Sheffield's health leader is expecting concerning new Covid-19 variants in the city.
Greg Fell, director of public health, said it was a matter of when, not if variants first recorded in South Africa and Brazil were found in Sheffield.
Mr Fell said: "The Kent B117 variant is the dominant strain. In Sheffield about 75% of all cases are B117, it is definitively more transmissible, that will have and upward impact on the seven-day infection rate. It's difficult to know where that will settle.
"There is some evidence that it is more serious in terms of the individuals infected, there is not robust data but certainly enough to give us pause for thought.
There are two other significant variants of concern. One is the South Africa variant, one is the Brazil variant - neither of which are in Sheffield that we know of at the moment. That is probably a matter of when, not if and both of those are of significant concern.
"The control measures are exactly the same, the issue is lapses and the need to be vigilant.
"The South African and Brazilian variants aren't thought to be particularly more dangerous to individuals, but the point is that antibodies from either the vaccination or from prior infection with a previous virus might not protect us as well, which is why they are both of concern."
He also said case rates and hospital activity were falling but warned of still a long journey towards returning to life without restrictions.
Two cases of the South African variant have now been detected in Leeds, leading to Coronavirus surge testing te be deployed.
What is surge testing?
Surge testing is increased and targeted coroanvirus testing used by the governmnet to try and detect new Covid-19 variants.
This includes door-to-door testing in some areas and enhanced contact tracing in specific locations in England.
It also involves testing people who do not have any symptoms.
Why is surge testing being used?
This is to identify different Covid-19 variants and mutations which cannot be traced back to international travel.
Other locations where surge testing is being usedAccording to the government, additional surge testing is being carried out in the following local authority areas:London
London Borough of Ealing (specific areas in and near to the W7 postcode area)
London Borough of Haringey (specific areas in and near to the N17 postcode area)
London Borough of Merton (Pollards Hill)
London Borough of Lambeth (specific areas in the SE21 and SW16 postcodes)
Metropolitan Borough of Sefton (specific areas in and near to the PR9 postcode)
Manchester City Council (specific areas in the following postcodes: M14, M15 and M16)
Hampshire County Council (specific areas in the RG26 postcode)
Bristol City Council (specific areas in the following postcodes: BS1, BS2, BS3, BS4, BS5, BS6, BS8, BS9, BS14 and BS16)
South Gloucestershire Council (specific areas in the following postcodes: BS16 and BS37)
Birmingham City Council (specific areas in and near to the B31 postcode)
Walsall Council (specific areas in and near to the WS2 postcode)
Worcestershire County Council (specific areas in and near to the WR3 postcode)
Yorkshire and the Humber
Middlesbrough Council (within parts of the TS7 and TS8 areas of Middlesbrough)