Just under two thirds of pupils at a West Midlands primary school connected to a swine flu outbreak returned to school on Monday.
Welford Primary School in Handsworth, Birmingham, reopened after closing for half-term a day early due to a high number of absences caused by illness.
Another four cases of swine flu associated with the school were announced on Monday.
The number of cases connected with the 420-pupil school stands at 83.
The HPA said the school had been thoroughly cleaned during the week-long holiday and all confirmed cases were being treated at home.
A Health Protection Agency spokeswoman said she thought the absences were due to parents being cautious.
Pupils who caught the flu virus before half-term and are now well again are not infectious to others
Dr Sue Ibbotson, HPA
The spokeswoman said the HPA wanted to reassure parents that steps had been taken to slow down the spread of the illness.
Earlier, the HPA's regional director for the West Midlands, Dr Sue Ibbotson, said the virus had an incubation period of up to seven days and the school had been closed for 10 days.
'Not infectious'
"This means that pupils who caught the flu virus before half-term and are now well again are not infectious to others and can return to school," she said.
The flu virus does not survive more than a day on hard surfaces and less on soft surfaces, she added, so the school could reopen as planned.
The school will be monitored and anyone with symptoms such as a high temperature, coughs and headaches is being asked to stay at home and contact a doctor by phone.
Parent Nicole Medourie brought her children back to the school but said she was nervous about doing so.
"You don't know if it has fully gone from the school or if someone else has got it. But hopefully everything is ok," she said.
Welford Primary closed for the half term break on 21 May, a day earlier than planned, after a high number of pupil absences were noted.
The first swine flu case was confirmed later that day with the number of people affected across the UK now standing at 246.
BBC News at Ten
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Pupils who caught the flu virus before half-term and are now well again are not infectious to others
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