Analysis 2: Matched Case-Control Analysis for Symptomatic Breakthrough Infections

Similar to the analysis of infections, we performed a matched case-control analysis for symptomatic infection (COVID-19). Cases were individuals who tested positive and exhibited COVID-19-related symptoms after 15 June 2021. Eligible controls were individuals who had not tested positive. Matching was performed as for the infection case-control analysis. Again, at least 1 and up to 20 controls per case were drawn from the entire population; 89% of cases were matched to 20 controls, 96% of cases had at least 10 controls, and 99% of cases had at least 5 controls; 99.8% of cases were matched to at least 1 control. A conditional logistic regression was fit to the data, adjusting for age and obesity, as specified in analysis 1.

Analyses were performed using R version 4.0.5. The analysis conformed to the STROBE checklist for case-control studies.