Four prisoners in Meru County were among the 688 Kenyans found to have Covid-19 as announced on Saturday.
County Public Health Director John Inanga said three were at Kangeta and one at Meru GK Prison.
They have been isolated within the facilities as public health officials look into the possibility of their transfer to Meru Teaching and Referral hospital.
"We tested all inmates. Some of the patients were imprisoned recently while another has been in prison for some time. We are also trying to get the history of the inmates to establish how they contracted the virus," Dr Inanga said.
With the four new cases, the total number of patients in isolation in Meru rose to nine.
Five patients are from North Imenti, one from Igembe South and three from Igembe Central.
Two of the patients are restaurant workers in Meru and Maua towns.
Meru Health executive Misheck Mutuma said more than 40 restaurant workers, who were in close contact with them, were quarantined.
Dr Inanga said 22 eatery workers in Meru town were taken into quarantine at Kenya Medical Training College (KMTC) as they could not self-isolate.
One tested positive on Wednesday, hence the need to observe the primary contacts, he noted.
Mr Mutuma said about 20 restaurant workers in Maua were under self-quarantine as one of them tested positive this week.
Meanwhile, the Meru health department has raised the alarm over residents' failure to observe Covid-19 containment guidelines amid increasing cases of local transmission.
Mr Mutuma cited traders, matatu and boda boda operators as some of those blatantly breaking the rules by operating without masks and carrying too many passengers.
"It is worrying that people are not ready to take personal responsibility. We may be forced to take stricter measures to contain the disease," he said.
He noted that by Saturday 11 people who died of Covid-19 in Nairobi and other counties were buried in Meru.