Identification of Contacts With High Rates of Missed and Asymptomatic Infection Following an Outbreak of Ebola Virus Disease: A Seroepidemiological Study in Likati Health Zone, Democratic Republic of the Congo

Abstract

Abstract Background Ebola virus disease (EVD) is often regarded as severe and highly fatal, but growing evidence suggests that subclinical or minimally symptomatic infections occur and frequently go undetected. During the 2017 outbreak in Likati Health Zone, only 8 cases were confirmed despite many reported exposures. We evaluated the extent of asymptomatic or unrecognized Ebola virus infection and associated factors among contacts of reported cases. Methods In November 2017, we conducted a cross-sectional community-based serosurvey among contacts originally identified through Ministry of Health records and newly identified through additional post-outbreak investigations. Participants provided blood samples and completed questionnaires on demographics, exposures, and symptoms within 4 weeks of symptom onset of the EVD case with whom contact was reported. Sera were tested for anti-EBOV nucleoprotein IgG by ELISA. Seropositive individuals were classified as asymptomatic (no symptoms) or unrecognized (≥1 symptom). Secondary attack rate (SAR) was estimated, and logistic regression assessed associations with sociodemographic factors, exposure level, and symptoms. Results Among 180 participants (79 originally identified; 101 newly identified), 33 (18.3%) were seropositive. Of these, 19 (58%) reported symptoms, and 14 (42%) were asymptomatic. Any EVD-related symptom was associated with higher odds of seropositivity (OR 2.48, P = .021), though no specific symptom was significant. Asymptomatic individuals had higher antibody titers than symptomatic seropositive contacts (P = .009). The overall SAR was 19.1% (95% CI: 15.9–23.0). High exposure level strongly predicted seropositivity (OR 11.2, 95% CI: 3.8–33.3). Conclusions Asymptomatic infections occurred, including among contacts missed during the response, highlighting the need for exposure-based serologic assessments in EVD investigations and raising questions about immune responses and the true disease burden in outbreak-affected settings.