A Covid-19 outbreak at Lincoln Prison is partly to blame for high levels of cases in the city, health bosses have said.
Lincoln recorded 479 cases per 100,000 people in the week between 4 and 10 December - one England's highest.
The county's assistant public health director Tony McGinty said the prison was among a number of specific outbreaks recorded recently.
These were "part of the reason why the numbers are quite high", he said.
Mr McGinty said the current outbreak meant rates in Lincoln also needed to be seen in context.
"When you are calculating rates per 100,000 people in cities the size of Lincoln, which has only got about 100,000 people, it doesn't take that many more additional cases to bump the rate up.
"We are seeing more cases than I want to see, but the effect on the rate is bigger in smaller places," he added.
He said in Lincolnshire as a whole rates were also above average, with 243 cases per 100,000 people, compared with about 181 nationally.
"Some of it is to do with outbreaks we are seeing in bigger settings, and I think the situation in Lincoln isn't helped by the fact [it] is very busy now at this time of year," he said.
Mr McGinty said he was not expecting any change to the county's current tier three status following a government review on Wednesday.
A Prison Service confirmed there were multiple cases at the prison, which holds more than 700 inmates, but did not say how many, according to the Local Democracy Reporting Service.