Monge S, et al. - In view of observations suggesting an increase in invasive Haemophilus influenzae serotype b (Hib) disease in the Netherlands in 2016 among children younger than 5 years, that coincided with the introduction of the hexavalent diphtheria, tetanus, and acellular pertussis-hepatitis B virus-inactivated polio virus/Hib vaccine (DTPa-HBV-IPV/Hib) from 2011 onwards, the effectiveness of the hexavalent vaccine was assessed to determine if this increase could be explained by decreasing effectiveness. Outcomes revealed no decrease in Hib vaccine effectiveness over time or by the introduction of the hexavalent DTPa-HBV-IPV/Hib vaccine thereby supporting the effectiveness of the current vaccination programme. Vaccine displayed high effectiveness but with age, it waned. Thus, for the increase in Hib disease, alternative explanations are needed to be assessed.

Methods

• Researchers performed a case-control study in the Netherlands.
• Using the surveillance records of the Netherlands Reference Laboratory for Bacterial Meningitis (Amsterdam), patients with a Hib infection (cases) were selected.
• They included cases with a Hib infection that began from Jan 1, 2003 to Dec 31, 2016 and who were younger than age 5 years.
• For each case, they selected 10 controls, matched by date of birth, from the national vaccination register (Praeventis).
• Praeventis, which details the vaccination records of children living in the Netherlands, was used to ascertain vaccination status.
• Using the last recorded vaccine dose, they classified the child as having received the hexavalent DTPa-HBV-IPV/Hib vaccine or a pentavalent vaccine (which excludes the hepatitis B virus component) or another vaccine.
• Conditional logistic regression was used to estimate the effectiveness of these vaccines.

Results

• In this study, 159 (94%) of 170 reported cases and 1590 matched controls were included as 11 cases could not be cross-matched with vaccination records from Praeventis.
• Median age of 1·5 years (IQR 0·8–2·9) was noted.
• Vaccination was reported in 91 (57%) of 159 cases compared with 1408 (89%) of 1590 controls.
• Observations suggested the overall vaccine effectiveness of 92·8% (95% CI 88·7–95·4), with no differences between the year of disease onset (p=0·9670).
• Type of vaccine given conferred no differences: vaccine effectiveness of the pentavalent and other vaccines was 91·8% (95% CI 86·1–95·1) vs 94·0% (89·0–96·8) for the hexavalent vaccine (OR 0·72, 95% CI 0·36–1·45; p=0·3591).
• Children aged 1–2 years showed the highest vaccine effectiveness at disease onset (97·1–99·0%) while children aged 3–4 years displayed the lowest vaccine effectiveness at disease onset (60·7–82·3%; p=0·0008).