Almost 100 children at a school in Montgomery County received an incorrect dose of the coronavirus vaccine at a clinic last week, the county health department said Monday.

The parents of the 98 students at South Lake Elementary School in Montgomery Village were being notified Monday night that the incorrect doses were administered at a clinic at the school Wednesday, the health department said.

The doses of Pfizer vaccine given at the clinic were diluted more than recommended, according to the health department. A makeup clinic will be held at the school Nov. 17 to give the affected children an additional dose.

The health department said it learned Saturday about the dilution. The recommendation to administer an additional dose as quickly as possible came after consultation with Pfizer and the state health department, the county officials said.

James Bridgers, the acting County health officer, expressed confidence that the dilution was an “isolated situation.” He said the staff had already been given more training on preparing and administering children’s doses.

Since the federal government approved a children’s vaccine, almost 20,000 children ages 5-11 have been vaccinated at clinics sponsored by the county school system or by physicians and pharmacies, the county health department said.

Even at standard concentrations, the children’s dose approved by the Food and Drug Administration is normally lower than that used for recipients 12 and older. The standard children’s dose is 10 micrograms compared with the 30-microgram dose approved for older recipients.

The county department said more pediatric doses had been administered in Montgomery than in any other Maryland jurisdiction.

It was not immediately clear how the mistake was made, or how much the vaccine was diluted. In its statement, the health department reported no ill effects on children.