Vaccine effectiveness against onward transmission of SARS-CoV2-infection by variant of concern and time since vaccination, Belgian contact tracing, 2021

Abstract

Background: During the first half of 2021, we observed high vaccine effectiveness (VE) against SARS-CoV2-infection. The replacement of the alpha-'variant of concern' (VOC) by the delta-VOC and uncertainty about the time course of immunity called for a re-assessment.

Methods: We estimated VE against transmission of infection (VET) from Belgian contact tracing data for high-risk exposure contacts between 26/01/2021 and 14/12/2021 by susceptibility (VEs) and infectiousness of breakthrough cases (VEi) for a complete schedule of Ad26.COV2.S, ChAdOx1, BNT162b2, mRNA-1273 as well as infection-acquired and hybrid immunity. We used a multilevel Bayesian model and adjusted for personal characteristics (age, sex, household), background exposure, calendar week, VOC and time since immunity conferring-event.

Findings: VET-estimates were higher for mRNA-vaccines, over 90%, compared to viral vector vaccines: 66% and 80% for Ad26COV2.S and ChAdOx1 respectively (Alpha, 0-50 days after vaccination). Delta was associated with a 40% increase in odds of transmission and a decrease of VEs (72-64%) and especially of VEi (71-46% for BNT162b2). Infection-acquired and hybrid immunity were less affected by Delta. Waning further reduced VET-estimates: from 81% to 63% for BNT162b2 (Delta, 150-200 days after vaccination). We observed lower initial VEi in the age group 65-84 years (32% vs 46% in the age group 45-64 years for BNT162b2) and faster waning. Hybrid immunity waned slower than vaccine-induced immunity.

Interpretation: VEi and VEs-estimates, while remaining significant, were reduced by Delta and waned over time. We observed faster waning in the oldest age group. We should seek to improve vaccine-induced protection in older persons and those vaccinated with viral-vector vaccines.

Keywords: Bayesian analysis; Contact Tracing; Covid19; Infection; Infectiousness; SARS-CoV2; Susceptibility; Transmission; Vaccine effectiveness.

Fig. 1

Baseline Odds Ratio (95% CI) for susceptibility (upper) and infectiousness (bottom) by age group, Belgian contact tracing, 26/01/2021–14/12/2021.

Fig. 2

(Top) VE-susceptibility (95% CI) and (Bottom) VE-infectiousness (95% CI) by vaccine brand and previous infection and by VOC, 0–50 days after vaccination (alpha = black, delta = orange, hybrid immunity not presented), Belgian contact tracing, 26/01/2021–14/12/2021. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

VE-susceptibility (95% CI) by vaccine brand and previous infection and by time since vaccination (0–50 days (black) and 150–200 days after vaccination/infection (green), Delta), Belgian contact tracing, 26/01/2021–14/12/2021. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

VE-infectiousness (95% CI) by vaccine brand and previous infection and by time since vaccination (0–50 days (black) and 150–200 days after vaccination/infection (green), Delta), Belgian contact tracing, 26/01/2021–14/12/2021. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

(Top) VE-susceptibility (95% CI) and (Bottom) VE-infectiousness (95 %CI) for the different age groups (index case and HREC from the same age group, 0–50 (black) and 150–200 (green) days after full vaccination, fully vaccinated with BNT162b2 no previous infection, Delta), Belgian contact tracing, 26/01/2021–14/12/2021. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

VE-Transmission (95% CI) for females aged 65–84 years old without previous infection, by vaccine brand, VOC and time since vaccination (0–50 and 150–200 days), Belgian contact tracing, 26/01/2021–14/12/2021. The first column presents VE-estimates against susceptibility (protection of the HREC by vaccination against exposure from an unvaccinated index case), the first row presents VE-estimates against infectiousness (protection of unvaccinated HREC through reduced infectiousness of an index case by vaccination).

Fig. 7

VE-Transmission (95% CI) over time since vaccination by age groups and sex (HREC and Index case fully vaccinated with BNT162b2, from the same age group and same sex), Belgian contact tracing, 26/01/2021–14/12/2021.