New coronavirus cases involving the Indian variant have more than doubled in a week as Boris Johnson said it could pose a “serious disruption to our progress” coming out of lockdown.

Data from Public Health England (PHE) published on Thursday shows a steep rise in cases associated with B1617.2, which has been designated as a “variant of concern”, from 520 last week to 1,313.

Speaking at a Downing Street press conference on Friday, England’s chief medical officer, Professor Chris Whitty, said the variant was more transmissible than the Kent variant of concern, which has now spread to become a dominant variant globally.

Mr Johnson said a range of measures were being taken including accelerating second doses of vaccines for people over 50 and the clinically vulnerable to eight weeks after the first dose, up from 12.

– How many cases have been detected in the UK?

There are 1,313 confirmed cases of the B1617.2 variant in the UK.

The cases are spread across the country, with the majority in the North West, mainly in Bolton, Sefton in Merseyside, Blackburn in Lancashire.

But there is a “realistic possibility” that the Indian coronavirus variant could be as much as “50% more transmissible” than the Kent strain, the Scientific Advisory Group for emergencies (Sage) has said, prompting concerns the variant could “dominate” over the others in the UK.

– What is being done?

Boris Johnson told a Downing Street press conference that second doses – which give people maximum protection against Covid-19 – will be brought forward from the planned 12-week interval to eight weeks.

First doses are also being prioritised for anyone eligible who has not yet come forward including the over 40s.

But Prof Whitty said local “surge vaccinations”, which would make first doses more widely available by offering them to all over-18s, was not being pursued because there is a “finite” supply of doses and that younger people were at less risk from the virus.

Mobile testing units have been deployed in Bolton and door-to-door PCR Covid-19 testing has been offered to 22,000 residents.

A vaccine bus has been set up to increase uptake among those who are eligible and a rapid response team of 100 nurses, public health advisers and environmental health officers has been sent in.

Mr Johnson said the army would be deployed on the streets of Blackburn and Bolton handing out tests to help surge testing efforts, while vaccination centres will extend their opening hours.

Surge testing has also been deployed in Sefton, after cases of the Indian variant were confirmed in the Formby area, as well as in parts of London.

– Will bringing forward second jabs help tackle the rise?

When the national vaccine programme began in the UK, officials took the bold decision to delay administering booster shots so that elderly and vulnerable people could more quickly receive their first shots.

Studies have since shown that giving the second dose 12 weeks after the first, instead of 21 days, can produce a stronger immune response.

Thus it remains to be seen whether bringing forward the date of the second dose will help curb rising infections.

Mr Johnson said: “I believe we should trust in our vaccines to protect the public whilst monitoring the situation as it develops very closely because the race between our vaccination programme and the virus may be about to become a great deal tighter and it’s more important than ever therefore that people get the protection of a second dose.”

– Is the B1617.2 variant driving the second wave in India?

On Thursday, India recorded 4,000 deaths and 343,144 new Covid-19 infections over the last 24 hours.

Scientists believe B1617.2 may be more transmissible than the UK variant (B117), which was first detected in Kent last year, and may be linked to the second wave in the country.

Professor Robert Dingwall, who is a member of Nervtag (New and Emerging Respiratory Virus Threats Advisory Group) – an expert committee which advises the Government on the threat posed by new respiratory viruses, said that while B1617.2 may be capable of becoming the dominant variant in the UK, the risk of a surge in deaths or hospital admissions remains low.

– Will vaccines still work?

At present, there is no evidence the B1617.2 variant is resistant to current vaccines.

Although deemed to be more transmissible, it does not feature the E484K mutation found in the South African variant of the virus, which could help the virus evade a person’s immune system and may affect how well coronavirus vaccines work.

Prof Dingwall told PA: “This variant seems to be better managed by the vaccines than the South African variant so that vaccinated people have only a very low risk of infection that is likely to be mild.”

– What other variants of concern have been identified in the UK?

All viruses undergo small genetic changes as they make copies of themselves in the host.

Most of these mutations are harmless but some can make the disease more infectious or threatening, and evade protection gained through infection or vaccination.

PHE has designated the UK, India, South Africa and Brazil (P1) variants as being “of concern”.

Current vaccines have been designed for earlier versions of coronavirus, but scientists believe they should still work, but may be less effective.

But experts are confident existing vaccines can be tweaked to better tackle emerging mutations.

The UK Government has a deal with biopharmaceutical company CureVac to develop vaccines against future variants, and has pre-ordered 50 million doses.