Projecting the end of the Zika virus epidemic in Latin America: a modelling analysis

Abstract

Background: Zika virus (ZIKV) emerged in Latin America and the Caribbean (LAC) region in 2013, with serious implications for population health in the region. In 2016, the World Health Organization declared the ZIKV outbreak a Public Health Emergency of International Concern following a cluster of associated neurological disorders and neonatal malformations. In 2017, Zika cases declined, but future incidence in LAC remains uncertain due to gaps in our understanding, considerable variation in surveillance and the lack of a comprehensive collation of data from affected countries.

Methods: Our analysis combines information on confirmed and suspected Zika cases across LAC countries and a spatio-temporal dynamic transmission model for ZIKV infection to determine key transmission parameters and projected incidence in 90 major cities within 35 countries. Seasonality was determined by spatio-temporal estimates of Aedes aegypti vectorial capacity. We used country and state-level data from 2015 to mid-2017 to infer key model parameters, country-specific disease reporting rates, and the 2018 projected incidence. A 10-fold cross-validation approach was used to validate parameter estimates to out-of-sample epidemic trajectories.

Results: There was limited transmission in 2015, but in 2016 and 2017 there was sufficient opportunity for wide-spread ZIKV transmission in most cities, resulting in the depletion of susceptible individuals. We predict that the highest number of cases in 2018 would present within some Brazilian States (Sao Paulo and Rio de Janeiro), Colombia and French Guiana, but the estimated number of cases were no more than a few hundred. Model estimates of the timing of the peak in incidence were correlated (p < 0.05) with the reported peak in incidence. The reporting rate varied across countries, with lower reporting rates for those with only confirmed cases compared to those who reported both confirmed and suspected cases.

Conclusions: The findings suggest that the ZIKV epidemic is by and large over within LAC, with incidence projected to be low in most cities in 2018. Local low levels of transmission are probable, but the estimated rate of infection suggests that most cities have a population with high levels of herd immunity.

Keywords: Connectivity; Epidemic; Latin America and the Caribbean; Mathematical modelling; Zika virus.

Fig. 1

Schematic of the meta-population model structure that focuses on the northern part of South America and the Caribbean islands. Each city consists of individuals who are assumed to be susceptible (S), pre-infectious (E), infectious (I) or recovered (R) from ZIKV infection. Movement of pre-infectious individuals between cities is modelled assuming different population flows, where a gravity model is illustrated. Movements to cities outside of the plotted area are not illustrated

Fig. 2

Reported Zika incidence (cases per 1000) within Latin America for (a) 2016 and (b) 2017. c Timing of peak incidence. d Total number of cases reported for each country for each calendar year (on a log 10 scale), according to the case classifications submitted by each country

Fig. 3

Comparisons of the time-series data for all Latin American countries (red) and normalised model output of the number of infections (blue). The countries are ordered by the type of surveillance data available: a Confirmed and suspected, b Confirmed, and c Suspected cases

Fig. 4

Comparisons of observed and model fit for ZIKV peak incidence in the 31 countries in Latin America. a Bayesian posterior checks that the estimated peak timing are consistent with the data; values between 0.01 and 0.99 indicate that the model and data are from the same distribution. b Quantile plot of the Bayesian posterior probabilities. c Comparison of the observed timing of the peak and estimated timing of the peak (with 95% CI). d Comparison of the estimated timing of the peak and the cross-validated estimates of peak timing (with 95% CI on the horizontal and vertical)

Fig. 5

The estimated probability of Zika cases in each country (and states in Brazil and Mexico). a Probability of more than 10 cases. b Median estimate of Zika cases in 2018. c The estimated time series of Zika cases within the five major cities of Colombia