An unvaccinated worker infected with a COVID variant sparked an outbreak at a US nursing home where nearly all the residents had been inoculated, said a study out on Wednesday.

The dozens of cases, including 22 among fully vaccinated residents and staff, highlighted the importance of broad vaccine coverage and prevention measures, according to the research released by the Centers for Disease Control and Prevention.

During the March outbreak at a nursing home in the state of Kentucky, 46 cases were identified, with three residents dying, including two who were not vaccinated, the study said.

The spread was traced back to a worker who had symptoms and was not vaccinated. The variant was R.1, which is "not currently identified as a CDC variant of concern or interest," the paper said. 

While the researchers noted the flare up of the disease showed the strong effect the vaccine had in preventing symptoms in the sick, they also noted its limits. 

"This underscores the importance... that all persons, including those who have recovered from COVID-19, be vaccinated," the authors wrote. 

"A continued emphasis on strategies for prevention of disease transmission, even among vaccinated populations, is also critical," they added.

Their findings, released alongside a study of a similar outbreak at a Chicago nursing home, pointed to the results of vaccinated and unvaccinated people mixing.

Though 90 percent of the 83 residents at the Kentucky home had received their doses, only half of the 116 workers had been inoculated by the time the outbreak was spotted.