Exploring the impact of vaccination coverage on yellow fever incidence: a country-level analysis of lagged effects in South America (2015–2023)

Abstract

Background: Yellow fever (YF) remains a significant public health concern in South America, particularly in Brazil, Colombia, Ecuador, and Peru. Vaccination campaigns are essential for controlling disease transmission, but their effects may vary due to differences in coverage, healthcare systems, and surveillance. Understanding lagged effects—how vaccination in one-year influences cases in the next—can guide more effective strategies. Methods: We conducted a secondary data analysis using World Health Organization data from 2015 to 2023. Annual vaccination coverage (%) and confirmed YF cases were extracted for Brazil, Colombia, Ecuador, and Peru. Vaccination coverage values were shifted forward by one year to assess lagged effects. Pearson’s correlation coefficient was calculated for each country, with significance set at P < 0.05. Analyses were performed in R (v4.3). Results: From 2015 to 2023, Brazil reported the highest total number of cases (n = 2,278), followed by Peru (n = 163), Colombia (n = 9), and Ecuador (n = 3). Vaccination coverage ranged from 43.2%–99.2% (Brazil), 54.3%–90.0% (Colombia), 69.8%–96.2% (Ecuador), and 50.2%–75.0% (Peru). Overall correlation between coverage and cases was r = –0.32 (p = 0.058). Lagged correlation analysis showed significant associations for Colombia (r = −0.917, P = 0.001) and Ecuador (r = 0.722, P = 0.043), but not for Brazil (r = −0.550, P = 0.158) or Peru (r = 0.051, P = 0.905). Conclusion: Lagged correlation analysis suggests that vaccination effects on YF incidence vary by country. Sustained coverage appears most protective in Colombia, while results in Ecuador highlight possible surveillance or reporting challenges. Tailored strategies and improved data systems are needed to optimize YF control. © 2025 Korean Society of Global Health.