Prior vaccination with recombinant Vesicular Stomatitis Virus - Zaire Ebolavirus vaccine is associated with improved survival among patients with Ebolavirus infection.

The second largest Ebolavirus disease (EVD) outbreak ever recorded is currently ongoing in Eastern Democratic Republic of the Congo (DRC). This is the first outbreak for which the recombinant Vesicular Stomatitis Virus - Zaire Ebolavirus (rVSV-ZEBOV) candidate vaccine has been widely administered, using a ring vaccination strategy. We examined whether prior vaccination with rVSV-ZEBOV impacts viral load, organ impairment, and survival among patients with EVD admitted to Ebola Treatment Units (ETUs) in the DRC. We conducted a retrospective observational study of patients admitted to the ETUs in Butembo and Katwa, Eastern DRC, between 30 March and 10 August 2019. We included 257 patients, of whom 44 had been vaccinated prior to admission and 213 were unvaccinated. Vaccinated patients were admitted to hospital sooner than unvaccinated patients (median 2 days (IQR 1.8-4) versus 4 days (IQR 3-6), p < 0.001). Vaccinated patients had a lower viral load at admission compared to those who were unvaccinated: 6.0 log10 cp/mL (IQR 5.0-7.1) versus 6.9 log10 cp/mL (IQR 5.4-7.6), p = 0.017. In a longitudinal analysis of daily viral load measurements, vaccinated patients had an overall viremia 1.32 log10 cp/mL (95% CI 0.58-2.1) lower than unvaccinated patients over the course of their infection. Acute kidney injury at admission was less common in vaccinated patients: OR 0.44 (95% CI 0.20-0.94), p = 0.027. Mortality in vaccinated patients was 10/44 (23%) compared to 117/213 (55%) unvaccinated patients (OR 0.24, 95% CI 0.11-0.51, p < 0.001). In conclusion, prior rVSV-ZEBOV vaccination was associated with reduced severity of infection and improved survival among patients with confirmed EVD treated at ETUs in Eastern DRC. These findings support the efficacy of the rVSV-ZEBOV vaccine in modulating EVD severity.