Frustration grows amid measles outbreak quarantines across several states

The measles outbreak among children in South Carolina is still growing, even though a small number of the quarantined school kids have been allowed back in class.

South Carolina's measles outbreak has grown to 15 cases, state health officials reported Friday, a small increase from a few days ago.

The new cases are among those who'd been exposed to the virus at school, but developed symptoms in quarantine. It's not known if they're adults or children, but 139 students are still currently in quarantine in the state.

"We are fortunate that those we have identified as being exposed to measles are complying with our guidance to quarantine for the period that they could not expose others," Dr. Linda Bell, epidemiologist for South Carolina, said in a statement to NBC News.

Those who aren't vaccinated against measles are recommended to stay home, away from others for a 21-day quarantine. That’s how long it can take to develop measles symptoms — high fever, red eyes and a rash — after being exposed.

"We recognize that quarantining is challenging for families and communities, and we continue to strongly encourage getting vaccinated, which would make quarantining unnecessary due to the MMR vaccine’s proven effectiveness for decades," Bell said, referring to the measles-mumps-rubella shot.