Trends in age-specific varicella incidences following the introduction of the general recommendation for varicella immunization in Germany, 2006–2022

BACKGROUND: In Germany, general childhood varicella vaccination has been recommended since 2004. A feared effect of low vaccination coverage is a possible shift in incidence from children to teenagers and young adults who are at higher risk of severe outcomes. If true, this shift would possibly necessitate changes to the national immunization strategy. We aimed to evaluate the effects of the general vaccination recommendation on age-specific varicella incidences in Germany in general and examine specifically whether a shift from children to teenagers (15 to 19 years) has occurred. METHODS: Trends in age-specific incidences were evaluated using triangulation with the following datasets: national mandatory notification data (N) (2014–2022), billing data of the statutory health insurance associations (I) (2009–2017) and data from a doctor's sentinel system (S) (2006–2017). Similar clinical case definitions were used in N and S, while I used ICD-10-codes. Age groups were stratified as available in all three systems. Incidences per year were calculated based on the total population (N), the number of statutory health insured (I), and extrapolated from S to the total population. RESULTS: During all years of observation, age-specific incidences have dropped significantly across all age-groups for S und I. The age groups (under 10 years) with initially highest incidences were the ones with the strongest reductions (under 1 year: -90%, 1–4 years: -95.5%, 5–9 years: -89.2% for S; -67.7%, -78%, -79.3% for I). A single 53.1% increase in the low incidence in S among 15–19-year olds observed in 2017 compared to 2016 could not be confirmed in N or I. Increases in incidences during the first two years of N are probably due to improved notification behaviour over these years. In 2019, all age-specific incidences increased (N), with 15 to 19-year olds showing the highest relative increase (28.2%). CONCLUSIONS: Since the introduction of the general vaccine recommendation against varicella, incidences across all age-groups have declined significantly. Available data indicate no evidence for a shift in disease incidence to older age groups. Every incidence increase beyond childhood age should however be followed up closely. So far, children and adolescents have both benefitted from the current vaccination strategy.