A school in Southampton has been closed for at least two days after 56 staff members called in sick.
Hundreds of pupils at Redbridge Community School have also fallen ill in a suspected flu outbreak.
The secondary school, which has more than 1,000 pupils, was shut after half of its staff became ill.
Headteacher Jason Ashley said the outbreak began on Wednesday when 62 pupils were sent home.
More pupils and teachers succumbed to the “incredibly virulent” strain the next day, he added.
The decision was taken to close the school on Friday and Monday after Public Health England (PHE) recommended the premises be deep-cleaned to prevent the spread of the virus.
Mr Ashley told the BBC: “I’ve been in the profession for over 24 years and I’ve never seen a virus spread so quickly around a school. It was like a domino effect.”
Speaking to local newspaper the Southern Daily Echo, he added that the decision was also taken due to staffing levels because a “disproportionate” number of employees became infected.
Another school, Portesham Primary in Dorset, closed on Tuesday and was due to reopen on Monday following a “serious outbreak of a flu-type virus”.
Eight flu outbreaks were reported in schools in the week ending 13 January, according to PHE.
