The outbreak began at Welford Primary School last week
Fifty children and adults being treated for swine flu in an outbreak linked to a primary school are responding well to treatment, health officials say.
Forty-five of the cases linked to Welford Primary School, in Handsworth, Birmingham, are children, the Health Protection Agency (HPA) has said.
It is the single largest outbreak of the virus in the UK.
The HPA stressed it was a "very mild illness" in "the very vast majority of cases" but the source was unknown.
The 420-pupil school is closed for the half-term holiday. It closed on Thursday, a day earlier than scheduled, after a higher than normal number of illnesses was noted.
Searching for source
A statement issued by the school at the time said the school would have a "deep-clean" during the holiday.
The relative of a Birmingham pupil with confirmed swine flu said her sisters and brothers had probably also been infected
The HPA said all the patients were being treated at home with anti-viral drugs.
Sue Ibbotson, from the HPA, said: "Those who have had symptoms have been and are being given anti-virals both for the pupils and any contacts in the family who are poorly.
"We've also offered anti-virals to people who are not sick in an attempt to stop the spread to them."
She added the HPA was still investigating how the virus got into the school.
She said: "We haven't ruled out that the most likely cause is travel related, that somebody has been somewhere where the virus is around and about.
"We haven't found that yet, but we're still looking."
The school was informed on Thursday evening that one of those taken ill was a confirmed case of swine flu.
The HPA said it invited all parents of pupils without symptoms into the school on Saturday and Sunday to collect anti-virals for their children.
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