SARS-CoV-2 Transmission, before Delta Variant became Dominant, of Asymptomatic Primary Cases in Families. Asymptomatic Primary Cases More Frequently Give Origin to Asymptomatic Secondary Cases

An observational, longitudinal and retrospective study of families, in which there were at least two polymerase chain reaction (PCR) confirmed COVID-19 cases (one primary case or index and at least one secondary case), was conducted from March 15 to December 25, 2020, in a general medicine office in Toledo, Spain.