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The problem
Intervention: Concurrent administration of separate MMR and varicella vaccine injections during a single visit to infants aged 12-23 months
Who it was tested with: 3,035 Korean infants aged 12-23 months (52.3% male; median age 12 months) who received MMR + V and hepatitis A vaccines between 2015 and 2024.
The same group is measured before the intervention and again afterwards, with a baseline and an endline and sometimes a midline. Change is visible, but other causes cannot be ruled out.
"Using electronic health records, we identified predefined adverse events (AEs), including fever and healthcare visits. Adjusted relative risks (aRRs) were estimated using conditional Poisson regression." The study employed a rigorously designed self-controlled case series method comparing risk and baseline periods within the same individuals to determine causality, which is appropriate for evaluating vaccine safety signals in routine practice.
Self-controlled case series design captures associations during risk periods but does not establish absolute incidence rates. Study used specific vaccine strains common in Korea and may not apply to other vaccine formulations. Long-term safety outcomes beyond the immediate post-vaccination period were not evaluated.
Across countries and designs
1 study · 1 country
