Director of Public Health Greg Fell says 70 per cent of all cases in Sheffield are the Kent variant but new mutations from South Africa and Brazil are a worry.
Mr Fell told a council meeting: “The Kent variant is basically now the dominant strain, it’s good in a fight and it’s taken over the original virus.
“It’s more transmissible, possibly more lethal if you look at the case fatality rates. The data on that aren’t strong but certainly there are some signals there.
“And there are two other variants of concern, one is the South Africa variant, second is the manaus variant from Brazil. They are not in Sheffield yet, but it’s a matter of time and when, not if, sadly.
“Both of them are deeply concerning, because we have great concerns about the vaccine effectiveness in both of those variants and about the possibility of people being reinfected who already have antibodies to the initial virus.
“The manaus is deeply worrying because prevalence is high and there are many tens of thousands of people who are now being reinfected which suggests the antibodies to the first bout will not protect you against the manaus. The South Africa variant is somewhat similar, slightly less serious.”
Mr Fell said the danger was other new variants could “undo all good work that has been done”.
“The nightmare scenario is a variant emerges that is more transmissible, we’ve seen what that can do; more dangerous and vaccine resistant, that’s something that would undo all of the good work.
“It’s in all of our interest to keep viruses circulating as low as humanly possible. Vaccine resistant variants could emerge anywhere in the world, and then travel anywhere in the world really quickly, so nobody’s safe until we’re all safe.”
The government has identified 105 cases of the South Africa variant, including two in Scarborough.