Methods: We conducted a retrospective cohort study using linked data from the Colorado Birth Registry Database and the Colorado Immunization Information System of live births between 2008-2016. Our population was limited to singleton, first births with first varicella vaccination documented in the immunization registry. Our primary outcome was receipt of at least one influenza vaccination in children ≤ 2 yrs. Exploratory variables included maternal (number of prenatal visits, urban vs rural residence), and infant factors (term birth, admission to neonatal intensive care [NICU] at birth). Multivariable logistic regression was used to assess the association between these factors and influenza vaccination.
Results: Among 126,763 births in the cohort, 50.2% were vaccinated against influenza by 2 yrs of age. Mothers of unvaccinated children were older (27 vs 26 yrs), married (67.8% vs 66.8%), and more likely to have at least some college education (25.4% vs 24.1%). A higher proportion of infants admitted to the NICU or who received oxygen were unvaccinated compared to vaccinated (8.5% vs. 8.0% and 2.5 vs 2.1 respectively), p<0.001 for all. There were no differences between urban vs rural residence. In adjusted/stratified analyses, an increase in pre-natal visits was associated with a decrease in early influenza vaccination (IR=0.992, 95% CI 0.986- 0.998, p = 0.0084 for Hispanic mothers and IR=0.984, 95% CI 0.973-0.996, p = 0.0069 for non-Hispanic mothers). After adjusting for maternal age, preterm birth, and oxygen at birth, children admitted to the NICU were less likely to be vaccinated (IR=0.915, 95% CI 0.873-0.959) against influenza by 2 yrs.
Conclusion: There were statistically significant differences in maternal and neonatal factors between unvaccinated and vaccinated children with influenza in the first 2 years of life, but the differences were too small to be clinically significant. Ongoing studies are needed to devise strategies to target early influenza vaccination.
S. Rao, GSK: Investigator , Research grant .