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

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 placebo or SCB-2019 recipients.SPECTRA 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 anti-SARS-CoV-2 N-antigen IgG/IgM testing to detect new COVID-19 infections.154 eligible COVID-19 index cases (51 vaccinees, 103 placebo) were included. The secondary attack rate per household for symptomatic COVID-19 infection was 0.76% (90% CI: 0.15-3.90) if the index case was a SCB-2019 vaccinee compared with 5.88% (90% CI: 3.20-10.8) for placebo index cases, a relative risk reduction (RRR) of 79% (90% CI: -28-97). The RRR of symptomatic COVID-19 per household member was similar: 84% (90% CI: 28-97). Impact on attack rates in household members if index cases were symptomatic (n = 130; RRR = 80%; 90% CI: 7-96) or asymptomatic (n = 24; RRR = 100%; 90% CI: -76-100) was measurable but the low numbers undermine the clinical significance.In this prospective household contact study vaccination with SCB-2019 reduced SARS-CoV-2 transmission compared with placebo in households and in household members independently of whether index cases were symptomatic or not.