Assessing Zika Virus Transmission Within Households During an Outbreak in Martinique, 2015-2016

Abstract

Since 2015, Zika virus (ZIKV) has caused large epidemics in the Americas. Households are natural targets for control interventions, but quantification of the contribution of household transmission to overall spread is needed to guide policy. We developed a modeling framework to evaluate this contribution and key epidemic features of the ZIKV epidemic in Martinique in 2015-2016 from the joint analysis of a household transmission study (n = 68 households), a study among symptomatic pregnant women (n = 281), and seroprevalence surveys of blood donors (n = 457). We estimated that the probability of mosquito-mediated within-household transmission (from an infected member to a susceptible one) was 21% (95% credible interval (CrI): 5, 51), and the overall probability of infection from outside the household (i.e., in the community) was 39% (95% CrI: 27, 50). Overall, 50% (95% CrI: 43, 58) of the population was infected, with 22% (95% CrI: 5, 46) of infections acquired in households and 40% (95% CrI: 23, 56) being asymptomatic. The probability of presenting with Zika-like symptoms due to another cause was 16% (95% CrI: 10, 23). This study characterized the contribution of household transmission in ZIKV epidemics, demonstrating the benefits of integrating multiple data sets to gain more insight into epidemic dynamics.

Keywords: Zika virus; asymptomatic infections; final size model; household transmission.

Figure 1.

Observed and expected statistics for households in Martinique during a Zika virus outbreak, 2015–2016. A) Distribution of the sizes of the 68 recruited households. B) Timing of household inclusion in the survey. Each black dot indicates recruitment of a household; the dotted line represents the duration the household was followed. The gray bars represent the epidemic curve for Zika in Martinique, given by the number of Zika-like cases reported in Martinique in a sentinel network of general practitioners (14). C) Observed and expected secondary clinical attack rate in households recruited in the study as a function of household size in the data (gray) and for the baseline (white) and the frequency-dependent (black) models. D) Expected secondary clinical attack rate in a typical household in Martinique, as function of household size (i.e., once the effect of the selection bias has been removed). The secondary clinical attack rate is the proportion of household contacts who exhibit symptoms. Predictions are given for the baseline (white) and the frequency-dependent models (black). Whiskers represent 95% confidence intervals.

Figure 2.

Expected distributions (boxplot) and observed values for the secondary clinical attack rate (defined as the proportion of nonindex household members with Zika-like symptoms) in the participating households recruited between December 2015 and September 2016, the seroprevalence among blood donors in Martinique in March and June 2016, and the proportion of pregnant women presenting Zika-like symptoms who tested positive for ZIKV infection during February–November 2016. The black dots represent the observed values in the data. The boxplots show the 2.5%, 25%, 50%, 75%, 97.5% quantiles of the distributions.