Stricter coronavirus rules are likely to remain in place in South Yorkshire for more than 28 days, according to Sheffield's director of public health.
More than 1.4m people in the county will be living under tier three restrictions from 00:01 on Saturday.
The restrictions must be reviewed after 28 days but Greg Fell said he feared four weeks "will not be long enough".
Communities Secretary Robert Jenrick has said restrictions will remain "as long as they are absolutely necessary".
In Barnsley the infection rate in the seven days to 19 October was 486 cases per 100,000 people, in Sheffield 415, in Rotherham 407 and in Doncaster 393. The average area in England had 117.
Mr Fell said: "I think the numbers would have to get an awful lot lower than they are now for us to be moved back in to tier two.
"The numbers are [currently] quite high and they are going up so we've got to turn that corner and get them going back down again.
"I hope I'm wrong but I sense it will take more than a few weeks for the numbers to get down to a low level."
Julia Burrows, Director of Public Health Barnsley, said it was too soon to say when the town would be ready to move out of Tier 3, and it would "depend on people adhering to the new guidelines".
Meanwhile, in a letter to South Yorkshire residents, Sheffield City Region Mayor, Dan Jarvis MP, urged people to "do their bit" and stick to the new restrictions.
The tier three alert level means people cannot mix with other households, and pubs and bars will be closed - unless they are serving substantial meals.
In South Yorkshire businesses such as betting shops, adult gaming centres, casinos and soft play centres must close and gym classes will not be allowed, although gyms can remain open.
Households are also banned from mixing outdoors in private gardens or anywhere inside and people are advised against travelling into or out of the area.
After the tighter restrictions for South Yorkshire were announced on Wednesday, Mr Jenrick said: "The restrictions we have agreed together will only be in place for as long as they are absolutely necessary. They will be reviewed jointly in 28 days time.
"The government is totally committed to working with local leaders as we tackle this challenge, for the benefit of all the people of South Yorkshire."