After years of holding steady, American vaccination rates against once-common childhood diseases have been dropping.

Nationwide, the rate of kindergartners with complete records for the measles vaccine declined from around 95 percent before the pandemic to under 93 percent last year, according to the Centers for Disease Control and Prevention. Immunization rates against polio, whooping cough and chickenpox fell similarly.

Average rates remain high, but those national figures mask far more precipitous drops in some states, counties and school districts.

In those areas, falling vaccination rates are creating new pockets of students no longer protected by herd immunity, the range considered high enough to stop an outbreak. For a community, an outbreak can be extremely disruptive. For children, measles and other once-common childhood diseases can lead to hospitalization and life-threatening complications.

Immunization rates fell in most states early in the pandemic, and continued to fall in the years that followed.

States, not the federal government, create and enforce their own vaccine mandates, but the incoming Trump administration could encourage anti-vaccine sentiment and undermine state programs. The president-elect’s nominee for health secretary, Robert F. Kennedy Jr., has spread the false theory that vaccines cause autism, among other misinformation.

But immunization rates had been falling for years before Mr. Kennedy’s recent political rise.

There are now an estimated 280,000 kindergartners without documented vaccination against measles, an increase of some 100,000 children from before the pandemic.

“These pockets are just waiting for an introduction of measles,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. “It’s trouble waiting to happen.”

Why rates are falling

As the pandemic strained trust in the country’s public health system, more families of kindergartners formally opted out of routine vaccines, citing medical, philosophical or religious reasons. Others simply didn’t submit proof of a complete vaccination series, for any number of reasons, falling into noncompliance.

The shifts in exemptions mostly fall along political lines. In states that supported Mr. Trump for president in November, the number of students with official exemptions have increased on average (rising everywhere but West Virginia). Exemption rates rose in a few states that supported Vice President Kamala Harris — including Oregon, New Jersey and Minnesota — but stayed relatively flat or fell in most.

The pattern for noncompliance looks different: The rate of children with no vaccination record shot up in both red and blue states.

Not all children with missing records are unvaccinated. Some are in the process of getting their shots, delayed because of the pandemic, and others just never submitted documentation. Schools are supposed to bar out-of-compliance students from attending, but whether they do varies from state to state and school to school.

Surveys reveal a new and deep partisan division on this issue. In 2019, 67 percent of Democrats and Democratic leaners told Gallup that childhood immunizations were “extremely important,” compared with 52 percent of their Republican counterparts. Five years later, the enthusiasm among the Democratic grouping had fallen slightly to 63 percent. For Republicans and G.O.P. leaners it had plunged to 26 percent.

Today, 31 percent of Republicans say “vaccines are more dangerous than the diseases they were designed to protect.” Just 5 percent of Democrats say the same.

“There seems to be a divide in terms of people’s feelings about science and skepticism towards the government,” said Dr. Natasha Bagdasarian, chief medical executive for Michigan. “I think some of those divisions are becoming apparent in vaccination rates.”

Lawmakers in numerous states have tried to roll back school vaccine mandates, but most changes have been minor: Louisiana required schools to pair any mandate notifications with information about exemption laws; Idaho allowed 18-year-old students to exempt themselves; and Montana stopped collecting data from schools on immunizations.

But there are a few places where state-level policy changes, or lack thereof, appear to have had a direct effect on rates.

In Mississippi, which had long held the country’s highest kindergarten measles vaccination rate, a federal judge ordered the state to allow religious objections; the state’s vaccination rate fell. In contrast, West Virginia’s governor vetoed a bill that would have loosened school vaccine policy; the state now has the highest rate.

Rates rose in Maine and Connecticut, two states that eliminated nonmedical exemptions during the pandemic. They also rose in Alabama, according to C.D.C. data, though the state declined to comment on why.

Vulnerable pockets

Epidemiologists say that when vaccination rates slip under 90 percent for measles, outbreaks become significantly harder to contain. At some point below that, spread becomes almost inevitable if measles is introduced.

There are thousands more schools with vaccination rates below 90 percent compared with just five years ago, according to a New York Times analysis of detailed data from 22 states.

Schools with falling rates can be found in red and blue states, in large urban districts and in small rural ones.

Measles vaccination rates dropped from 83 percent to 75 percent in Yavapai County in Arizona; from 93 percent to 78 percent in Pacific County on the coastline of Washington; from 97 percent to 93 percent in Union County, N.J., just outside New York City — places that span the political spectrum.

These numbers capture vaccination rates only for kindergartners, often partway through the school year, so they include students who may have finished their vaccine series later or will go on to finish it. And across the U.S., most students remain protected against childhood diseases.

But high rates nationally don’t help places no longer protected by herd immunity, as evidenced by recent outbreaks of childhood diseases. Measles and whooping cough cases both climbed last year; polio partly paralyzed a man in New York in 2022.

Growing anti-vaccine sentiment is only part of the public health challenge. In the Minneapolis public schools, completion rates for the measles, mumps and rubella vaccine among kindergartners dropped from around 90 percent to 75 percent. The district’s exemption rate barely moved; instead, far more students had incomplete vaccination records.

Few of those students’ families are strongly anti-vaccine, said Luisa Pessoa-Brandao, director of public health initiatives with the Minneapolis Health Department. Some are immigrants who moved into the district recently, missing either shots or records. Others missed regular doctor visits during the pandemic and got out of the habit of preventative care.

“I think we’re going to be catching up for a while,” Ms. Pessoa-Brandao said.

While vaccination rates were dropping in Minneapolis, they climbed in neighboring St. Paul Public Schools, from around 91.4 percent to around 93 percent, according to state data.

The district attributed the rise to strict new procedures started in 2021, including letters and phone calls to families in their native languages; more vaccines available on district grounds; and monthly compliance reports — an extra mile that not every district is able or willing to go.

There are still parents who opt out. But during a measles outbreak last year, a few changed their mind, said Rebecca Schmidt, the St. Paul district’s director of health and wellness.

“The fear of measles,” she said, “is sometimes greater than the ease” of getting an exemption.

Data for all 50 states