Soaring cases of the COVID Indian variant have forced the government to consider bringing forward vaccine second doses in some areas.
The moves came after fresh data showed that variant infections had more than doubled in the past week.
Public Health England (PHE) on Thursday said there are 1,313 cases in the UK, up from the 520 recorded the previous week.
In response to the rise, the government has announced a range of new measures – and is considering shortening the 12-week gap between vaccine jabs.
A statement from the Department of Health and Social Care said that "most cases are in the North West of England, with some in London".
Surge testing, increased genomic sequencing and enhanced contact tracing have all been introduced in affected regions.
Earlier, Prime Minister Boris Johnson earlier said he was "anxious" about the variant, and was "ruling out nothing" to tackle it.
The strain was designated a "variant of concern" last week by PHE, when cases again more than doubled, from 202 to 520 in the previous seven days.
Some 100 extra nurses have been sent to Bolton to help with the situation there.
The COVID-19 strategic response director at PHE, Dr Susan Hopkins, said: "Cases of this variant are rising in the community and we are continuously monitoring its spread and severity to ensure we take rapid public health action.
"We need to act collectively and responsibly to ensure that variants do not impact on the progress we have all made to drive down levels of COVID-19 and the increased freedom that brings.
"That means you should pay attention to and act on the local health advice in your area. Testing and isolating when required not only limits spread, it helps us to better understand how the variant behaves in the community which is vital to taking effective and proportionate action moving forward.
"If you’re asked to take a test please do. The way to limit the spread of all variants is the same. Keep your distance, wash your hands regularly and thoroughly, cover your nose and mouth when inside, keep buildings well ventilated and meet people from other households outside."
Health Secretary Matt Hancock said: "We are monitoring the situation very carefully and will not hesitate to take further action if necessary.
"It is imperative we all continue to be vigilant, and if you live in one of the 15 areas where we’ve introduced surge testing, make sure you get a free PCR test. And everyone who’s eligible needs to come forward and get a jab."
Boris Johnson said the COVID-19 variant "has been spreading" and the UK wants to "grip it".
"We want to make sure that we take all the prudential, all the cautious steps now that we could take," he said.
"So there are meetings going on today to consider exactly what we need to do."
The PM’s spokesman said there were "no plans" to reintroduce the tiered system of virus restrictions used in England towards the end of last year.
Despite concerns about the Indian variant, the PM said he "can see nothing that dissuades me from thinking we’ll be able to go ahead on Monday and indeed on 21 June everywhere" in terms of easing coronavirus restrictions in England.
Next Monday will see step three of the country’s roadmap out of lockdown come into effect.
Pubs and restaurants will be able to reopen indoors and people will be allowed to mix indoors as two households or under the rule of six.
People will also have the choice whether to socially distance with close family and friends they meet up with.
On 21 June, step four, the government wants to end "all limits on social contact".