As measles cases pop up across the country this winter — including several in California — one group of children is stirring deep concerns among pediatricians: the babies and toddlers of vaccine-hesitant parents who are delaying their child’s measles-mumps-rubella shots.

Pediatricians across the state say they have seen a sharp increase recently in the number of parents with concerns about routine childhood vaccinations who are demanding their own inoculation schedules for their babies, creating a worrisome pool of very young children who may be at risk of contracting measles, a potentially deadly yet preventable disease.

“Especially early on, when a parent is already feeling really vulnerable and doesn’t want to give something to their beautiful baby who was just born if they don’t need it, it makes them think, ‘Maybe I’ll just delay it and wait and see.’” said Dr. Whitney Casares, a pediatrician and author who has written on vaccination for the American Academy of Pediatrics. “What they don’t realize is if they don’t vaccinate according to the recommended schedule, that can really set their child up for a whole lot of risks.”

It is difficult to know how widespread such delays have become. California keeps careful track of the rate of kindergartners who have been vaccinated against measles, but does not have comprehensive data for children at younger ages.

Dr. Eric Ball has seen the shift firsthand. At his Orange County pediatric practice, Ball said, he has noticed an increase in parents asking about delays since the COVID-19 pandemic, as politicization of and misinformation about that vaccine has seeped into discussions about routine childhood vaccinations, including measles-mumps-rubella, known as MMR.

Rather than an outright refusal, however, these vaccine-hesitant parents express a softer kind of reluctance, asking if it’s possible to use an “alternative schedule” of vaccines, rather than sticking to the Centers for Disease Control and Prevention’s recommendations. Sometimes they seek to delay the shots by a few months, and sometimes by several years.

“I have patients who have three kids, and they vaccinated the first two kids on schedule. And then since COVID, with their third kid, they are like, ‘I don’t know if this is safe. I want to wait until the kids are older’, or ‘instead of doing two shots today, I want to do one shot,’” said Ball. “It just prolongs the time where you have a child who’s unprotected and potentially can get sick from these diseases.”

He tries his best to explain to parents the importance and safety of vaccines, including MMR. He even brings out his own children’s vaccine records to prove his point, and he is often successful. But not always.

At Children’s Hospital Los Angeles, attending pediatrician Dr. Colleen Kraft said about half of parents are questioning the CDC’s recommended vaccine schedule — a significant increase since the pandemic.

“Even my most reasonable parents ask questions. So it’s definitely in the mainstream,” she said. She also worries about her patients who are behind on vaccines because they missed so many appointments during the pandemic and are only now returning to her office.

In Marin County, parents’ requests to delay vaccinations have become so frequent that Dr. Nelson Branco said last month his practice decided to tighten vaccine requirements as cases of both measles and pertussis have spread. Babies seen by doctors in the practice will need to have their first set of vaccines completed by 4 months of age. The primary series of vaccines against the most serious and common diseases, including measles, must be completed by 24 months.

If parents don’t agree, they must leave the practice.

“Kids are doing a lot of things that are high risk before they’re 5 and are required to be vaccinated to attend kindergarten, said Branco. “They’re getting on international flights, they’re going to Disneyland where there are lots of kids,” leaving young children vulnerable to measles when they could be protected.

The CDC recommends that the first dose of MMR be given when a baby is 12 to 15 months old. Usually this happens at a child’s 12-month well visit. A second dose is then given at 4 to 6 years of age.

At least 95% of people in a community must be vaccinated to achieve a level of “herd immunity” that protects everyone in a community, including those who cannot get the vaccine because they are too young or are immunocompromised, according to the World Health Organization.

Low vaccination rates have led to measles outbreaks in several states over the last decade, most recently in Florida.

Nationally, the rate of kindergartners fully immunized against the measles dropped from 95% in the 2019-20 school year to 93% in 2022-23, according to the CDC.

But there is overall good news in California. Since the state’s 2015 ban on parents’ personal beliefs as a reason to skip vaccinating children before school, the measles vaccination rate for kindergartners has grown from 92% in the 2013-2014 school year to 96.5% in 2022-2023.

