The present analysis utilizes data on the immunization status of low birth weight infants enrolled in a large individually randomized, double–masked, placebo–controlled trial of neonatal vitamin A supplementation within 72 hours of birth. This study was conducted in Faridabad and Palwal districts in the state of Haryana, North India from June 2010 to July 2012 [23]. The trial procedures and details of study area have been described previously [23,24].

The trial was approved by the ethics review committees of the Society for Applied Studies, World Health Organization (WHO) and by the state government of Haryana. It is registered with ClinicalTrials.gov, number NCT01138449. Permission was taken from all the concerned investigators of the primary trial for this secondary data analysis

Study teams identified pregnant women through household surveillance at intervals of 3 months in areas allocated to them. The pregnancies identified were followed up until delivery and birth outcomes were reported to the co–ordinators who then informed the enrolment workers immediately [24]. For each live birth identified, the study team visited the family, explained the trial, screened the infant against a pre–defined eligibility criteria (infants aged ≤3 days at screening, could suck or feed and family intended to stay in the study area for at least 6 months) and obtained written consent from at least one parent ie, mother/ father of eligible infants. The enrolled infant was weighed by the study team members who were trained and standardized.

At enrolment, information was collected primarily on household characteristics (social class, religion, wealth quintile), infant characteristics (birth weight, sex, place of delivery, personnel conducting delivery, multiple births), maternal characteristics (number of living children, age, education, occupation) and father’s education. Each enrolled infant was allocated a home visit worker for further follow up until 12 months of age. All infants were contacted when aged 29 days and at 3, 6 and 12 months and at each such visit, information was collected on vital status and immunization.

At each visit, the study team member looked for written documentation of vaccines administered to the infant. The documents reviewed were maternal and child health card, immunization card of the infant or any slip(s) issued by the facility where vaccination was done. The study team made several attempts to obtain written documented evidence of vaccination. This included a wait time, to ensure the mother ample time to search for the missing record, telephoning the father for any relevant information, and also postponing the visit to a later date. If the immunization card was still not available the team helped mother to report accurate dates by referring to important events or festivals. Also, the mother was asked to recall which vaccines were given, at what body site and the mode of vaccination (oral or injection). An infant was categorised as “not vaccinated” when the mother reported infant had never been vaccinated.

The primary outcomes were full immunization by one year of age and delayed vaccination with DPT1 and DPT3 in LBW infants. In concordance with the guidelines of the National Immunization Program in India, an infant was considered “fully immunized” if he/she had received BCG, 3 doses of DPT, OPV each and measles by one year of age [25]. Hepatitis–B immunization was not considered as a part of full immunization as the vaccine was not introduced during the time of trial in the state of Haryana [26]. There is no standard approach to the assessment of delayed vaccination and several definitions have been described [27,28]. However, previous studies have considered DPT–1 and DPT–3 vaccination as acceptable points to assess delay [13,21].

Operationally, “delayed vaccination” was defined as having received the vaccine after 4 weeks of recommended/due time [21,29,30]. In India BCG is to be given at birth; OPV–1 and DPT–1 at 6 weeks of age; OPV–2 and DPT–2 at 10 weeks of age, OPV–3 and DPT–3 at 14 weeks of age; measles at 9 months of age [25]. For the primary analysis, a delay in DPT–1 was considered when the LBW infant was vaccinated later than 10 weeks of age and for DPT–3, when the infant was vaccinated at >18 weeks of age.

Additionally, sensitivity analysis was done to assess whether delayed DPT3 vaccination reflected delayed DPT1 vaccination. Starting with follow–up at receipt of DPT1 vaccination, an infant was labelled as having a “delayed receipt” of DPT–3 when it was given >12 weeks after DPT–1 (according to National Immunization Schedule, the time interval between DPT–1 and DPT–3 should be 8 weeks).

For the analyses, infants with known vaccination status, dates of vaccination and with complete data on covariates were included. Infants who were lost to follow–up or died before the vaccination due date, were excluded. This principle was followed for all the time points of analysis. Data analysis was performed using STATA version 11 (Stata Corporation, College Station, TX). Proportions were calculated for all categorical variables used in the analysis. Median (interquartile range; IQR) was calculated for delay in vaccination (in days), from the recommended time, for each of the vaccine that was considered in the analysis. Chi–square test was done to compare proportions and Wilcoxon–Mann–Whitney 2– sample rank sum test to compare medians across the two birth weight categories.

