Half of all care homes in Bournemouth, Christchurch and Poole have outbreaks of coronavirus, health bosses say.
About 15% of all cases in the council area (BCP) are now from care settings, according to the county's director of public health.
Sam Crowe said two-thirds of infections in the over 60s were from care homes, resulting in an "extremely concerning" number of cases in that age group.
Overall, cases in BCP are decreasing but "not fast enough", Mr Crowe said.
He said: "In the last week, we've had about 3,000 cases, around 750 of those would have been in people over the age of 60 and of that 750, there are 450 connected with our care providers.
"This time around, instead of one or two positive cases, the homes are finding, suddenly, it's gone through many, many residents and many, many staff and much, much quicker."
On Thursday, University Hospitals Dorset NHS Trust said Bournemouth and Poole's hospitals were "absolutely full" and due to transfer some patients to a Nightingale Hospital in Exeter.
Mr Crowe said the number of cases in BCP "ramped up very, very quickly over the holiday period" because of "social mixing".
He said: "We all have to remember, while we still have high levels of Covid circulating in the community, that feeds through very, very quickly into cases in more vulnerable sections of our community and contributes to the pressure in our hospitals and care system.
"So if we don't turn the tap off by getting our cases down, then that sector is going to be under pressure for some time to come."