Eleven people have been diagnosed with measles in Lebanon County, Pennsylvania health officials said Wednesday.

After three people with measles were hospitalized in the central Pennsylvania county, state Department of Health staff conducted contact tracing in the area, located east of Harrisburg and north of Lancaster.

They identified several more Lebanon County residents with “mild” cases of the highly contagious virus, which can sicken nine in 10 unvaccinated people if they’re exposed.

Of the 11 residents with measles, 10 were unvaccinated; state officials couldn’t confirm the vaccination status of one patient. None of the patients are infectious anymore, health officials said.

Health officials did not share details on when the first cases were identified, and officials did not identify specific communities affected, citing privacy concerns.

The cases follow an outbreak in nearby Lancaster County this winter that sickened eight county residents and infected some in neighboring counties. In all, 23 cases of measles have been identified in Pennsylvania this year. In all but one case, the people infected were unvaccinated.

Health officials stressed that the most effective protection against measles is a vaccine. On Tuesday, health department staff vaccinated 26 people at a special clinic in Lebanon.

Measles vaccination rates have declined across the country in recent years, and vaccination rates for kindergarteners in Lebanon County were at 93.2% last school year, just under the 95% vaccination threshold required to prevent community spread of the virus.

Measles causes a rash, high fever, cough, and a runny nose. Serious complications can include pneumonia, brain swelling, and death.

Unvaccinated people are particularly at risk: one in five will be hospitalized from measles. About one to three of every 1,000 children with the virus die, according to the Centers for Disease Control and Prevention.

Pennsylvanians can get the measles, mumps, and rubella (MMR) vaccine at primary care or pediatric practices, as well as at federally qualified health centers and rural health centers.

The vaccine is available for free at state health centers for under- and uninsured children, as well as children who receive Medicaid, aged 18 and younger.

Uninsured and underinsured adults can also get free MMR vaccines at state health centers.

Anyone who believes they may have had measles in the past month should call the state Department of Health toll-free at 877-724-3258.