Officials are "proactively managing" the spread of the infection, adding the "general risk is low, even if the disease itself is extremely serious", Health Secretary Wes Streeting told the BBC.
Two people have died - a 21-year-old university student and Juliette, a sixth form pupil.
Susan Hopkins, chief executive of the UK Health Security Agency, said there was an "explosive nature" to the outbreak, while deputy chief medical officer Dr Thomas Waite said it was the quickest growing outbreak he had seen in his career.
A targeted vaccine programme for 5,000 students is being set up at the University of Kent in Canterbury.
