Appendix S1. Evolution of Zika prevalence in a dengue hyper-endemic municipality in Southern Mexico after the outbreak of 2015 to 2017.

The Zika virus (ZIKV) produced extensive outbreaks in the last decades including a large epidemic across Latin America (2015-2017). Due to Zika is trassmited for the same mosquito-vector of dengue diseases, both diseases coexist in the same areas. During the outbreak, the real magnitude of the epidemic was underestimated due to the high proportion of clinical mild cases and asymptomatic infections, and the similarity of its clinical manifestations with dengue. Another contributing factor was that confirmatory molecular testing (RT-PCR) is only useful during the first 5-7 days after the onset of the symptoms and is not widely available across regions.

Population-based serosurveys provide an accurate prevalence estimator to understand the real magnitude of epidemics, however, in dengue-endemic regions, Zika seroprevalence estimations are difficult due to the extensive cross-reactivity of the elicited antibodies in both diseases. Studies of the first Zika epidemics used seroprevalence of IgG against the NS1 Zika protein using enzyme-linked immunosorbent assays (ELISA). The commercial tests demonstrated high diagnostic performance in dengue non-endemic populations. As the Zika epidemic advanced in dengue-hyperendemic areas, most of these ELISA depicted low specificity. Other techniques are also affected, including neutralization tests, which is considered the gold standard test in flavivirus infections. Conversely, the ELISA based on the domain III of E protein (EDIII) reported higher specificity in flavivirus-endemic populations, although there are no commercially available anti-EDIII IgG ELISAs.

Commercial ELISAs have important advantages in large sample size serosurveys, such as reproducibility, high throughput and low cost. We used MNT and anti-EDIII IgG ELISA to adjust the cutoff value of the commercial ELISA and correct for possible biases due to cross-reactive antibodies. This study is the supplementary material of the study conducted to describe the extension of the Zika outbreak in a dengue hyper-endemic municipality in southern Mexico