Multivariable logistic regression was performed to identify factors associated with full immunization and delayed vaccination. Bivariate analysis was first done for all explanatory variables and those with a P–value of <0.20 were then included in the final multivariable logistic regression model [31,32]. A P–value of <0.05 was considered statistically significant in the final regression model. Explanatory variables considered were household characteristics (wealth index, religion. social class); maternal and paternal characteristics (maternal age, maternal education, maternal occupation, paternal education); birth related characteristics (place of delivery, personnel conducting delivery, multiple births, and number of living children) and infant characteristics (birth weight and sex).

Additionally, to assess the association of birth weight on study outcome(s) ie, “full immunization “and “delayed vaccination”, regression analysis was done with birth weight as the exposure variable (in dichotomous form ie, ≥2500 and <2500 g) and adjustment done for other covariates. Assessment for effect modification (ie, potential interaction) between birth weight and all covariates was done using interaction term in the model. Likelihood ratio test was used to compare models with or without the interaction term. Sensitivity analysis was also conducted where data collected only from immunization cards were analysed to document the determinants of full immunization and delayed vaccination in low birth weight infants. Analysis to document the determinants of full immunization and delayed vaccination in normal birth weight infants was also undertaken on an exploratory basis.

A total of 44 984 infants were recruited in the primary trial, within 72 hours of birth, out of which 10 644 (23.7%) were low birth weight infants. This subset of LBW infants was analysed for the primary outcome(s). However, to give a general sense of how these low birth weight infants compared to their normal birth weight counterparts, a comparative description of the characteristics have been presented ( Table 1 ). The proportion of female infants was more in the LBW category (55.5%) compared to normal birth weight (NBW) infants (45.6%) ( Table 1 ). The mean (SD) birth weight of normal and low birth weight infants was 2914.0 (421.0) and 2193.1 (224.3) grams respectively. A similar proportion of LBW and NBW infants were born at home (44.6% vs 42.8%). Most of the LBW infants had mothers with low literacy (50.3% of mothers reported having no education or not completing primary school) and 11% had mothers with age less than 20 years ( Table 1 ). In NBW infants, low maternal literacy was observed in 44.4% and around 7% (2388/34,340) had mothers aged less than 20 years.

Figure 1 shows the flow of infants recruited in the primary trial, starting from the time of recruitment ie, within 2 days of birth, until the age of 1 year. Out of the 10 644 LBW infants that were enrolled, 847 died by the end of one year (infant mortality rate of 79.6/1000 live births). A large proportion of the deaths occurred in the first 6 weeks of life (n = 404/847; 47.4%). In normal birth weight infants, there were a total of 971 deaths in the first year of life, resulting in an infant mortality rate of 28.3/1000 live births.

Out of the total 10 644 LBW infants that were enrolled in the trial, immunization data was available for 10 517 (98.8%). In 77.8% infants, data was obtained through “immunization card” and in rest; it was elicited through reliable history. Low birth weight infants had a comparatively lower immunization uptake compared to normal weight infants, both in terms of the proportion that received a particular vaccine and also in appropriateness of timing of receiving vaccine ( Table 2 ). Around three–fourth received BCG vaccine (75.9%) and one–fifth (20.3%) received zero–dose of polio vaccine. The proportion that received DPT–1, DPT–2, DPT–3 and measles vaccine was 74.1%, 58.3%, 45.4% and 35.6% respectively. Less than one–third (29.7%) were fully immunized by one year of age ie, had received BCG, three doses of DPT and OPV each and measles vaccine.

There was a delay in the time of receipt of the vaccines compared to the recommended time as per the National Immunization Schedule. The median (interquartile range; IQR) delay for BCG, DPT–1, DPT–2 and DPT–3 was 41 (19–75), 30 (12–63), 46 (23–89) and 62 (34–112) days respectively. Around 65% of the LBW babies had delay in receiving BCG, 52% in DPT–1, 68% in DPT–2 and 81% in DPT–3. For measles vaccine, the median (IQR) delay from the recommended time was 24 (9–46) days and around two–fifth infants (43.5%) had delay in receiving the vaccine.

Out of 9779 LBW infants that were alive at the age of 1 year, 2913 (29.7%) were fully immunized. There was a dose response relationship between wealth quintiles and full immunization status. Those LBW infants who were from the lowest wealth quintile had the lowest odds compared to those in highest wealth quintile (adjusted odds ratio (AOR) 0.39; 95% confidence interval (CI), 0.32–0.47] ( Table 3 ). Belonging to a Muslim family (AOR 0.41; 95% CI, 0.35–0.48), mother’s age <20 years (AOR 0.62; 95% CI, 0.52–0.73) delivered by unskilled attendant (AOR 0.77; 95% CI, 0.64–0.91) and being a female (AOR 0.84; 95% CI, 0.77–0.92) decreased the odds. With increasing number of living children a women had, the odds of fully immunizing the recently delivered LBW baby decreased; the lowest odds in those with ≥4 children (AOR 0.58; 95% CI, 0.43–0.77) ( Table 3 ).

