of rVSVΔG-ZEBOV-GP Vaccine: The Trial to Introduce a Vaccine Against Ebola (STRIVE)

Summary: We assessed immunogenicity of rVSVΔG-ZEBOV-GP vaccine in adults in Sierra Leone with and without asymptomatic malaria parasitemia. A robust immune response persisted for 9-12 months in both study groups. Responses in participants with asymptomatic malaria parasitemia were somewhat lower. Abstract Background The effect of malaria infection on rVSVΔG-ZEBOV-GP (ERVEBO®) immunogenicity is unknown. Methods The Sierra Leone Trial to Introduce a Vaccine against Ebola (STRIVE) vaccinated 7998 asymptomatic adults with rVSVΔG-ZEBOV-GP during the 2014-6 Ebola epidemic. In STRIVE’s immunogenicity substudy, participants provided blood samples at baseline, 1, 6, and 9-12 months. Anti-glycoprotein (GP) binding and neutralizing antibodies were measured using validated assays. Baseline samples were tested for malaria parasites by PCR. ≥1 incubated in the rGP coated wells with a serial-diluted and incubated with goat anti-human IgG horseradish peroxidase conjugate. The enzymatic reaction was stopped using a sulfuric acid solution. Optical density was measured on an ELISA plate reader and titer (GP-ELISA units/mL) were calculated from the standard curve using a four-parameter logistic curve fit. Neutralizing antibodies to the rVSVΔG-ZEBOV-GP vaccine strain were measured using the validated PRNT assay. Serum was diluted from 1:5 to 1:10,240 and mixed with an equal volume of diluted rVSVΔG-ZEBOV-GP vaccine for final dilutions of 1:10 to 1:20,480. Neutralization proceeded over an 18-hour period at 2ºC to 8ºC after which the serum/virus mixture was used to inoculate Vero cells. Viral adsorption was done at 37 ± 2ºC for 60 minutes followed by a methylcellulose overlay, and subsequently cells were incubated at 37 ± 2ºC for 2 days. Plaques were visualized by crystal violet stain and counted using the ViruSpot from Autoimmun Diagnostika GMBH. A neutralizing titer was defined as one that resulted in a 60% reduction in viral plaques in the presence of serum compared with virus control without serum. assessing whether 95% CIs overlapped or not. Imputation was not performed for missing data. Baseline positive results were defined as ≥200 EU/mL for GP-ELISA and as any reaction above the lower limit of quantification (LLOQ) for PRNT; the LLOQ was 36.11 EU/mL for GP-ELISA, and 35 for PRNT. For measurements below LLOQ, samples were assigned a result of 1/2 LLOQ (e.g. 18.06 EU/mL for ELISA). The STRIVE immunogenicity data demonstrate that vaccination with rVSVΔG-ZEBOV-GP elicited robust immune responses in vaccinated persons by one-month post-vaccination that persisted, albeit at slightly lower GMTs, through 9-12 months post-vaccination. These results are consistent with other studies that reported immune responses to rVSVΔG-ZEBOV-GP that used the same validated ELISA and PRNT assays.[22, 33] This study demonstrates that in participants with asymptomatic malaria parasitemia at baseline, a somewhat lower response to vaccination is achieved. Although most participants with asymptomatic malaria infection met seroresponse definitions at most post-vaccination time points, we found significantly lower PRNT seroresponse and trends toward lower GP-ELISA seroresponse and lower PRNT and GP-ELISA GMTs. However, although the number of participants with high malaria parasite density was small, we saw no clear dose-response relationship. Thus, the clinical and practical relevance, if any, of these findings regarding asymptomatic malaria infection is unclear. It is plausible that malaria infection, especially symptomatic infection, could reduce the immune response to vaccination; this has been reported occasionally with vaccines against other infectious diseases [26]. However, our results should be interpreted in light of the clinical trial results from Guinea [4] and the vaccine effectiveness results from the 2018-2020 outbreak in the Democratic Republic of Congo [9]. Although neither specifically assessed the impact of malaria infection on vaccine effectiveness, both showed that rVSVΔG-ZEBOV-GP provided excellent protection against Ebola in malaria-endemic settings. Some complexities in our data that bear on the finding of lower vaccine response in participants with baseline malaria parasitemia merit further discussion. First, our observation that baseline GPELISA titers were higher in malaria-positive participants was unexpected. The lack of increased baseline PRNT seropositivity in malaria-positive participants argues against these participants having been exposed to Ebola, no Ebola (C and D) GMTs (A and C) and seroresponse percentages ≥2fold increase from baseline and ≥200 EU/ml, which was the defini(cid:127)on that best differen(cid:127)ated vaccine from placebo recipients in the Partnership for Research on Ebola Vaccines in Liberia (PREVAIL) clinical trial; (B and D) in par(cid:127)cipants in the immunogenicity substudy of the Sierra Leone Trial to Introduce a Vaccine against Ebola (STRIVE).