Study setting and population

This retrospective cohort study used population-based administrative data in Alberta, a Canadian province with a population of approximately 4.5 million (Government of Alberta, 2022). We assessed the immunization coverage of a province-wide cohort of children born in Alberta in 2008 when they were at the start of kindergarten and end of grade one, including children who received their education through homeschooling (typically < 2% of the population of Albertan schoolchildren are homeschooled in a given school year) (Van Pelt, 2015). All routine childhood immunizations in Alberta are covered by a universal provincially funded health care insurance plan (Bell et al., 2015). The recommended vaccines for preschool children during the study period were diphtheria, pertussis, tetanus, polio, Haemophilus influenzae type b (DTaP-IPV-Hib); meningococcal conjugate (Men C); pneumococcal conjugate (PCV); and measles, mumps, rubella (MMR), or MMR-varicella (MMRV) (see Table ​Table1).1). We excluded (1) children with health care cancellation by age 7 (death or migration from the province), (2) First Nations children living on reserves, who receive vaccines through federal programs, and (3) children living in the town of Lloydminster (whose vaccines are administered by the neighbouring province). We included only one child of any multiple births (e.g., twins), selected randomly, to ensure the independence of observations.

Alberta recommended routine preschool childhood immunization schedule that applies to a 2008 birth cohort

^aThis schedule reflects changes to the vaccine schedule that occurred in 2012 that applies to the 2008 birth cohort. Specifically, the 2^nd dose of varicella was added for 4- to 6-year-old children in Aug 1, 2012. Therefore, children who turned 4 years old before Aug 1, 2012, would not be eligible to receive a second dose of varicella vaccine, and thus would have received MMR vaccine instead of MMRV

^bAlthough these doses are offered to children at 4–6 years of age, it is recommended that they receive them before they begin school, in order to ensure that they are fully protected