An outbreak of chikungunya fever started in the Americas at the end of 2013. Figure 1 below shows territories that reported at least one autochthonous case by 15 June 2014. The first territory reporting cases was Saint Martin (9 December 2013). Further reports quickly followed from two other French territories, Martinique on 19 December 2013 and Guadeloupe on 28 December 2013. By 15 June 2014, 16 territories had reported at least one autochthonous case.
Figure 1: Areas that reported at least one laboratory-confirmed authochthonous case of chikungunya fever.
Our Unit supported the Institut de Veille Sanitaire (InVS, the equivalent of the Centers for Disease Control and Prevention in the US) and the Cellule inter-régionale d’épidemiologie (CIRE) Antilles-Guyane to monitor the emergence of chikungunya in the Caribbean and inform policy making. Working in close collaboration with InVS, CIRE, INSERM UMR S 1136 and Institut Pasteur Guyane, we developed mathematical and statistical models to better understand the local dynamics of transmission and ascertain the risk of contagion to other regions of the Americas. Analysing surveillance data from the first three island reporting cases, we estimated that local transmission of chikungunya fever in the Caribbean was very effective, the mean number of cases generated by a human case ranging from two to four. Looking at the dynamics of regional spread, we found that there was a strong spatial signature in the regional epidemic, with the risk of transmission between areas estimated to be inversely proportional to the distance rather than driven by air transportation. This simple distance-based model successfully predicted observed patterns of spread (see Figure 2 below). The spatial structure allowed ranking areas according to their risk of invasion. This characterisation may help national and international agencies to optimise resource allocation for monitoring and control and encourage areas with elevated risks to act. Figure 2: For different time points in the epidemic, the grey bars give the predicted probability the area may be invaded in the following 75 days. Red bars indicate territories that were effectively affected in the followings 75 days. The dark grey bars give the predicted probability the area was already invaded but had not reported autochthonous cases yet at the time the analysis was done.
- Cauchemez S, Ledrans M, Poletto C, Quenel P, de Valk H, Colizza V, Boëlle PY. Local and regional spread of chikungunya fever in the Americas. Eurosurveillance 2014; 19: online publication.