Multiple factors are believed to have contributed to the UK increase. One of these has been the distribution since late 1999 of combination Hib vaccines containing acellular pertussis (DTaP-Hib), which have lower immunogenicity for the Hib component than the equivalent whole cell pertussis containing preparations (DTwP-Hib). An analysis of vaccines received by fully immunised children presenting with invasive Hib disease in the UK, compared with healthy controls matched by date of birth, has been conducted (3). Significantly more cases than controls in the time period studied received all three doses of their infant primary course as DTaP-Hib, compared with two or three doses of another Hib vaccine (Conditional odds ratio (OR) 6.77; 95% confidence interval (CI) 3.26, 14.07) (3). This is the first study to show a reduction in clinical protection with these vaccines, within the context of an accelerated 2, 3, and 4 month infant immunisation schedule without a booster dose. In Germany however, where DTaP-Hib is routinely administered with a fourth dose in the second year, no similar increase in Hib incidence has occurred (4).

There are many differences in the way Hib vaccines are given throughout the EU. The choice of combinations used, age at immunisation, spacing of the primary course, and late first year or early second year boosters are all variables known to affect immunogenicity. While Germany's experience would suggest that a booster dose is more effective in controlling disease (4), it should be noted that the Netherlands, which saw a four fold rise in cases last year, administers Hib at 2, 3, 4, and 11 months of age (2). These observations raise questions about the long term impact of all conjugate Hib vaccines and therefore for the ongoing control of Hib in Europe. Answers that will ensure optimal delivery of immunisation schedules will only be found through ongoing commitment to high quality surveillance for this and other vaccine preventable diseases.