[1] UCL, Res Dept Infect & Populat Hlth, London WC1E 6JB, England
[2] Cent & North West London NHS Fdn Trust, Mortimer Market Ctr, London WC1E 6JB, England
[3] Publ Hlth England, Virus Reference Dept, 61 Colindale Ave, London NW9 5EQ, England
[4] Publ Hlth England, Ctr Communicable Dis Surveillance & Control CIDSC, 61 Colindale Ave, London NW9 5EQ, England
[5] Publ Hlth England, Modelling & Econ Unit, 61 Colindale Ave, London NW9 5EQ, England
[6] London Sch Hyg & Trop Med, Dept Infect Dis Epidemiol, Keppel St, London WC1E 7HT, England