Figure 2. Selection process of participants in a study of SARS-CoV-2 secondary attack rates in vaccinated and unvaccinated household contacts during replacement of Delta with Omicron variant, Spain. Index case-patients were those who first showed clinical symptoms of infection in a specific household and sought diagnosis or treatment at a primary healthcare center. Contacts were defined as persons who had spent >15 min with the index case-patient in an indoor space without intervention measures, such as masks, during the 48 hours before COVID-19 diagnosis was confirmed for the index case-patient. Contacts with no RT-PCR results and negative Ag-RDT were excluded from the study. Ag-RDT, rapid antigen detection tests; RT-PCR, reverse transcription PCR.