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The problem
Intervention: One-dose and two-dose varicella vaccination
Who it was tested with: 551,980 children and adolescents born between 2007 and 2025 in Chaoyang District, Beijing, China.
Children or communities are compared with a group that did not receive the intervention, or followed over years. This is the design that best separates what the intervention did from everything else that changed.
"A retrospective cohort study was conducted among 551,980 children and adolescents born between 2007 and 2025 using varicella surveillance data from the Chaoyang District Center for Disease Control and Prevention. Incidence rates were calculated, and vaccine effectiveness was estimated using Cox proportional hazards models." The use of a large cohort with comparison groups (vaccinated vs unvaccinated, one-dose vs two-dose) and statistical modelling to estimate effectiveness constitutes an experimental design.
Retrospective study design limits causal inference; no information on vaccination timing, intervals between doses, or potential confounders affecting vaccination coverage; data covers a historical period with varying vaccination policies.
Across countries and designs
5 studies · 3 countries