Compared to infants with illiterate parents, those with mothers [AOR 2.39; 95% CI, 1.97–2.91] and fathers [AOR 1.49; 95% CI 1.22–1.83] who were educated until secondary school or higher (≥12 years of schooling) had increased odds of full immunization. Mother’s age was also an important determinant. Compared to mother’s aged 20–25 years, those aged 26–30 years [AOR 1.38; 95% CI, 1.19–1.58] and >30 years [AOR 1.49; 95% CI, 1.15–1.95] had higher odds of getting their child fully immunized. Also, delivery at a government health facility [AOR 1.29; 95% CI, 1.08–1.54] increased the odds ( Table 3 ).

In the sensitivity analysis, using data documented through immunization cards, 15.2% of LBW infants were fully immunized by 12 months of age. The determinants of full immunization in these infants were essentially similar to those obtained when combined data obtained through immunization cards and reliable histories were analysed (Table S1 in Online Supplementary Document). Lower wealth quintile, belonging to a Muslim family, mother’s age <20 years, delivered by unskilled attendant, ≥4 living children for the woman and female sex of the infant were associated with decreased odds. Unlike in the combined analysis wherein father’s education was associated with increased odds of the child for being full immunized, in the sensitivity analysis, father’s education did not emerge as a statistically significant determinant. However, mother’s education and delivery at the government health facility were associated with increased odds.

Lowest wealth quintiles [AOR 1.51; 95% CI, 1.25–1.82], Muslim religion (AOR 1.41; 95% CI, 1.21–1.65), mother aged <20 years (AOR 1.31; 95% CI, 1.11–1.53) and birth weight <2000 g (AOR 1.20; 95% CI, 1.03–1.40) were associated with higher odds of delayed vaccination with first–dose of DPT (DPT–1) vaccine ( Table 4 ). On the other hand, higher maternal education (AOR 0.59; 95% CI, 0.49–0.73) and delivery in a government facility (AOR 0.81; 95% CI, 0.68–0.96) were associated with lower odds of delay for DPT–1. Interestingly, maternal education status was the only variable that was significantly associated with delay in receiving third dose of DPT (DPT–3). Infants of mothers with ≥12 years of schooling (ie, secondary school complete and higher education) had lower odds of delay (AOR 0.57; 95% CI, 0.43–0.76) for DPT–3 compared to those with illiterate mothers ( Table 4 ).

In the sensitivity analysis, using data obtained through immunization cards, 46.6% and 79.2% of the LBW infants had delay in vaccination for DPT–1 and DPT–3 respectively. Lower wealth quintiles, Muslim religion, mother aged <20 years and birth weight <2000 g were associated with higher odds of delayed vaccination with first–dose of DPT (DPT–1) vaccine (Table S2 in Online Supplementary Document(Online Supplementary Document) ). On the other hand, higher maternal education (AOR 0.56; 95% CI, 0.45–0.71) and delivery in a government facility (AOR 0.71; 95% CI, 0.57–0.86) were associated with lower odds of delay for DPT–1. For DPT–3, higher maternal education was associated with reduced odds of delayed vaccination (AOR 0.56; 95% CI, 0.41–0.76).

Low birth weight was associated with reduced odds of full immunization (AOR 0.85; 95% CI, 0.81–0.90) ( Table 5 ). For both DPT–1 and DPT–3, a statistically significant interaction was obtained between birth weight and sex (P–value for interaction = 0.0006 and 0.020 respectively). Low birth weight was associated with increased odds of delayed vaccination for DPT–1 (AOR 1.18; 95% CI, 1.10–1.28) and DPT–3 (AOR 1.18; 95% CI, 1.04–1.33) in male infants but there was no such significant association in female infants. After adjusting for the late vaccination with DPT–1, birth weight had no significant association with delay in DPT–3 vaccine ( Table 5 ).

As part of the exploratory analysis, determinants of full immunization and delayed vaccination were also documented for normal birth weight infants. Lower wealth quintiles, belonging to Muslim community, mother’s age <20 years and female sex were associated with low odds to full immunization, largely similar to that observed in low birth weight infant. Higher maternal education and delivery at a government facility were associated with increased odds of full immunization and decreased odds of delayed vaccination. They are presented in Table S3 and S4 in Online Supplementary Document(Online Supplementary Document) .