Transmission Dynamics of Zika Virus in Island Populations: A Modelling Analysis of the 2013-14 French Polynesia Outbreak

Abstract

Between October 2013 and April 2014, more than 30,000 cases of Zika virus (ZIKV) disease were estimated to have attended healthcare facilities in French Polynesia. ZIKV has also been reported in Africa and Asia, and in 2015 the virus spread to South America and the Caribbean. Infection with ZIKV has been associated with neurological complications including Guillain-Barré Syndrome (GBS) and microcephaly, which led the World Health Organization to declare a Public Health Emergency of International Concern in February 2015. To better understand the transmission dynamics of ZIKV, we used a mathematical model to examine the 2013-14 outbreak on the six major archipelagos of French Polynesia. Our median estimates for the basic reproduction number ranged from 2.6-4.8, with an estimated 11.5% (95% CI: 7.32-17.9%) of total infections reported. As a result, we estimated that 94% (95% CI: 91-97%) of the total population of the six archipelagos were infected during the outbreak. Based on the demography of French Polynesia, our results imply that if ZIKV infection provides complete protection against future infection, it would take 12-20 years before there are a sufficient number of susceptible individuals for ZIKV to re-emerge, which is on the same timescale as the circulation of dengue virus serotypes in the region. Our analysis suggests that ZIKV may exhibit similar dynamics to dengue virus in island populations, with transmission characterized by large, sporadic outbreaks with a high proportion of asymptomatic or unreported cases.

Fig 1. Human-vector transmission model schematic.

S^H represents the number of susceptible people, E^H the number of people incubating the virus, I^H the number of infectious people, R^H the number recovered people. Similarly, S^V represents the proportion of mosquitoes currently susceptible, E^V the proportion in their incubation period, and I^V the proportion of mosquitoes infectious. Mosquitoes are assumed to remain infectious for life. β_V is the transmission rate from humans to mosquitoes; β_H is transmission from mosquitoes to humans; 1/α_H and 1/α_V are the mean latent periods for humans and mosquitoes respectively; 1/γ is the mean infectious period for humans; 1/δ is the mean lifespan of mosquitoes; and N is the human population size.

Fig 2. Comparison of reported cases and fitted model trajectories.

Black dots show weekly reported confirmed and suspected ZIKV cases from sentinel sites. Blue line shows median of 2,000 simulated trajectories from the fitted model, adjusted for variation in reporting over time; shaded region shows 95% credible interval.

Fig 3. Estimated growth in effective reproduction number as susceptible pool increases over time.

(A) Tahiti, (B) Sous-le-vent, (C) Moorea, (D) Tuamotu-Gambier, (E) Marquises, (F) Australes. Line shows median from 1,000 samples of the posterior distribution, shaded region shows 95% credible interval.