Estimating transmissibility of Zika virus in Colombia in the presence of surveillance bias

Abstract

The 2015-2016 Zika virus outbreak in the Americas presented significant challenges in understanding the transmission dynamics due to substantial reporting biases, as women of reproductive age (15-39 years) were disproportionately represented in the surveillance data when public awareness of relationship between Zika and microcephaly increased. Using national surveillance data from Colombia during July 27, 2015-November 21, 2016, we developed a Bayesian hierarchical modeling framework to reconstruct the true numbers of symptomatic cases and estimate transmission parameters while accounting for differential reporting across age-sex groups. Our model revealed that the detection rate of symptomatic cases among women of reproductive age was 99% (95% CI: 98.7-100), compared to 85.4% (95% CI: 84.7-86.1) in other demographic groups. After correcting for these biases, our results showed that females aged 15-39 years remained 82.8% (95% CI: 80.2-85.2%) more susceptible to Zika symptomatic infection than males of the same age, independent of differential reporting areas. Departments with medium-high altitude, medium-high population density, low coverage of forest, or high dengue incidence from 2011-2015 exhibited greater Zika risk. This study underscores the importance of accounting for surveillance biases in epidemiological studies to better understand factors influencing Zika transmission and to inform disease control and prevention.

Fig. 1. Epidemic curve for the zika outbreak in Colombia from July 27, 2015 to November 21, 2016.

A Weekly numbers of cases across six age-sex groups. B Cumulative numbers of cases across age-sex groups. C Proportions of monthly cases across age-sex groups.

Fig. 2. Zika Attack Rate Heat Map Across Demographic Groups in Colombia.

The maps display the Zika virus attack rates stratified by age group and gender across municipalities in Colombia. A–C represent attack rates for females, while (D–F) represent males. Each panel is further subdivided into three age categories: 0–14 years (Panels A and D), 15–39 years (B, E), and 40+ years (C, F). The intensity of the purple shading corresponds to the attack rate, with darker shades indicating higher attack rates.

Fig. 3. Temporal distribution of Zika cases across colombian regions by gender and age group.

This figure illustrates the monthly temporal trends in Zika virus cases stratified by gender and age groups across five regions of Colombia during the epidemic period (2015–2016). Each panel corresponds to a specific region: (A) Amazon, (B) Andean, (C) Caribbean, (D) Orinoquia, and (E) Pacific. The y-axis represents the number of cases, while the x-axis spans the time from July 2015 to November 2016. Colored lines represent different demographic groups: females aged ≤14 years, 15–39 years, and ≥40 years, and males in the same age brackets.

Fig. 4. Estimated attack rates from the model.

A The estimated attack rates for age-sex groups. B The estimate attack rates for regions. The black circle and blue diamond are the observed, and model-predicted with correction for reporting differences respectively.

Fig. 5. Estimates of covariate effects on transmissibility of Zika.

The blue points and horizontal bars are the posterior median of odd ratios and 95% credible intervals.