In Reply Ms Zhu and colleagues express concerns regarding potential bias and confounding in the study evaluating the safety of repeated tetanus-containing vaccines in pregnancy.
First, they point to the accuracy of methods for identifying gestational age. The pregnancy episode algorithm was used to identify live births in our study but was not the method used for identifying gestational age. The validation data presented were specific to the pregnancy episode algorithm for identifying live births. We stated in the article that we limited the cohort for birth outcomes to “records that contained information on the neonate (ie, weight and gestational age).” In the Vaccine Safety Datalink, gestational age data come from electronic medical record and state birth registry data, which are based on clinician assessment. The clinician uses all available data to determine this estimate, including estimated due date based on ultrasound and last menstrual period data and can adjust this estimate based on the infant’s appearance at birth.
Sukumaran L, Omer SB. Repeat Tdap Vaccination and Adverse Birth Outcomes—Reply. JAMA. 2016;315(12):1286. doi:10.1001/jama.2015.19087