The latest lockdowns have been a “success” but the UK is “still not out of the woods”, the head of the official statistics body has said.

Prof Sir Ian Diamond, the head of the Office for National Statistics (ONS), said there had been “very strong reductions everywhere”, although in the north-east and east of England the decline had “flattened off, potentially”, unlike in the south-west and south-east, which have continued with a decline in cases.

Diamond told BBC Radio 4’s Today programme: “I think this lockdown has been a success but at the same time, while we have seen major reductions, we are still relatively high. I’m in very much the view that we should do everything we can not to blow it nationally.

“We have done fantastically well in the last couple of months but we are not completely out of the woods yet.”

He added that it was “very difficult” to work out the difference between the lockdown impact and the effect the vaccine was having, but that it was clear both were working in reducing the numbers. There would also probably be a seasonal effect to the viruses’ prevalence.

But Prof Ravindra Gupta, a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said: “There will be people out there who have not been tracked and traced who have the variant and who may have transmitted it.

“We need to have redesigned our vaccines for the coming year with some of the key mutations present in those vaccines so actually we can prevent transmission of those variants if they were to take off in the coming months.”

A mystery carrier of the P1 variant of concern was found in Croydon, south London, after a team of 40 people spent days tracking them down, it was announced on Friday.

Gupta, an expert in clinical microbiology at the University of Cambridge, said it was always a possibility that the Manaus P1 variant could become dominant in the UK, but that it was unlikely at the moment due to low transmission rates.

There is growing concern over other variants that could be largely resistant to current vaccines.

But a scientist heading the body tracking new variants said she was “optimistic” about the UK “staying ahead” of the virus’ developments via adapted vaccines.

Prof Sharon Peacock, the head of the Covid-19 Genomics UK consortium, told the Times new variants were “very unlikely to send us back to square one”.

The UK has had by far the most deaths related to Covid in Europe, and there were at least 2,000 excess deaths from Covid each week over January and February.