Children having their second dose of the swine flu jab may develop a high fever, European regulators have said.
Parents and doctors are being urged to keep watch on a child's temperature after research found they were more likely to suffer a fever above 38C (100F) after the second dose compared with after the first.
Children were also more likely to suffer soreness at the site of injection as well as drowsiness, irritability and loss of appetite after the second dose. British children under 10 in high-risk groups are being given two 0.25ml doses of the Pandemrix vaccine, half a normal adult dose. The doses are at least three weeks apart.
Experts at the European Medicines Agency (EMEA) said the "findings were unexpected, as they were not seen with the mock-up vaccine". But the Agency said the second dose boosted the child's immune response against swine flu.
More than three million healthy children aged six months to five across the UK will be offered the swine-flu jab after those in high-risk groups (such as with asthma, cancer or diabetes) have been inoculated. Pandemrix, made by GlaxoSmithKline, is being given out widely as the main vaccine; Baxter's Celvapan is reserved for people with an egg allergy.
The EMEA said: "Parents and carers of young children (below six) vaccinated with Pandemrix should be aware that fever may occur, and that this fever can be high (above 38C). They should monitor the child's temperature after each vaccination, and give a medicine such as paracetamol to control the fever as necessary.
"But the Agency noted that the second dose increases the immune response against pandemic influenza. The Agency has recommended that this information be included in the prescribing information, and be taken into consideration when deciding whether to give a second dose to children. This follows the review of new GlaxoSmithKline data from an ongoing clinical trial in children aged from six months to three years."
Some 1.6 million doses of the vaccine have been given in England so far, a rise of 600,000 in the past week. The Chief Medical Officer, Sir Liam Donaldson, has said under-fives are suffering particularly badly with swine flu, which can cause trouble with breathing. Under-fives are about three times as likely to end up in hospital with swine flu as those aged 16 to 64. Figures show an estimated 22,000 new cases of swine flu in England in the past week, down 46,000 the week before.