More than 1 in 6 toddlers don't complete early childhood vaccine series. Here's why.

Vaccination is a routine part of health care, but many childhood vaccines require three or four doses over time for optimal protection. Now, a new study finds that 1 in 6 toddlers started the recommended vaccination series but didn't complete it, leaving them vulnerable to a host of serious illnesses.

The news comes just months after a new report from the Centers for Disease Control and Prevention (CDC) found that childhood vaccination rates dropped for the third year in a row.

But what's behind children not receiving their complete vaccination series and how serious is it? Pediatricians break it down.

What the study says

The study, which was published in the journal Pediatrics, found that while many toddlers in the U.S. start their routine vaccination series, a large number don't actually complete it.

What are the key findings?

The study analyzed data from the 2019 National Immunization Survey-Child and looked at immunization rates of more than 16,000 children between the ages of 19 and 35 months for the vaccine series that protects kids from 11 illnesses — diphtheria, tetanus, acellular pertussis, pneumococcal infections, Haemophilus influenzae type b, hepatitis B, polio, measles, mumps, rubella and varicella.

The researchers found that while nearly 73% of toddlers completed the seven-vaccine series, 17.2% (1 in 6) started the series but didn't complete it. About 1.1% of children weren't vaccinated at all, and nearly 10% didn't start one or more of the seven vaccines.

The researchers didn't ask parents why their children didn't complete their vaccine series, but found links between those who didn't finish the series and moving across state lines, having a larger number of children in the house and not having insurance coverage.

"More than one in six US children initiated but did not complete all doses in multi-dose vaccine series, suggesting children experienced structural barriers to vaccination," the researchers wrote. "Increased focus on strategies to encourage multi-dose series completion is needed to optimize protection from preventable diseases and achieve vaccination coverage goals."

What experts think

Doctors say they're not shocked by the findings. "Is it surprising? No. Disappointing? Yes," Dr. Danelle Fisher, a pediatrician and chair of pediatrics at Providence Saint John’s Health Center in Santa Monica, Calif., tells Yahoo Life.

"The results of the study are not surprising to me, given both the concerns about vaccine hesitancy as well as the number of visits needed in the early childhood years to get children fully vaccinated and protected," Dr. Melissa Stockwell, chief of the Division of Child and Adolescent Health at New York-Presbyterian/Columbia University Irving Medical Center, tells Yahoo Life.

It's important to note that the data is from 2019, and doctors say things haven't improved since then. "Unfortunately, things have gotten worse since the pandemic as distrust of the COVID vaccine reinvigorated some parents to delay or not give vaccines to their children, especially since they were stuck at home," Dr. Victoria Regan, pediatrician with Children’s Memorial Hermann Pediatrics in Texas, tells Yahoo Life. "Also, with children at home and not in day care or school, parents did not get reminders to keep their child’s vaccines up to date."

Some families fell behind and while many caught up, others never did, Fisher says.

There are a few additional factors other than what the researchers flagged that can be standing in the way of children being fully vaccinated, Stockwell says. One is vaccine safety. "Parents are trying to do the right thing by their child, but they may not know what the right thing is since there is so much disinformation out there," Fisher says. "The second is not perceiving risk for their child — wondering whether the vaccine is needed."

Finally, access is important. "Primary care practices may not be offering appointments at times that work for families, and this may particularly adversely affect families who cannot take off time from work without losing income," Stockwell says.

Why it matters

When children aren't up to date on their vaccines, "it raises the risk that they'll get sick with totally preventable illnesses," Fisher says. "It's important that our kids get this protection," she adds. Regan agrees. "Vaccines do work, are proven safe and prevent children from dying," she says.

But vaccines don't just protect the children who receive them — they also provide herd immunity to communities, Fisher points out. As vaccine levels drop, it raises the risk that certain illnesses may circulate and make children — and adults — sick. (Measles, for example, requires a 95% vaccination rate to protect the other 5%.)

"The first thing a parent should do is to make sure their child is up to date on all vaccines," Fisher says. "It's very important to get them in a timely fashion. I wish they were effective in the first dose, but they're not."

Fisher says that doctors are "always happy to help catch kids up."

She also points out that lack of health insurance shouldn't keep kids from getting fully vaccinated. "All states have programs that parents can access to help," she says. "Lack of health insurance shouldn't be a barrier."

While the findings are not great, Stockwell says they indicate that "hesitancy is a spectrum."

"The good news is that only 1.1% were completely unvaccinated and only 9.9% had not initiated at least one vaccine," she says. "That means the rest started the series, and the focus needs to be on how to support families on completing the series in a timely fashion."

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