its guidance, cutting by a third the length of time previously left between jabs, which was 12 weeks. Scientists have been forced to weigh up the benefit of getting those most at risk from the virus fully vaccinated sooner, with the higher level of effectiveness thought to be gained by delaying second doses by up to 12 weeks. New coronavirus cases involving the strain known as B1.617.2, which has helped fuel India’s devastating outbreak, have more than doubled in a week in England. London and the North West have seen the biggest rise in cases of the variant. A Government source told The Telegraph its growing spread was a "concern" and warned that scientists were still not certain about how transmissible the strain is, nor how effective vaccines are against it. Mr Johnson confirmed he will proceed with step three of his roadmap out of lockdown on Monday as planned, at which point six people or two households will be allowed to meet inside. Acknowledging the increased risk from this new variant, he said there was no evidence at present to suggest the vaccines will be less effective against it, but said the situation would continue to be monitored. The military, led by Colonel Russ Miller, will be deployed to Bolton and Blackburn with Darwen, the areas worst-hit by the new variant, to support local leaders in managing the response, Mr Johnson revealed. This will include a vaccination surge for second doses, and targeted new activity to accelerate vaccine uptake among eligible cohorts. Increased surge testing will also be rolled out. While the Government decided to change its advice on the gap between doses, it rejected other calls for the vaccine to be offered to all adults over 18 in Indian variant hotspots. Earlier in the day Nadhim Zahawi, the vaccines minister, explained the rationale against awarding jabs to younger age groups in the worst-hit areas. He told the BBC it takes three weeks to build protection from a first dose and to have any effect on transmission of the virus. His explanation raises concerns that the surge vaccination strategy may not take effect quickly enough to curb a major outbreak. In Bolton, which has a particularly high rate of the Indian variant, the leader of the council called for the jab to be offered to young people in the area. David Greenhalgh told BBC Radio 4's The World at One: "The vast majority of our cases are in their teens, 20s and 30s at the moment. "If we can get vaccinations to (those aged) 16-plus, which are licensed by Pfizer, then it will make a total transformation of transmission as it moves forward." He confirmed there had been talks between council leaders and the Government about surge vaccinations, describing the discussions as "very, very constructive". "This is an issue of capacity but… all the soundings are is that they are looking to progress that as soon as possible," he said. More vaccine doses have been sent to Bolton, while 800,000 PCR tests have been sent to 15 separate areas of England, including parts of London and Merseyside. London and the North West have seen the biggest rise in cases of the variant, with Public Health England (PHE) data showing it has been responsible for four deaths as of May 12. Blackburn with Darwen Council said on Thursday that it would be offering vaccines to all over-18s from next week following the increase in cases, but later said that although additional vaccine clinics are being set up, the jab will only be offered to those eligible under current Government guidance. It is understood the NHS asked the council to remove the tweet, as advice on vaccine prioritisation had not changed. Greater Manchester Mayor Andy Burnham also weighed in, calling for "extra vaccine supplies" to be extended to "the younger working-age population, the student population". He added: "That is what is needed if we are to make the most decisive and effective intervention into this situation that we can right now. "We recognise the pressure on vaccine supplies all over the country, but we have been moving at a pace where we have been treating all areas equally, and I think the time has now come to recognise areas with the highest case rate do need to be able to move more quickly down the ages." Bedford Borough Council has also called for vaccines to be made available for over-16s in the face of the variant. In the Formby area of Sefton, new drive-through and walk-through test centres were set up on Friday, specifically to identify the Indian variant. The latest case rate in Sefton was 53.9, up from 26 the previous week, with 149 new cases. Measures have also been brought in elsewhere, including in parts of London. Hounslow is the London borough with the highest rate at 48.2 per 100,000 people in the seven days to May 9, with 131 new cases.