Acambis, the British vaccine maker, agreed a £276m takeover by Sanofi-Aventis, the French pharmaceuticals group. The cash offer of 190p per share represents a 64 per cent premium to Acambis’s closing price.

Sanofi Pasteur, the French company’s vaccines arm, was already collaborating with Acambis on commercialising three vaccines – against dengue, West Nile virus and Japanese encephalitis.