Estimating the basic reproductive number and the impact of containment measures during the Italian chikungunya outbreak
Caterina Rizzo, Piero Poletti2, Marco Ajelli, Thomas Seyler,4, Andrea Pugliese3, Marta Luisa Ciofi
degli Atti1,5, Stefania Salmaso, Stefano Merler
- National Center for Epidemiology Surveillance and Health Promotion, Istituto Superiore di Sanità, Rome, Italy
- Fondazione Bruno Kessler, Trento, Italy
- Department of Mathematics, University of Trento, Trento, Italy
- European Programme for Intervention Epidemiology Training (EPIET), European Centre for Disease Prevention and Control (ECDC), Stockholm, Sweden
- Ospedale Pediatrico “Bambino Gesù”, Roma, Italy
Abstract
During summer 2007, Italy experienced the first outbreak of Chikungunya virus (CHIKV) documented in a temperate climate country. The infection is transmitted through the bite of an
infected Aedes mosquito. A total of 217 laboratory confirmed CHIKV cases were reported from 15 July 2007 to 28 September 2007. In order to assess control measures implemented we investigated the transmission dynamics of the CHIKV outbreak. As a first step, we estimated the basic reproduction number (R0) of the outbreak.
We used a system of ordinary differential equations to model the dynamics of the outbreak and assumed homogenous mixing between host and vector populations. We used two methods to
estimate R0. The first method relied on the estimation of the intrinsic growth rate and parameters such as the duration of viremia in humans and the extrinsic and intrinsic incubation period. The
second method required an estimate of the biting rate and the total number of vectors. We estimated these parameters by fitting the model to the epidemic curve using a least-square algorithm.
Using the intrinsic growth rate, R0 had a mean value of 3.62 (95%CI: 3.11-4.21). Fitting the model to the epidemic curve, R0 had a mean value of 3.93 (95%CI: 3.17-4.79). The two distributions of
R0 were not significantly different.
These results suggest that without vector control measures the attack rate in the initial affected villages could have been as high as 90%. Considering the effectiveness of control measures
implemented during the outbreak, our results strongly suggest that the use of pyrethroids against adult mosquitoes and anti-larval products from 18 August 2007 onwards, together with an active
information campaign on personal protection, limited the outbreak.