Methods
Study population
We randomly selected mother-infant dyads at birth, from 30 Anganwadi served communities, representative of the population residing in rural, urban, and resettlement colonies of Chandigarh, India. Data collection occurred during January 2017–October 2018.
Study staff first identified potential participants i.e., pregnant women, from Anganwadi records. For inclusion into the study, they had to be residing in Chandigarh, India at the time of delivery, and intending to reside in Chandigarh for at least a year after delivery. Study staff contacted each individual up to four times prior to the delivery date to invite their participation. If potential participants declined to participate or Study staff were not able to reach them, the next eligible individual from the enrollment log was contacted to participate in the study.
Enrolment of pregnant women was done at delivery in the hospital labour room. The following were excluded from enrolment: those with an acute febrile illness, those with known hemophilia or other blood dyscrasias characterized by potential for excessive bleeding, or those with a health condition necessitating immune-suppression medication. Infants born from participants were enrolled immediately after delivery.
Each enrolled infant underwent four scheduled follow-up visits at the ages of 3, 6, 9, and 12 months. Visits were scheduled during routine working hours, and mentioned on the Study records provided to the family. In addition, Study staff contacted the family telephonically at least 48 h prior to the scheduled visit, to remind them about the follow-up visit.
Derived variables
The main outcome in this study was being ‘lost to follow-up’. If caregivers of infants declined to follow-up during telephonic reminders, or failed to follow-up despite reminders, or could not be contacted prior to, or after the scheduled visit date elapsed, the Study staff recorded the reasons for loss to follow-up. The major categories were: (1) no longer interested to participate in the research study, (2) moved away from Chandigarh, (3) phone was disconnected (i.e., the number was no longer operational), or (4) other reasons.
The predominant independent variables were infant’s sex, mother’s age (trichotomized as 18–24 years, 25–29 years, and 30–42 years), caste [scheduled casted/scheduled tribe, other backward caste (OBC), or other], religion (dichotomized as Hindu or not), and monthly income (< 10,000 INR, 10,000–24,999 INR, 25,000–49,999 INR and ≥ 50,000 INR, which correspond to < $133, $133–$333, $334–$666 and > $666, respectively per current exchange rates). We also considered when the drop-out occurred, as the study check points were at birth, and age of three, six, nine and twelve months.
Statistical analysis
We summarized the number of individuals lost to follow-up, by reason. We ran two Rao-Scott chi-square tests to compare demographic information for those who were lost to follow-up versus those who were not. Reasons for loss to follow-up were also depicted graphically across study check points. The chi-square tests accounted for the survey design (clustering at Anganwadi centers). A p value < 0.05 was considered significant. Analyses were processed in SAS version 9.4 (SAS Institute, Cary, NC).
Ethical approval
This study was approved by the University of Michigan Health Sciences and Behavioral Sciences Institutional Review Board (HUM00104905), the Institutional Ethics Committee of the Postgraduate Institute of Medical Education and Research, Chandigarh, and the Health Ministry Screening Committee of the Government of India. Enrolled mothers provided written, informed consent to participate. Either parent provided written, informed consent for enrolment of their infants.