Near disappearance of childhood invasive Haemophilus influenzae type b disease in Singapore

Background: Haemophilus influenzae type b (Hib) conjugate vaccines have significantly limited Hib's disease impact in every country where it was introduced. We previously estimated invasive Hib disease incidence in Singapore at ∼4.4 per 100,000 children <5 years (from 1994 to 2003, period 1), but the vaccine was not included in the national childhood immunization schedule until May 2013 (although it was available privately). The current study aims to describe changes in Hib disease incidence and vaccine coverage prior to the introduction of the vaccine.

Methodology: We retrospectively reviewed all invasive Hib cases from January 2004 to December 2012 (period 2) and estimated population-based incidence rates. Vaccine coverage was estimated from vaccine sales for 1994-2003, and from National Immunisation Registry for 2004-2010.

Results: There were 8 cases of invasive Hib disease in period 2, of whom 5 were <5 years. Invasive Hib incidence for period 2 was 0.57 per 100,000 children <5 years, representing an 86.4% reduction compared to period 1 (rate ratio 0.14, 95% confidence interval 0.06-0.26). However, for the later part of period 2 (2008-2012), incidence was 0.2 per 100,000 children <5 years; this represented a 95% reduction from period 1 (rate ratio 0.05, 95% confidence interval 0.01-0.18). Between periods 1 and 2, national Hib vaccine coverage rose from 22% to >80%, with >99% of children receiving combination Hib-containing vaccines.

Conclusions: Childhood invasive Hib disease has nearly disappeared from Singapore, despite the absence of universal mass vaccination. We believe this is likely due to excellent uptake of combination vaccines.