At least a third of the world’s schoolchildren are unable to access remote learning when their schools are closed, according to new figures by the U.N. Children’s Fund (UNICEF).

Experts and advocates are concerned about what that means for the long-term impact of the coronavirus pandemic, as schools and governments struggle to keep education on track.

Earlier this year, the vast majority of schoolchildren worldwide — more than 1.5 billion — were affected by lockdowns that closed schools, UNICEF said Thursday. Of those children, roughly 463 million were unable to access online instruction through radio, television or the Internet, the agency said.

The real number may be “significantly higher than estimated,” UNICEF cautioned.

As of earlier this month, more than a billion students were still affected by school closures.

“The sheer number of children whose education was completely disrupted for months on end is a global education emergency,” Henrietta Fore, UNICEF’s executive director, said in a statement. “The repercussions could be felt in economies and societies for decades to come.”

The distribution of the impact mirrors existing patterns of inequality. Students in sub-Saharan Africa were particularly affected. Nearly half of all schoolchildren in the region — more than 120 million students — have not been able to access remote learning, according to UNICEF’s data.

Roughly three-quarters of children worldwide who have missed out on remote learning opportunities live in rural areas, according to UNICEF.

Another demographic often left behind by remote learning is children with intellectual disabilities and special learning needs. One-size-fits-all materials may not work for students with a range of learning issues, including autism, attention-deficit/hyperactivity, and hearing or visual impairments. A June report by the U.N. Educational, Scientific and Cultural Organization (UNESCO) found that 40 percent of low- and lower-middle-income countries have not provided support to disadvantaged learners affected by temporary school closures.

That number is probably incomplete, UNESCO said, because of a lack of data on the topic.

“Inadequate data means we are missing a huge part of the picture,” Manos Antoninis, the director of UNESCO’s Global Education Monitoring Report, said in a statement accompanying the report. “It is no wonder the inequalities suddenly exposed during covid-19 took us by surprise.”

Students with disabilities were already “less likely to attend school, more likely to be out of school, less likely to complete primary school, and, therefore, less likely to possess basic literacy skills,” according to a July report by the World Bank Group and Inclusive Education Initiative. “The covid-19 pandemic magnified the systemic inequalities that exist in the inclusion and protection of children with disabilities.”

As countries try to decide whether to reopen schools — and if so, when and how — UNICEF cautioned that remote learning remains laden with obstacles, even for the children who do, at least in theory, have unimpeded access to it.

“Even when children have the technology and tools at home, they may not be able to learn remotely through those platforms due to competing factors in the home including pressure to do chores, being forced to work, a poor environment for learning and lack of support in using the online or broadcast curriculum,” UNICEF said in a statement.

The agency urged governments to make schools a top priority for reopening, noting that the youngest students have fallen behind. At least 70 percent of children younger than primary age cannot access a remote learning option, largely because of a lack of suitable online options for young children, according to the report.

Of the alternatives to virtual learning, television could reach the largest number of students, according to UNICEF. Although it presents some challenges — namely the sharing of course materials — television could reach almost 930 million students worldwide and potentially “has a significant role to play in delivering education during school closures,” the agency said.

Some countries have undertaken creative initiatives, like offering supplementary materials designed for different kinds of learners or trying to rely on a range of written, audio and visual materials, UNESCO’s Antoninis told The Washington Post.

“But the loss of the daily contact means that all of these substitutes for education are nothing but partial education,” he said. The pandemic has underscored “the importance of teachers, who are often underestimated by governments and people who allocate resources.”

It has also laid bare the persistence of gender inequality, as the burden of child care still falls disproportionately on women and girls, Antoninis said.

“Many people had been harboring hopes that technology would be the answer for the future of education,” he said. “That aspiration has probably been sent crashing down.”

U.N. Secretary General António Guterres has warned that closing schools could cause a “generational catastrophe.” Policymakers, however, have struggled to balance access to education with the concerns among families, teachers and other workers in schools that reopening will spread the virus.

While children on average are not as severely affected by covid-19, they can still spread the virus — even when asymptomatic. In some countries, including South Korea, classes recently resumed only to shut down again after a spike in cases.