But those postponing vaccinations have created a potential vulnerability gap in a child’s first four years.

One in 5 unvaccinated people who get measles in the U.S. will be hospitalized. Since there is no good treatment for measles, doctors can often do little more than offer supportive care. One in 1,000 children with measles will develop brain swelling that can leave a child deaf or with an intellectual disability; 1 to 3 children in 1,000 will die, according to the CDC.

Measles is so contagious that 90% of people close to an infected person will catch it if they are not immune, according to the CDC. The virus can remain contagious in a room or on a surface for up to two hours after the infected person has left.

In the Children’s Hospital Orange County primary care network, which has more than 130 pediatricians, the share of 15-month-olds with an MMR vaccine has been dropping consistently over the past last few years, from 98% in 2019, down to 93.5% in 2023.

For years in the early 2000s, anti-vaccine sentiment was at an all-time high after the publication of a now-debunked and retracted study that falsely tied the MMR vaccine to autism. In December 2014, an unvaccinated 11-year-old was hospitalized with measles following a visit to Disneyland. Over the next few months, measles spread to 125 people across seven states.

The outbreak helped galvanize support for vaccination nationwide. A year after the Disneyland outbreak, California passed its ban on personal exemption.

“The pendulum swung back the other way, and we had a few years where vaccination rates were really high,” said Ball. But the rumors and rhetoric surrounding the COVID vaccines have caused the pendulum to swing in the other direction. “We’re back to dealing with conspiracy theories, things that people heard on the internet, or something that their cousin’s neighbor’s roommate said. It’s really hard.”

A Pew Research poll conducted in March 2023 found that 88% of Americans are confident that the benefits of an MMR vaccine outweigh the risks, a percentage that has remained fairly consistent since before the pandemic.

But support for all school-based vaccine mandates has fallen; 28% now say that parents should be able to decide not to vaccinate their children, even if it causes health risks for others, up from 16% in October 2019. Among Republicans, the share has more than doubled, from 20% in 2019 to 42% in 2023.

Support for the MMR vaccine was lower among parents with young children, the poll found. About 65% of parents with children under age 5 reported that the preventative health benefits of MMR were high — compared to 88% of all adults — and 39% said the risk of side effects was either medium or high; half said they worried about whether all childhood vaccines are necessary.

Tara Larson, a former ER nurse who lives in Santa Monica, said she became concerned about childhood vaccination when she was pregnant last year. She started watching anti-vaccine documentaries, reading vaccine safety inserts, and following several social media accounts “to make us an informed vaxxer. We’re not anti-vax,” she said.

Larson decided that she wanted to delay vaccinating her son until he was 3 months old, to limit him to just three vaccines in his first year that she felt were essential, and to spread them out so that he would only get one shot per month. “By the time he starts playing on the playground and goes to school, he’ll need to start his course of Hep B, but why overload his course of vaccines right now?” she said.

The first pediatrician she saw refused to follow her requested schedule. But, Larson said, “in my gut, I just felt like this is the right thing to be doing for our baby, and I left.” After weeks of searching, she found a holistic provider who charges a $250 monthly fee and agreed with her approach.

She said she hasn’t yet decided whether to give her son, who is now 8 months old, the MMR vaccine when he becomes eligible. “I think some doctors will say to wait until they’re 3, but that was when there wasn’t a resurgence of measles,” she said. “That’s my next thing to dive into.”

But there’s no scientific basis and no known benefits to delaying vaccines except in very rare medical circumstances, said Casares, whose pediatric practice is in Oregon.

Casares said the problem is that parents have an “exposure bias.” They often consume an onslaught of information on social media about the risks, but very little about the benefits of vaccines or the enormous risks of the diseases themselves. She said in a country such as the United States, where vaccination rates are fairly high, most people don’t see the ravages that the diseases can cause if rates fall.

This article is part of The Times’ early childhood education initiative, focusing on the learning and development of California children from birth to age 5. For more information about the initiative and its philanthropic funders, go to latimes.com/earlyed.