Factors associated with severity of adverse events following yellow fever vaccination during the biggest outbreak of the disease recorded in Brazil, 2016-2017.

Objective: to analyze characteristics, incidence and factors associated with serious adverse events (SAEs) following yellow fever vaccination during an outbreak of the disease in Brazil (2016-2017).

Methods: this was a case-control study using data from the National Immunization Program Information System (SI-PNI); SAE were considered to be cases, and non-serious adverse events (NSAE) were considered to be controls.

Results: we analyzed 135 SAE cases and 1,058 controls; of the 135 SAE, 79 (58.5%) were males and median age was 28 years [09-49]; incidence in January 2017 reached 1.3 case per 100,000 vaccine doses administered; there was statistical association with males (Odds Ratio [OR]=1.73 - 95%CI 1.20;2.48), primary vaccination (OR=1.65 - 95%CI 1.01;2.71), and being 60 years of age or older taking as reference those aged under 5 (OR=4.4; p-value <0.02).

Conclusion: SAE owing to yellow fever vaccine showed a greater chance of occurring in men, the elderly and primary vaccination.