Fig. 1From: Protective action of natural and induced immunization against the occurrence of delta or alpha variants of SARS-CoV-2 infection: a test-negative case-control studySelection process of study population starting from candidates to vaccination identified at December 27, 2020, until the case identification (i.e., individuals for whom an infection caused by delta or alpha variant was ascertained from April 3, 2021, until July 16, 2021), and their inclusion by means of a matching design jointly with negative controlsBack to article page