Impact of vaccination with SCB-2019 COVID-19 vaccine on transmission of SARS-CoV-2 infection: a household contact study in the Philippines

ABSTRACT Background An exploratory household transmission study was nested in SPECTRA, the phase 2/3 efficacy study of the adjuvanted recombinant protein-based COVID-19 vaccine SCB-2019. We compared occurrence of confirmed COVID-19 infections between households and household contacts of infected SPECTRA participants who were either placebo or SCB-2019 recipients. Methods SPECTRA trial participants at eight study sites in the Philippines who developed rRT-PCR-confirmed COVID-19 were contacted by a study team blinded to assignment of index cases to vaccine or placebo groups to enroll in this household transmission study. Enrolled households and household contacts were monitored for three weeks using rRT-PCR and rapid antigen testing to detect new COVID-19 infections. Results Observation of the households of 154 eligible COVID-19 index cases, 130 symptomatic and 24 asymptomatic at diagnosis, revealed household secondary attack rates for any COVID-19 infection of SCB-2019 index cases of 0.76% (90% CI: 0.15–3.90) compared with 5.88% (90% CI: 3.20–10.8) in placebo index case households, a relative risk reduction of 79% (90% CI: -28–97). The relative risk reduction of symptomatic COVID-19 was 84% (90% CI: 28–97) for household contacts of all COVID-19 infected index cases, and 80% (90% CI: 7–96) for household contacts of index cases with symptomatic COVID-19. Conclusions In this prospective household contact study vaccination with SCB-2019 reduced SARS-CoV-2 transmission in households, so decreasing infections of household contacts, compared with placebo.