The flu season is now under way, health experts have warned, with the number of cases in school children doubling in just one week.
GP consultations for flu-like illness among five to 14-year-olds increased by 50 per cent in the last week, with rates rising from 31 to 47 cases per 100,000 people.
Flu rates among the general population also rose from 14 to 23 cases per 100,000 people, with the highest rates occurring in England and Wales.
Forty five outbreaks of flu were reported to the HPA last week, with 43 occurring in primary and secondary schools.
Dr Richard Pebody, head of seasonal flu surveillance at the HPA said: "We are seeing an increase in flu activity mainly among school children indicating the start of this year’s flu season."
He urged people at higher risk of developing complications from flu, including pregnant women, to get vaccinated.
"Flu vaccination is still the most effective way of preventing flu and it is not too late to get it so we would encourage all those who are in ‘at risk’ groups to get vaccinated as they are more vulnerable to developing complications from flu.
"These include people with underlying conditions such as heart problems, diabetes, lung, liver or renal diseases and those with weakened immune systems, as well as older people and pregnant women."
Typical flu symptoms include sudden onset of fever, cough as well as sore throat, aching muscles and joints.
The best treatment for flu in healthy people is to stay at home, rest, drink plenty of fluids and take pain relievers such as paracetamol, said the HPA. Children under 16 should not be given any medicines containing aspirin.
Doctors have been given the go-ahead to prescribe anti-viral drugs for those at risk of complications.
Dr Pebody added: "Although unpleasant, flu is a self-limiting illness and if you have flu it is best to stay at home until you are well. If people in at risk groups develop symptoms consistent with flu, or if anyone’s symptoms persist or become more severe, we advise they seek medical advice."
Good cough and hand hygiene, such as covering your nose and mouth with a tissue when coughing and sneezing, disposing of tissues as soon as possible and regularly cleaning hands can help prevent the spread of flu virus and reduce the risk of transmitting the disease.
For more information on flu, or to take part in the annual flu survey: flusurvey.org.uk