Back in December, just as millions of families in Britain were preparing to embark on much-awaited Christmas festivities, news intensified of a new Covid-19 variant that appeared to be much more transmissible than others in circulation.

The strain first identified in Kent ultimately dashed hopes of a gradual relaxation of restrictions in the new year, and was blamed for the devastating second wave of the virus that hit the UK in the winter.

Now, a week after the country reported some of the lowest infection rates since the summer of last year, the rapid spread of a family of variants first discovered in India is threatening to do much the same.

“This new variant could pose a serious disruption to our progress and could make it more difficult to move to ‘Step 4’ in June,” UK prime minister Boris Johnson said at a press conference on Friday.

From Monday, England will move to the next phase of its road map out of lockdown with pubs and restaurants allowed to serve customers indoors and other indoor entertainment venues such as cinemas allowed to reopen.

But the decision to speed up the vaccination programme — reducing the gap between first and second doses of the vaccine to just eight weeks — and the prioritisation of all those over the age of 40 who have not yet come forward for vaccination is a sign of how seriously the government is taking the spread of the Indian variants.

The government had already announced on Thursday that surge testing and enhanced contact tracing will be rolled out in hotspots where the variant had been found.

But even as public health officials adapt their plans to contain its spread, scientists and experts in the genetic make-up of viruses are still trying to assess very limited data on the characteristics of the main strain that is causing concern in Britain — B.1.617.2.

Evidence does suggest the variant is growing much more rapidly than any other imported strains, or the UK’s own “variant of concern”, B.1.1.7, (the ‘Kent’ variant) when it first emerged in December.

There have been 1,768 sequenced cases of B.1.617.2, according to the latest data from the Covid-19 Genomics UK Consortium. Cases are clustered around Bolton, Blackburn with Darwen, Bedford and London and some parts of the Midlands, where the number of cases associated with other strains has continued to fall.

England’s chief medical officer Chris Whitty confirmed on Friday that B.1.617.2 is more transmissible than the Kent variant but added that it was not yet known whether it was significantly more transmissible.

And even if it is more easily spread, scientists do not yet know whether the increase in cases will translate into greater hospitalisations and more severe illness. Scientists also want to know whether the variant is able to escape either natural or vaccine-induced immunity.

“This is a data-free zone,” said one senior public health official. “I am concerned but there’s no reason to change what we’re doing at the moment.”

Chart showing that there are early signs that B.1.617.2 may be pushing the national trend in cases back upwards

“If it is more transmissible, there is a possibility it will replace B.1.1.7. They’re in an arms race with one another,” the public health official added.

Underpinning nervousness about the variants is modelling released by government advisers this week that presented a sombre picture of the possible repercussions of the emergence of a more virulent or vaccine-resistant strain of Covid-19.

A variant that is as transmissible as the B.1.1.7 strain first identified in the UK, and which has some vaccine resistance, “could easily lead to another peak of double or treble the size of that seen in January 2021 if no interventions were taken”, according to the paper released by the UK’s Scientific Pandemic Influenza Group on Modelling (SPI-M). It concluded that “variants remain a major risk to the road map” out of lockdown.

Chart showing that according to one model, a much more transmissible variant could produce a higher peak in hospital admissions than either of the UK’s two waves to date

But the public health official warned that the early models of this kind “should not be taken as gospel”, in part because “we’d never allow it to get that far”.

The good news is that a growing body of scientific study does suggest that the existing crop of vaccines are able to “neutralise” the B.1.617 family of variants, and protect against severe disease, although there may be a marginal increase in infections compared with the strain first found in Kent.

That said, most of this lab work has focused on B.1.617.1, the variant first identified in India, with limited data available specifically on B.1.617.2 which has its own unique characteristics.

A paper released by academics in India on Thursday found that recipients of the Serum Institute’s Covishield vaccine did produce antibodies when confronted with the B.1.617.1 variant, and concluded that “vaccine-induced antibodies are likely to be protective to limit the severity and mortality of the disease in vaccinated individuals”.

Chart showing that the share of cases and deaths among the elderly is falling in parts of India, suggesting the vaccines are providing some protection against B.1.617.2

But as fears rise that this variant poses the biggest threat yet to a return to international travel and unfettered social interactions, attention is also turning to whether UK officials did enough to heed warning signs from India and nip the strain’s spread in the bud more quickly.

The most glaring area of inaction was the decision not to impose any limits on travel between India in the UK. The decision not to place India on the UK’s red list meant that hundreds of cases were imported last month which seeded the bigger community outbreaks happening now. India was added to the UK’s red list last week.

With surge testing already under way in those areas where the variant is more prevalent, the question now is whether health officials should introduce other measures such as vaccinating everyone over the age of 18 in parts of the country where the variant is spreading so rapidly.

Chart showing that cases are rising again in north-west England where B.1.617.2 is circulating, but that rise is concentrated among younger, less vaccinated age-groups

Muge Cevik, a clinician and researcher in virology at Scotland’s University of St Andrews, said the idea of offering “surge vaccinations” to younger people in variant hotspots “makes a lot of sense”.

“If you look at the numbers of B.1.617.2 — it’s spreading in unvaccinated populations so you’ll be reducing transmission,” she said. Imposing harsher restrictions like localised lockdowns, however, “will probably need to be a last resort”.

“Going forward we need to think about what is a proportionate response,” she added.