Shamez N Ladhani1, Ray Borrow2, Nick J Andrews3. 1. Immunisation Department, Public Health England, London NW1 5EQ, UK. Electronic address: shamez.ladhani@phe.gov.uk. 2. Meningococcal Reference Unit, Public Health England, London NW1 5EQ, UK. 3. Statistics, Modelling and Economics Department, Public Health England, London NW1 5EQ, UK.