School of Public Health, Kyoto University, Kyoto, Kyoto, Japan.
PeerJ. 2022 Jan 19;10:e12767. doi: 10.7717/peerj.12767. eCollection 2022.
In Japan, routine two-dose immunization against varicella has been conducted among children at ages of 12 and 36 months since 2014, and the vaccination coverage has reached around 90%. To understand the impact of routine varicella vaccination, we reconstructed the epidemiological dynamics of varicella in Japan.
Epidemiological and demographic datasets over the past three decades were analyzed to reconstruct the number of susceptible individuals by age and year. To estimate the annual risk of varicella infection, we fitted a balance equation model to the annual number of cases from 1990 to 2019. Using parameter estimates, we reconstructed varicella dynamics starting from 1990 and modeled future dynamics until 2033.
Overall varicella incidence declined over time and the annual risk of infection among children younger than 10 years old decreased monotonically starting in 2014. Conversely, varicella incidence among teenagers (age 10 to 14 years) has increased each year since 2014. A substantial number of unvaccinated individuals born before the routine immunization era remained susceptible and aged without contracting varicella, while the annual risk of infection among teenagers aged 10 to 14 years increased starting in 2011 despite gradual expansion of varicella vaccine coverage. The number of susceptible individuals decreased over time in all age groups. Modeling indicated that susceptibility rates among pre-school children aged 1 to 4 years will remain low.
Routine varicella vaccination has successfully reduced infections in pre-school and early primary school age children, but has also resulted in increased infection rates among adolescents. This temporary increase was caused both by the increased age of susceptible individuals and increased transmission risk among adolescents resulting from the dynamic nature of varicella transmission. Monitoring susceptibility among adolescents will be important to prevent outbreaks over the next decade.
自 2014 年以来，日本已针对 12 个月和 36 个月龄的儿童常规开展两剂次水痘疫苗接种，接种覆盖率已达 90%左右。为了解常规水痘疫苗接种的效果，我们重建了日本水痘的流行病学动态。
分析过去 30 年的流行病学和人口统计学数据集，按年龄和年份重建易感人群数量。为了估计水痘年感染风险，我们拟合了一个平衡方程模型，对 1990 年至 2019 年的年发病数进行拟合。利用参数估计值，我们从 1990 年开始重建水痘动态，并对 2033 年之前的未来动态进行建模。
水痘总体发病率随时间下降，2014 年以来 10 岁以下儿童的年感染风险呈单调下降趋势。相反，自 2014 年以来，青少年（10 至 14 岁）的水痘发病率逐年上升。在常规免疫时代之前出生的大量未接种人群仍然易感，且未感染水痘，但自 2011 年以来，尽管水痘疫苗覆盖率逐渐扩大，10 至 14 岁青少年的年感染风险仍呈上升趋势。所有年龄组的易感人群数量随时间减少。模型预测，1 至 4 岁学龄前儿童的易感性将持续较低水平。
常规水痘疫苗接种成功降低了学龄前和小学低年级儿童的感染率，但也导致青少年感染率上升。这种暂时增加既源于易感人群年龄的增加，也源于水痘传播的动态特性导致青少年间传播风险增加。监测青少年的易感性对于预防未来十年的暴发至关重要。
