The number of hepatitis A cases has increased to 13, after Public Health Wales confirmed it was investigating more in connection with the Glyn-Gaer outbreak.
The viral infection was initially detected at Glyn-Gaer Primary School, near Gelligaer, earlier in May.
The latest case concerns a second pupil at Ysgol Rhydywaun, in Rhondda Cynon Taf.
Heather Lewis, Consultant in Health Protection for Public Health Wales, said: “This new case is a pupil at Ysgol Rhydywaun, where there was already a previously confirmed case.
“Although hepatitis A is usually a mild illness in children, a safe vaccine to prevent it is available. To reduce the number of additional cases vaccination is being offered to all pupils and staff who may be at risk over the next week.
“We are confident that to date, all the public health actions needed to control this outbreak have been taken and that the risk to the public remains low.
“We appreciate any new case may cause concern but none of the new cases were unexpected and we will see further cases arising from this outbreak as the incubation period for hepatitis A is up to 50 days.”