The former first daughter serves as vice chair of the Clinton Health Access Initiative (CHAI), a global health organization that aims to save lives and reduce disease burden in low- and middle-income countries.

Since the pandemic began, millions of children around the globe have missed some or all of their essential vaccines—25 million during 2021 alone, according to the WHO.

That’s why CHAI and like-minded organizations will be working with the WHO to bridge the gap, in an effort dubbed The Big Catch-Up, Clinton said Tuesday at Fortune’s Brainstorm Health conference in Marina del Rey, Calif.

The pandemic-era rise in vaccine hesitancy—and outright rejection of vaccines—is “unfortunate,” she said, noting that she had tempered her words.

“No one should die of polio, measles, or pneumonia—including in this country, where we also need people to vaccinate their kids,” Clinton said.

As of Dec. 31, 2022, only 10% of U.S. children ages 6 months through 4 years had received one or more COVID shot, according to the U.S. Centers for Disease Control and Prevention. And it’s not just COVID vaccine uptake that’s lacking. Not even 70% of U.S. kids ages 2 and under were considered fully vaccinated during 2020–2021, having received a full set of shots for diphtheria, tetanus, pertussis, polio, measles, and other diseases common before the vaccine era.

Slated to last 18 months, The Big Catch-Up is shaping up to be “the largest childhood immunization effort ever,” Clinton said.

She warned that the world is less prepared for the next pandemic than it was before COVID.

“I think we are less prepared today than we were, arguably, in January 2020—partially because of the lack of trust and confidence in not only our scientists, but in science itself, and certainly in public health professionals,” Clinton said. “We all deserve to hopefully not be as unprepared as I worry we are at the moment.”