LONDON (AP) -- The European Medicines Agency warns that young children given GlaxoSmithKline's swine flu shot may get a fever after their second dose.

In a statement issued Friday, the European drug regulator said data from GlaxoSmithKline PLC showed a higher number of children aged six months to 3 years had a fever after their second dose of the Pandemrix vaccine, compared with the first. Kids were also more likely to have side effects like muscle pain, drowsiness, and irritability.

The European regulator recommends children get two doses of swine flu vaccine, though Glaxo says one dose is enough.

Glaxo's vaccine contains an adjuvant, a chemical compound to boost the immune response. It is sold across Europe and Canada.

Another formulation of Glaxo's vaccine, without the adjuvant, is available in the U.S. Vaccines with adjuvants usually cause more side effects. No flu vaccines with adjuvants are licensed in the U.S.

Last month, Glaxo advised health authorities not to use one batch of its Canadian-manufactured swine flu vaccine in case it triggered life-threatening side effects like anaphylactic shock.

In a press briefing on Thursday, the World Health Organization's flu chief Keiji Fukuda said that more than 150 million doses of swine flu vaccine have been distributed in more than 40 countries and that they had not seen any evidence to suggest the vaccines caused worrying side effects.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.