Characteristics of the study population and immunisation status.

SIDS: Sudden Infant Death Syndrome.1The information on citizenship was missing for 7 infants.2Causes of death judged to be compatible with sudden unexpected death after the revision of death certificates.SIDS：婴儿猝死综合征（Sudden Infant Death Syndrome）。1. 7名婴儿的公民身份信息缺失。2. 经死亡证明修订后，判定其死因符合突发性意外死亡。