Study suggests “superspreaders” fueled the 2014–15 Ebola outbreak

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A new study has highlighted the importance of disease “superspreaders” in infectious disease outbreaks. The team investigated the 2014–15 Ebola epidemic discovering that just 3% of individuals were responsible for an estimated 61% of cases. Better identifying these patients could lead to targeted, individual control measures, as opposed to current population-level interventions.

It has previously been reported that some infected patients transmit disease to a disproportionately large amount of susceptible individuals, a phenomenon termed “superspreading”. The phenomenon had been implicated in the recent Ebola epidemic, however systematic characterizations of spatiotemporal disease dynamics had been lacking.

In addition, previous outbreak models have focused primarily on population-level estimates, which analyze the average number of secondary infections produced by an individual, overlooking important heterogeneity in the infected population.

The researchers therefore created a new statistical framework allowing them to investigate the importance of superspreaders in the recent Ebola epidemic, and also to measure how superspreading changed over time as control measures were implemented.

The team discovered that superspreaders played a significant role in sustaining the 2014–15 Ebola outbreak, with just 3% of patients responsible for approximately 61% of the infections. Moreover, the researchers suggested this may be a conservative estimate, as the study only assessed individuals who had been buried safely.

The group also tried to better profile superspreaders, in order to allow these individuals to be targeted. They discovered that superspreaders were more likely to be community-based cases, compared with those in health care facilities. In addition to this, age appeared to be an important predictor.

Author Benjamin Dalziel, from Oregon State University (OR, USA), explained “In the recent Ebola outbreak it’s now clear that superspreaders were an important component in driving the epidemic,

“We now see the role of superspreaders as larger than initially suspected. There wasn’t a lot of transmission once people reached hospitals and care centers. Because case counts during the epidemic relied heavily on hospital data, those hospitalized cases tended to be the cases we ‘saw.’

“However, it was the cases you didn’t see that really drove the epidemic, particularly people who died at home, without making it to a care center. In our analysis we were able to see a web of transmission that would often track back to a community-based superspreader.”

It has been suggested that a better understanding of the superspreading phenomenon could lead to better targeted and effective interventions, which focus on individuals rather than whole populations. This not only applies to Ebola; superspreading has also been implicated in the severe acute respiratory syndrome outbreaks from 2003 and the more recent Middle East respiratory syndrome epidemic in 2013 suggesting its importance in any future disease outbreaks.

Dalziel concluded: “As we can learn more about these infection pathways, we should be better able to focus on the types of individual behavior and demographics that are at highest risk for becoming infected, and transmitting infection.”

Source: Lau MSY, Dalziel BD, Funk S, McClelland A et al. Spatial and temporal dynamics of superspreading events in the 2014–2015 West Africa Ebola epidemic, Proc. Natl Acad. Sci. USA, doi:10.1073/pnas.1614595114 (2017) (Epub ahead of print); http://oregonstate.edu/ua/ncs/archives/2017/feb/disease-%E2%80%9Csuperspreaders%E2%80%9D-were-driving-cause-2014-ebola-epidemic

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