Two doses of the same combined vaccine — for measles, mumps and rubella — are required to protect children from the disease. For a whole community to be safe, including babies and others who have yet to be vaccinated — so-called herd immunity — 95 percent immunization coverage is required. But global coverage is much lower: In 2017, worldwide coverage was reported at 85 percent for the first dose and 67 percent for the second, Unicef said.

Nearly 169 million children worldwide missed out on the first dose, with an average of more than 21 million children missing out every year from 2010 to 2017. More than 2.5 million children in the United States, and more than half a million children in France and Britain, did not get the first measles vaccine dose between 2010 and 2017.

Could this number be high enough to affect Britain’s herd immunity? It depends, said Anna Schurich, an immunology lecturer at King’s College London.

“If you have pockets of parents not vaccinating and they all live close together, then even relatively low numbers of nonvaccinated children can become problematic, as they could potentially infect each other,” she said. “If unvaccinated children are very spread out, then the risk is, of course, lower,” she added.

Britain eliminated measles in 2016, the government said. But though the disease is no longer native to the country, Britain has experienced recent measles outbreaks, and immunity levels among mostly young people remain lower than required.

“Measles is one of the most infectious known diseases, so it can take only one infected person in an area with lower vaccination rates to cause an outbreak,” Public Health England, a government body, said in a statement in January.

Outbreaks of the disease have been reported in British cities such as Leeds, Liverpool, Birmingham and Surrey since October 2017, according to the National Health Service. But Manchester appears to be a focal point of the disease in the country, with 47 cases of measles there since the start of the year, Public Health England said last week. Last year, the city had three cases total, the BBC reported.