In 2014 and 2015, Ebola spread through West Africa like wildfire, affecting nearly 29,000 people and killing more than 11,000. During the course of the epidemic, researchers identified an experimental Ebola vaccine that provided 100 percent protection against the disease.
“The vaccine has been shown to be very efficacious in a field trial using a novel ring vaccination design,” said Dr. Elizabeth (Betz) Halloran, a professor of biostatistics at the University of Washington School of Public Health. In the ring vaccination design, the recombinant vesicular stomatitis virus–Zaire Ebola virus vaccine, or rVSV-ZEBOV, is given to first and second-degree contacts of those infected either immediately or within a day.
“It takes the vaccine trial to where the transmission is occurring,” added Dr. Halloran, also a core member of the Vaccine and Infectious Disease Division at Fred Hutchinson Cancer Research Center.
A collaboration of researchers, including Dr. Halloran, investigated whether this same ring vaccination strategy can curb future Ebola outbreaks or other emerging disease threats. Through an Ebola outbreak simulation, researchers found that ring vaccination can successfully contain outbreaks with moderate transmission potential.
“Our simulations show that a public health intervention strategy based on ring vaccination can also contain outbreaks,” Dr. Halloran said. “This could mean that not everyone would need to be vaccinated before an outbreak, but in the event of an outbreak, targeted vaccination campaign might be able to contain transmission.”
Researchers simulated an Ebola outbreak in the Pujehun district of Sierra Leone, assigning a model population of 375,000 people to one central town and some neighboring villages. The simulation was adjusted to reproduce the most important features of the 2014-15 Ebola outbreak that can be considered typical in terms of both transmissibility and key time periods.
Individuals were assigned to a specific household, which were then linked to create extended households, as are typical in rural Africa, the study noted. Microsimulations were used to model Ebola transmission within households and extended households, and in the general community, including contacts made through health care and during burial ceremonies.
The study, published recently in PLOS Neglected Tropical Diseases, not only showed that ring vaccination can successfully contain an outbreak with reproduction numbers up to 1.6, but it also identified additional interventions that could increase the ability to contain outbreaks with reproduction numbers up to 2.6.
- Shortening the time from symptoms onset to hospitalization to 2-3 days on average through improved contact tracing procedures.
- Adding a 2 km spatial component to the vaccination ring definition, targeting 2.7 villages on average.
- Decreasing human mobility by quarantining affected areas.
Plans are underway to create a mobile stockpile of this Ebola vaccine at the World Health Organization to contain future Ebola outbreaks, according to the study. The GAVI alliance has also pledged funds to buy 300,000 doses of the vaccine for such a stockpile.
“Based on the results here, the planned stockpile of 300,000 doses should be sufficient to implement ring vaccination policies for containing a timely detected Ebola outbreak at the source,” the researchers confirmed.
Ring vaccination was instrumental in the elimination of local outbreaks of smallpox during the eradication phase. “Targeted vaccination interventions,” like those outlined here, “should be adapted for other emerging infectious disease threats, as was done in the past for smallpox,” the study suggests.
Dr. Alessandro Vespignani, from Northeastern University, was the lead author of the study. He and Dr. Halloran were joined by experts from Northeastern University in Boston, the University of Florida in Gainesville, Doctors with Africa-CUAMM in Padua, Italy, and the Bruno Kessler Foundation in Trento, Italy.