Children experienced learning deficits during the Covid pandemic that amounted to about one-third of a school year’s worth of knowledge and skills, according to a new global analysis, and had not recovered from those losses more than two years later.

Learning delays and regressions were most severe in developing countries and among students from low-income backgrounds, researchers said, worsening existing disparities and threatening to follow children into higher education and the work force.

The analysis, published Monday in the journal Nature Human Behavior and drawing on data from 15 countries, provided the most comprehensive account to date of the academic hardships wrought by the pandemic. The findings suggest that the challenges of remote learning — coupled with other stressors that plagued children and families throughout the pandemic — were not rectified when school doors reopened.

“In order to recover what was lost, we have to be doing more than just getting back to normal,” said Bastian Betthäuser, a researcher at the Center for Research on Social Inequalities at Sciences Po in Paris, who was a co-author on the review. He urged officials worldwide to provide intensive summer programs and tutoring initiatives that target poorer students who fell furthest behind.

Thomas Kane, the faculty director of the Center for Education Policy Research at Harvard, who has studied school interruptions in the United States, reviewed the global analysis. Without immediate and aggressive intervention, he said, “learning loss will be the longest-lasting and most inequitable legacy of the pandemic.”

Before Covid, crises such as the 2005 earthquake in Kashmir, the 2014 Ebola outbreak in West Africa and enduring teacher strikes in Argentina showed that long-term school absenteeism could have lasting effects. But none had compared to Covid’s scope: About 1.6 billion children worldwide missed a significant amount of classroom time during the pandemic’s peak, according to Unicef.

To quantify the impact, investigators combined findings from 42 different studies published between March 2020 and August 2022, spanning middle- and high-income countries in the Americas, Europe and southern Africa. Global education deficits were equivalent to about 35 percent of a school year and remained “incredibly stable” in the years that followed, Mr. Betthäuser said, as students stopped losing additional ground but also failed to rebound.

Delays were worse in mathematics than in reading, Mr. Betthäuser said, possibly because math requires more formal instruction and because reading comprehension generally improves with brain development as children grow. Data shows that students of lower socioeconomic status shouldered much of the burden, likely because they faced noisy study spaces, spotty internet connections and economic turbulence.

Dr. Damon Korb, a developmental and behavioral pediatrician who founded the Center for Developing Minds, was unsurprised to discover that learning deficits were consistent across grade levels. He said that many young children whom he treated struggled to reintegrate to classrooms quickly because they needed to relearn basic socialization skills. And teenagers returned to schools bearing anxiety disorders “beyond anything I’ve ever seen in my career,” he said.

Dr. Korb said he hoped to see more granular research quantifying the delays among unique learners, such as students with attention disorders or autism, who were stuck behind computer screens and unable to access aides.

Deficits were more pronounced in middle-income countries like Brazil, Mexico and South Africa than in high-income ones such as Australia. Sweden, which mostly avoided school closures, showed no major deficits in academic performance, and Denmark also fared well. (Denmark closed schools, but Mr. Betthäuser said the country’s robust welfare structure might have buffered it against stressors at play elsewhere.)

Researchers excluded low-income countries from the analysis, saying they lacked sufficient data. Mr. Betthäuser said he suspected that losses could be even worse in those settings, and called for further research.

In the United States, one study showed that the average public elementary or middle school student lost the equivalent of a half-year of learning in math, and 6 percent of students were in districts that lost more than a full year. Standardized math test scores in 2022, when compared with those in 2019, showed the largest drop ever recorded in the three decades since the exam was first administered.

The findings challenge the perceptions of many parents, almost half of whom said in 2022 surveys that they did not believe their children had suffered any achievement loss during the pandemic, and only 9 percent of whom expressed concern about whether their children would catch up.

A separate review of test scores from 2.1 million students in the United States highlighted the impacts of economic disparity. Students at schools in communities with high poverty levels spent more of the 2020-2021 school year learning remotely than those at schools in wealthier communities did, and students in poorer schools experienced steeper declines in performance when they were remote.

But “assigning these deficits entirely to school closures would mean missing many mechanisms at play here,” said Sean Reardon, a professor of poverty and inequality in education at Stanford University. Disadvantaged students faced myriad distractions, as parents lost their jobs and others doing essential work became infected at outsize rates.

The analysts also found that, even within districts that were remote for most of the 2020-2021 year, poorer schools lost twice as much learning progress as wealthier schools in the very same districts.

“A kid’s ability to learn and a teacher’s ability to teach are shaped by so many factors beyond just whether they’re physically in the building,” Mr. Reardon said. “If everyone had fallen behind equally, all at once, it presumably wouldn’t affect your chance of getting into college. But when the effect is differential, that could exacerbate inequality into adulthood for the whole generation. That’s worrisome on a global level.”

Because children have a finite capacity to absorb new material, Mr. Betthäuser said, teachers cannot simply move faster or extend school hours, and traditional interventions like private tutoring rarely target the most disadvantaged groups. Without creative solutions, he said, the labor market ought to “brace for serious downstream effects.”

Children who were in school during the pandemic could lose about $70,000 in earnings over their lifetimes if the deficits aren’t recovered, according to Eric Hanushek, an economist at the Hoover Institution at Stanford. In some states, pandemic-era students could ultimately earn almost 10 percent less than those who were educated just before the pandemic.

The societal losses, he said, could amount to $28 trillion over the rest of the century.