Impact of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Vaccination and Pediatric Age on Delta Variant Household Transmission

Background The impact of SARS-CoV-2 vaccination status and paediatric age on transmission of the Delta variant is key to preventing COVID-19 spread. In Singapore, quarantine of all close-contacts, and quarantine-entry and exit PCR testing, enabled evaluation of these factors. Methods This retrospective cohort study included all household close-contacts between March 1, 2021 and August 31, 2021. Logistic regression using generalized estimating equations was used to determine risk factors associated with SARS-CoV-2 acquisition and symptomatic disease. Findings Among 8470 Delta variant-exposed household close-contacts linked to 2583 indices, full-vaccination of the index with BNT162b2 or mRNA-1273 was associated with significant reduction in SARS-CoV-2 acquisition by contacts (adjusted odds ratio [aOR]:0.56, 95% robust confidence interval [RCI]:0.44–0.71 and aOR:0.51, 95%RCI:0.27–0.96 respectively). Compared to young adults (18–29y), children (0–11y) were significantly more likely to transmit (aOR:2.37 [95%RCI:1.57–3.60]) and acquire (aOR:1.43 [95%RCI:1.07–1.93]) infection, taking into account vaccination status. Longer duration from completion of vaccination among contacts was associated with decline in protection against acquisition (first-month aOR:0.42, 95%RCI:0.33–0.55; fifth-month aOR:0.84, 95%RCI:0.55–0.98; p<0.0001 for trend) and symptomatic disease (first-month aOR:0.30, 95%RCI:0.23–0.41; fifth-month aOR;0.62, 95%RCI:0.38–1.02; p<0.0001 for trend). Contacts immunized with mRNA-1273 had significant reduction in acquisition (aOR:0.73, 95%RCI:0.58–0.91) compared to BNT162b2. Conclusions Among household close-contacts, vaccination prevented onward SARS-CoV-2 transmission and there was increased risk of SARS-CoV-2 acquisition and transmission among children compared with young adults. Time after completion of vaccination and vaccine type affected SARS-CoV-2 acquisition.