HIB VACCINE EFFICACY - THE UNITED KINGDOM (UK) 'NO BOOST' EXPERIENCE

J. McVernon, P.T Heath, M.PE Slack, M. Ramsay, E.Richard Moxon

Oxford Vaccine Group, Oxford, United Kingdom., StGeorge's Hospital Medical School, London, UK., PHLS Haemophilus Reference Laboratory, Oxford, UK., Communicable Disease Surveillance Centre, Colindale, London, UK

The UK is relatively unique in having implemented a 3 dose primary course of Haemophilus influenzae type b (Hib) conjugate vaccine without booster. Since this national programme commenced in 1992, the British Paediatric Surveillance Unit has conducted enhanced paediatric surveillance in the United Kingdom and Ireland with additional reporting by microbiologists. The case definition requires isolation of Haemophilus influenzae from a sterile site, with serotype confirmation through a reference laboratory. Non vaccinated cases have been reported since 1995. By 1st March 2000, 434 cases had been reported in vaccinated and 172 in unvaccinated children. 162 true vaccine failures (TVFs) were identified (140 post 3 doses), with an overall estimate of efficacy for 3 doses in infancy of 98% (98,99). By age, point estimates of efficacy in the UK show highest protection at age 5-11 months (99%, 95% CI 99,100) with the lowest efficacy (91%, 83,96) in 48-59 month olds. Correspondingly, the median age at disease presentation in 3 dose TVFs was 31.5 months (interquartile ranges 19,45) in 1999 - unchanged since 1997. Clinical risk factors, including chromosomal anomalies, malignancy, immunosuppression and minor immunoglobulin deficiency were associated with TVF in 53 children (33%). 20 were born prematurely (12%); a proportion not statistically larger than would be expected. Clinical presentation is predominantly with meningitis (93, 57%) followed by epiglottitis (28, 17%) and bacteraemia (18, 11%). 5 deaths occurred in fully immunised children (3.5%). Such surveillance offers novel insights into the duration of protection provided by conjugate vaccines in infancy without boosting. This is an issue relevant to considerations regarding the optimal implementation of other recently licensed related meningococal and pneumococcal vaccines.