To determine whether breast-feeding protects infants from infections, a case-control study was conducted. The cases were previously healthy children who were admitted to Yale-New Haven Hospital for an infectious illness at or before 90 days of age. The controls were chosen from the log of births and matched to the cases for five important demographic variables. In addition, logistic regression models were used to adjust the results for other potential confounders. To minimize the potential surveillance bias that might occur if formulafed and breast-fed infants with the same degree of illness have a different probability of being hospitalized, the case-control pairs were stratified by the severity of the medical condition of the case at the time of hospitalization. For the 281 case-control pairs, the matched odds ratio was .50 (95% confidence interval .32, .77; P .005), which indicates that breast-feeding is protective against infections. However, this apparent protective effect was diminished substantially when the data were stratified according to the severity of illness: the matched odds ratio for the 164 infants with serious illnesses was .79 (.47, 1.32; P .50), and for the 117 infants with mild illnesses it was .17 (.03, .44; P .001). These stratified results suggest that breast-feeding protects infants from hospitalization rather than from infections. Failure to consider the problem of surveillance bias may lead to erroneous conclusions about the protective effect of breast-feeding.
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机译：为了确定母乳喂养是否能保护婴儿免受感染，进行了病例对照研究。病例为先前健康的儿童，他们在90日龄或之前因传染病入读耶鲁-纽黑文医院。从出生对数中选择对照，并与五个重要人口统计学变量的病例相匹配。另外，逻辑回归模型用于调整其他潜在混杂因素的结果。为了最大程度地降低如果患病程度相同的配方奶和母乳喂养的婴儿住院的可能性不同时可能出现的监视偏见，病例对照对按当时病例的医疗状况的严重性进行分层住院。对于281个病例对照对，匹配的优势比为.50（95％置信区间.32，.77; P <.005），这表明母乳喂养可以预防感染。但是，当根据疾病的严重程度对数据进行分层时，这种明显的保护作用会大大降低：164名严重疾病的婴儿的匹配优势比为.79（.47，1.32; P <.50），而117名轻度疾病婴儿为.17（.03，.44; P <.001）。这些分层结果表明，母乳喂养可以保护婴儿免受住院治疗，而不是受到感染。不考虑监视偏见的问题可能导致关于母乳喂养的保护作用的错误结论。
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