LONDON—The pharmaceutical developer Novavax says it has asked the European Medicines Agency to extend the authorization of its coronavirus vaccine to children aged 12 to 17 amid a surge of disease across the continent. In a statement on Thursday, Novavax said its request is based on data from research in more than 2,200 adolescents aged 12 to 17 in the U.S., which found its vaccine to be about 80 percent effective against COVID-19. The study was done when the delta variant was the predominant virus in the U.S. The main side effects reported were pain at the injection site, headache, and tiredness. The EU drug regulator gave Novavax’s two-dose COVID-19 vaccine for adults the green light in December; the shot has also been cleared by Indonesia, Australia, and the World Health Organization, among others. The EMA has previously OK’ed vaccines made by Pfizer-BioNTech and Moderna for use in children from …