In a report on global immunization coverage, the U.N. agencies found that vaccination levels are stagnating, notably in poor countries or areas of conflict.

“Vaccines are one of our most important tools for preventing outbreaks and keeping the world safe,” the WHO’s director general, Tedros Adhanom Ghebreyesus, said in a statement.

“It’s often those who are most at risk – the poorest, the most marginalized, those touched by conflict or forced from their homes - who are persistently missed,” he said. “Far too many are left behind.”

The WHO/UNICEF report found that since 2010, vaccination coverage with three doses of diphtheria, tetanus and pertussis (whooping cough) vaccine and one dose of measles vaccine has stalled at around 86%.

The report said this was too low, since 95% coverage is generally needed to provide “herd immunity” to those who are not vaccinated.

In 2018 for example, the number of measles cases around the world more than doubled, to almost 350,000.

“Measles is a real-time indicator of where we have more work to do to fight preventable diseases,” said Henrietta Fore, UNICEF’s executive director. “An outbreak points to communities that are missing out on vaccines ... (and) we have to exhaust every effort to immunize every child.”

Almost half the world’s unvaccinated children are in just 16 countries: Afghanistan, Central African Republic, Chad, the Democratic Republic of Congo, Ethiopia, Haiti, Iraq, Mali, Niger, Nigeria, Pakistan, Somalia, South Sudan, Sudan, Syria and Yemen.

If these children fall ill, the report said, they are at risk of the most severe health consequences, and are least able to get the treatment and care they need.