A majority of babies are getting all their vaccines right on time, but the small minority of babies whose families are limiting or forgoing vaccination has grown.

Consistent vaccine-limiting "has become relatively common and stable over time — between 1.5% and 2% of families will not give all the vaccines that are recommended at a given visit, and they might limit it to just two vaccines," said Matthew F. Daley, the lead author of a study published April 29 in the American Journal of Preventive Medicine. 

Even though these families are delaying vaccines rather than forgoing them entirely, there's still cause for concern, said Daley, a senior clinician investigator at Kaiser Permanente Colorado's Institute for Health Research and an associate professor of pediatrics at the University of Colorado School of Medicine.

"The timing of vaccines is designed to protect kids when they're most vulnerable, and it also is to give vaccines at a time that we know that they'll be both safe and effective," Daley said. "A baby who doesn't get his or her first dose of vaccines until 4 months of age, I worry about. ... Waiting until 4 months of age, from a parent standpoint, you may think, 'Well, they're older, they can handle it better.' But, really, they're unprotected when they're most vulnerable."

In the study of 808,170 children born between 2004 and 2017, researchers found that 68.4% of children born in 2017 got all their recommended vaccinations on schedule before their second birthday. That was a significant uptick from children born in 2008, only 47.1% of whom got all their vaccines on time. However, the percentage of completely unvaccinated toddlers increased from 0.35% of babies born in 2004 to 1.28% of babies born in 2017. 

Meanwhile, 2.04% of babies born in 2017 were consistently receiving fewer vaccinations than recommended, leaving them unprotected for longer periods and potentially leaving them behind altogether.

"We find that kids who are delayed don't catch up," Daley said.

The study relied on electronic health records, immunization information systems, and parent surveys of 1,444 people who took their babies to at least three well-child visits at one of eight large medical organizations that are Vaccine Safety Datalink sites working with the Centers for Disease Control and Prevention. Researchers tracked each day that a vaccine was late for all vaccines recommended during the first 23 months of a child's life, except for hepatitis A and flu vaccines. Babies had a one-month grace period beyond the recommended vaccination window in which they could get vaccinated and be considered "on time." The study excluded babies who had a medical reason that they couldn't get vaccinated, such as a compromised immune system.

The caregiver survey included a measure of vaccine hesitancy, and the results showed a clear pattern: Only 5.8% of parents with fully vaccinated babies felt highly hesitant to vaccinate their children, while 56.5% of parents with highly undervaccinated children were highly hesitant to vaccinate. However, the survey also highlighted some access problems: The parents of mildly undervaccinated children were unlikely to say they were skeptical of vaccines, suggesting there were other factors at play.

"Everybody wants the best for their children," Daley said. "Given all the misinformation out there, people are just trying to make the best decision they can, and they feel like the best decision is to wait a bit, even though, based on all the data we have, that's probably not the best decision for their child and for society writ large."

Daley said that as a practicing pediatrician at Kaiser Permanente Colorado, he occasionally encounters "a family who says, 'My baby's 2 months old, and he just seems really young and little; can't we wait a month or two until we start the vaccinations?' Then I'll have that conversation about that most vulnerable period, when they're only a couple months old."

Still, to him, the main findings of the study were cheering. 

"Family medicine physicians and pediatricians have felt a bit on the defensive — they open the paper and read about vaccine hesitancy, they open the paper and read about COVID-vaccine hesitancy, but in some ways, what this data shows is that, yes, parental vaccine refusal and delay happens, but it's still pretty uncommon," Daley said. "What that means is that everything we're doing every day is working. The vast majority of parents want to vaccinate according to the recommended schedule."

The paper, "Temporal trends in undervaccination: a population-based cohort study," published April 29 in the American Journal of Preventive Medicine, was authored by Matthew F. Daley, Kaiser Permanente Colorado and University of Colorado; Liza M. Reifler, Jo Ann Shoup and Komal J. Narwaney, Kaiser Permanente Colorado; Elyse O. Kharbanda, HealthPartners Institute; Holly C. Groom, Kaiser Permanente Northwest; Michael L. Jackson, Kaiser Permanente Washington; Steven J. Jacobsen, Kaiser Permanente Southern California; Huong Q. McLean, Marshfield Clinic Research Institute; Nicola P. Klein, Kaiser Permanente Vaccine Study Center; Joshua T.B. Williams, Denver Health and Hospitals; Eric S. Weintraub and Michael M. McNeil, Centers for Disease Control and Prevention; Jason M. Glanz, Colorado School of Public Health.