It takes two doses of measles vaccine to protect children from the disease. A troubling new report from UNICEF finds that 2.5 million children in the United States and 169 million children worldwide missed out on their first dose between 2010 and 2017.
That's roughly 20 million children a year, on average.
This gap in protective coverage has triggered measles outbreaks around the globe, from high-income countries in the Americas and Europe to low- and middle-income countries in Asia and Africa.
As a result, in 2017, measles killed 110,000 people globally, mostly children, according to the UNICEF report. That's up 22% from the year before.
"If we are serious about averting the spread of this dangerous but preventable disease, we need to vaccinate every child, in rich and poor countries," Henrietta Fore, UNICEF's executive director, said in a news release.
The analysis is based on UNICEF and the World Health Organization's estimate of national immunization coverage of 194 countries for 2017.
The report says lack of access to vaccines, poor health systems, complacency and fear of vaccines have all contributed to the problem.
"The resurgence of measles is of serious concern, with extended outbreaks occurring across regions, and particularly in countries that had achieved, or were close to achieving measles elimination," Dr. Soumya Swaminathan, deputy director general for programmes at WHO, said in a recent news release. "We risk losing decades of progress in protecting children and communities against this devastating but entirely preventable disease."
Globally, coverage of the first dose of the measles vaccine was 85% -- far short of the 95% needed prevent outbreaks and make communities immune to the disease. Global coverage for the second dose was even lower, just 67%.
In 2017, Nigeria had the highest number of children under age 1 who missed their first vaccine dose. India, Pakistan, Indonesia and Ethiopia followed. These five countries accounted for about half of the children worldwide who missed their first dose, about 10.4 million children in total.
In countries where measles mortality is high, WHO recommends the first dose be given at 9 months of age. In countries where the risk of infection among infants is lower, such as the United States, the first dose is recommended at age 12 months in most cases.
The situation was better but still problematic in high-income countries such as the United States, France and the UK. Coverage for the first dose of vaccine was 94% and 91% for the second dose, according to the latest data.
Among high-income countries, the United States topped the list of children not vaccinated with the first dose.
As of this week, measles cases in the United States for this year reached the greatest number since the disease was declared eliminated in 2000. Experts blame the rise on misinformation about the virus and the vaccine that has led some parents to refuse to vaccinate their children.
"The anti-vaccine lobby has been telling parents that children are getting injured from vaccines or they're getting autism. We know none of those things are true," said Dr. Peter Hotez, a professor of pediatrics at the Baylor College of Medicine in Houston.
The UNICEF report noted the global measles crisis has continued into 2019. From January to March, provisional estimates show more than 100,000 measles cases were reported worldwide -- a 300% increase from the same period just one year ago.
UNICEF said efforts are underway to address the situation, including procuring more vaccines and helping countries identify unreached children.
"Existing strategies need to change: more effort needs to go into increasing routine immunization coverage and strengthening health systems," Dr. Seth Berkley, CEO of the global vaccine alliance Gavi, said in a WHO news release. "Otherwise we will continue chasing one outbreak after another."