Significant social clustering could to be a factor in the transmission of Ebola virus disease in West Africa, according to recent findings.
Researchers collected 78 Ebola virus genome sequences from more than 70% of the confirmed cases occurring in Sierra Leone during June. A transmission-oriented phylodynamic model, which assumes a time-based evolutionary reconstruction of the viral dynamics, was then applied to the genome sequences. Transmission chains using a Bayesian approach on the same genomic information.
With this data, researchers used a complementary Susceptible Exposed Infectious Removed (SEIR) network model to approximate clustering based on confirmed Ebola virus disease cases and deaths, deducing criteria for a clustered (φ>0) and nonclustered population (φ=0). SEIR models parameters were fit to the aggregate numbers of confirmed Ebola virus cases and laboratory-confirmed Ebola virus disease deaths documented in WHO Global Alert and Response news between May 27 and Aug. 31, 2014.
The researchers found that the best-fit phylodynamic model for the virus’ genomic data produced a basic reproductive ratio of 1.4 (95% highest posterior density: 1.1-1.8), with different model parameters producing similar measures. Using this data, researchers estimated a secondary infection ratio of 1.35, and both genomic analyses yielded similar estimated starting dates for the Sierra Leone outbreak. These dates were more recent than has been previously reported, but consistent with the first Sierra Leone infections reported at a funeral in May.
A fit of the SEIR model, which estimated the proportion of possible high-risk contacts of an infected person who is also a contact, found evidence of a clustered population (φ=0.71; 95% CI, 0.66–0.72), along with an estimated basic reproductive ratio of 1.29 (95% CI, 1.27–1.37).
The comparison of epidemiological data evaluated by models with and without clustering suggested compelling evidence for clustering in the contact network, the researchers wrote. Transmission events of infections spread through social clustering were correlated, and occurred within reciprocally associated subgroups.
“The clustering of transmission we infer has implications for the public health response such as the rate at which health resources, such as hospital beds, are required, for deriving realistic predictions about epidemic potential, and for the design of vaccine trials,” the researchers wrote.