One in six American parents has delayed or skipped some or all of the standard childhood vaccines, according to a new Washington Post poll released on Monday.

Most of those parents cite concerns about potential side effects as well as a lack of confidence in federal health authorities to ensure their safety.

Some nine percent have opted out of administering their kids with polio or MMR (measles, mumps, rubella) shots, a trend public health experts warn risks triggering a widespread return of potentially fatal illnesses that routine childhood vaccination had largely wiped out.

In 2025, the United States experienced its worst measles outbreak in more than 30 years, with more than 1,400 total confirmed cases and three deaths, including two young children.

The people who said they delayed or skipped vaccines were more likely to identify as Republican, be under 35, cite religious beliefs or homeschool their child.

The wide-ranging Washington Post-KFF poll said parents are even less likely to have vaccinated their children against Covid-19 or the flu: approximately half of parents did not get their children flu shots last year, and 56 percent said they were not confident that Covid-19 vaccines are safe for kids.

The vast majority of American parents still support vaccinations and 81 percent said public schools should still require measles and polio shots, the poll showed.

But while vaccinations have long been part of standard-issue medical care, resistance has mushroomed in the United States in recent years, stoked in large part by debunked claims linking shots to autism.

The US federal health secretary, Robert F Kennedy Jr, has played a significant role in fueling those fears by repeating the false claims and sowing doubts about vaccine safety.

As the top US health official, he has taken steps to curb access to Covid shots and recently ousted Sue Monarez from her post as director of the Centers for Disease Control and Prevention (CDC) over immunisation guidelines.

Monarez is set to testify before the Senate health committee this week.

Kennedy's revamped Advisory Committee on Immunization Practices (ACIP) – a scientific advisory board meant to make recommendations to the CDC, which he has staffed with vaccine skeptics after dismissing the entire previous body – is also set to convene at the end of this week.

On Monday the CDC announced an additional five ACIP members, including figures who have argued against Covid vaccine mandates and pushed unproven treatments for the disease. (AFP)