The second dose of the swine flu vaccine currently being given to children under 10 may result in a high fever, according to research.
European regulators have urged parents and doctors to monitor their child's temperature, in case it rises above 38C (100F) after a second injection.
As well as fever, the second dose has also been found more likely to cause soreness in the arm, drowsiness, irritability and loss of appetite.
The vaccine is being given to children under 10 in high-risk groups. They are getting two 0.25 ml doses of Pandemrix, half the normal amount, at least three weeks apart.
It will then be offered to three million children aged from six months to five, across the UK.
Experts at the European Medicines Agency (EMEA) said the "findings were unexpected as they were not seen with the mock-up vaccine", but added that the second dose boosted the ability to fight off swine flu.
Pandemrix, made by GlaxoSmithKline, is being given out widely as the main vaccine, while Baxter's Celvapan is reserved for people with an egg allergy.