1 Immunisation and Countermeasures Division, National Infection Service, Public Health England, 61 Colindale Avenue, London NW9 5EQ, United Kingdom. Electronic address: sema.mandal@phe.gov.uk.
2 Immunisation and Countermeasures Division, National Infection Service, Public Health England, 61 Colindale Avenue, London NW9 5EQ, United Kingdom.