Why carry out this study?

Rotavirus vaccination was introduced in Belgium in 2006.

Since then data have been collected annually in 10 hospitals in Belgium to measure the impact of the vaccination, assessing temporal changes in the proportion of positive rotavirus test results as compared with the pre-vaccination situation.

The present study looked at surprising results in the dataset and whether we could find an explanation for those through a modeling exercise by which we could improve the vaccine impact if we had to start elsewhere with this vaccination.

What was learned from the study?

The surprising results in the RotaBIS database were the weak response of the vaccine effect (direct and indirect) in the first year, the impact curve flattening after 3 years, and the small peaks appearing at years 9 and 11 post vaccine introduction.

The model showed that the selection of the starting date of the vaccination program and the level of vaccine coverage of the big transmitters of the virus during the next normal peak season determine the results of the exceptions.

Improvement of the vaccine effect in Belgium could have been achieved if the vaccination program had started 8–10 months before the peak season of the infection (in July in Belgium) and with a high coverage at the start. Under such conditions, a 90% drop in the proportion of positive rotavirus test results could have been reached after 5 years instead of the observed 70%, along with a better long-term vaccine effect.

These results provide valuable insights for those countries wishing to initiate rotavirus vaccination.