Introduction

In order to simplify its vaccination schedule and to be able to introduce new vaccines, Belgium is currently evaluating a switch from a 3+1 to a 2+1 schedule for the hexavalent vaccine. However, several neighbouring countries, such as France and the Netherlands, which switched to the 2+1 schedule several years ago, have recently experienced an increase in cases of haemophilus influenzae type b (Hib). To explore the potential causes of such increase, we compare trends and vaccination schedule of Hib in the European Union /European Economic Area countries.

Method

We used yearly Hib cases reported to ECDC between 2000 and 2023. We excluded Croatia and Bulgaria as data specific to serogroup b were not available. National immunisation schedules were retrieved from ECDC vaccine scheduler website,  yearly vaccination coverage and year of introduction of Hib vaccination from WHO/UNICEF Joint Reporting Form on Immunization. ECDC’s National Focal Points for vaccine preventable diseases were contacted by e-mail for year of introduction of the 2+1 schedule.

Results

Out of 28 countries, 16 used a 2+1 schedule (Table 1). Some of these countries experienced an increase in Hib cases (Figure1). However, the increase sometimes appeared before the introduction of the 2+1 schedule (e.g. Germany), immediately after (The Netherlands) or more than a decade later (Italy). In contrast, Slovakia, Austria and the Nordic countries, all with a 2+1 schedule, do not report any increase. Vaccination coverage is globally high but sometimes below 90% (e.g. Germany and Netherlands), which could also lead to an increase in cases. In countries with 2+1 schedule, age of booster is variable. France, Germany and Netherland, all countries with an increase in cases, administer the booster at 11 months. None of the countries that opted for a 2+1 will presently revert to a 3+1 schedule.

Conclusion

The causes of the increase in Hib in some European countries are probably multifactorial. In addition to vaccination schedule, coverage and age of booster, further elements that could be explored include quality of surveillance, attendance of childcare at young age and quality of surveillance.