Impact of Vaccination With the SCB-2019 Coronavirus Disease 2019 Vaccine on Transmission of Severe Acute Respiratory Syndrome Coronavirus 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 placebo or SCB-2019 recipients. Methods 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. Results 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. Conclusions 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.

Mucosal vaccines for SARS-CoV-2: scientific gaps and opportunities—workshop report

“…Speakers also highlighted the challenge of designing clinical studies to measure the impact of a vaccine on transmission. While reduced transmission of SARS-CoV-2 to household contacts has been demonstrated for some parenterally administered vaccines 35 , none of the current mucosal vaccine products or candidates have demonstrated efficacy against infection or transmission in humans, nor has this been required for authorization. CHIM may play a role in studies to dissect the underlying mechanism of transmission and correlates of protection, but these designs face major challenges, including long timelines for the production of challenge strains and difficulty establishing infection in individuals with pre-existing immunity 18,36 .…”

“…CHIM may play a role in studies to dissect the underlying mechanism of transmission and correlates of protection, but these designs face major challenges, including long timelines for the production of challenge strains and difficulty establishing infection in individuals with pre-existing immunity 18,36 . Assessing prevention of transmission in the context of a randomized controlled trial (RCT) is possible but operationally challenging and resource intensive 35 . The RCT design also depends on the endpoint of interest.…”