London(CNN) Children across the globe have seen their schools closed because of the coronavirus pandemic.
According to the World Bank, 1.6 billion students were out of school during the first peak of the pandemic in April 2020, and almost 700 million remained out as 2020 drew to a close.
It may take years for the full impact of these months of missed schooling to be known, so what can history tell us about the long-term effects of disruptions to education?
Nothing can be directly compared to the impact of the coronavirus pandemic, said Alberto Posso, professor of economics at RMIT University in Melbourne, Australia, but some parallels can be drawn. "As far as learning from history goes, I think the value is in the potential warning signs these things can give us," he said.
Posso looked at examples including the Ebola epidemic in West Africa, teacher strikes in Argentina in the 1980s and World War II in a piece for The Conversation.
Perhaps the most striking data came from a paper assessing the long-term education cost of World War II for children who were 10 years old during the conflict in Germany and Austria -- both participants in the war -- and comparable children in Switzerland and Sweden, countries that remained officially neutral.
The authors of the 2004 paper, Andrea Ichino and Rudolf Winter-Ebmer, concluded that "individuals experienced a sizable earnings loss some 40 years after the war, which can be attributed to the educational loss caused by the conflict."
"Austrian children missed around 20% of classes during the war and their earnings dropped by around 3%. German children lost around 25% of classes and had earnings dropped by around 5%," Posso told CNN, citing their findings.
More recent insights can be drawn from the experience of children whose education was disrupted in the early 1990s by the Bosnian War -- with the obvious caveat that life during conflict is very different to life during a peacetime pandemic.
Arnesa Buljusmic-Kustura, who was born in Sarajevo, was about four years old when the nearly four-year siege of the city began. She should have been starting kindergarten. Instead, she and her family were left sheltering in a basement with their neighbors.
"The war and genocide in Bosnia was taking place and for us in the city that really meant that we were completely cut off from the rest of the world," said Buljusmic-Kustura, 32, now a writer, researcher working on genocide education, and the deputy director of Remembering Srebrenica UK.
"We had no electricity, no water, really limited access to food and obviously we were subjected to daily bombing and shelling, as well as ongoing sniper attacks that were really targeting civilians and even children. So going to school was really unsafe," she told CNN.
Instead, the adults took it in turns to distract the children from the situation, with a story or by practicing letters, Buljusmic-Kustura said. "We didn't have formal schooling. What we did have was a community that came together and tried to engage us on a very different level."
When peace came, Buljusmic-Kustura was able finally to attend the school five minutes' walk from her home. By that time she was seven-and-a-half years old, and about two years late starting first grade.
In some parts of the city, impromptu classes had been held under cover for mostly older children as the siege went on, she said.
Many, though, were in the same position as Buljusmic-Kustura. "That first day of school I remember not being able to read or write or do those very basic things -- but neither did anyone else," she said.
"Because everybody had sort of experienced the same thing, there was a huge amount of understanding and a lack of this kind of competition that you now see. Because when you come from a place where you have experienced collectively something horrible, [people think] how do we rebuild our lives and how do we move forward?"
Buljusmic-Kustura moved to the United States as a refugee at the age of 12. She had to learn English from scratch but, after a few rocky months, she caught up with her peers, became a "straight-A student" and continued into further education.
She relocated to London from Chicago at the end of April 2019 and has since seen the education of her daughter disrupted by both the move and the pandemic.
Like many countries in Europe, the UK closed its schools to most children in March. They reopened from September to December but have since closed again, except for vulnerable children and those whose parents are key workers, as the country endures its third national lockdown.
Adi Jovovic, now 35, was also living in the heart of Sarajevo during the siege. "When the war started I should have started first grade. I didn't go to school at all for a little while," he said.
Eventually, Jovovic -- whose father worked with CNN reporting teams as a driver during the war -- started going to an informal "school" with adults who tried to step in as teachers.
"One day there would be grenades and bombs going off, so school would be canceled or we wouldn't go for a while, so it was really, really inconsistent -- similar to what kids are going through now, I guess, but even more inconsistent," he said. "I'll be honest with you, I didn't learn anything in that time."
Jovovic and his family moved to Jacksonville, Florida, in March of 1994, when he was nine. He started in second grade at a school for children with English as a second language, attended summer school, focused on learning English, and by the end of third grade was ready to move to a regular school.
Thanks in part to tutoring and a rigorous college prep school curriculum, Jovovic went on to high school and then to the University of Florida, gaining first an undergraduate degree and then a master's degree in chemical engineering. He now lives in Valdosta, Georgia, and is vice president of operations for a pecan processing facility.
"I still don't read and write Bosnian well, which is obviously a shame," he said. Jovovic blames the early disruption to his education for that lack of facility in a language he speaks fluently.
"Not having that fundamental education ... probably set me back a little bit. Now, ultimately I was able to catch up here but it's tough and I certainly feel for the kids who are going through it right now," he said.
"Virtual is nice but it's not the same as being in school. There's the social aspect of it they are not getting, which I argue is as important as the academia."
Posso, the economics professor, agrees the impact of school closures has been lessened where children can access remote lessons using technology like Zoom. But where social inequalities already exist, the pandemic is making them worse, he said.
"Even within the UK there's going to be some segments of society [where] a lot of the poorer kids may not have access to good internet, a computer or educated parents who can help out with the home schooling. And as a result they can be quite disadvantaged," said Posso.
"The distinction is not going to be as clear as German versus Swiss kids, but within the UK I think we might see differences that maybe highlight existing social structures that are already unequal -- in other words, making inequality worse in the future."
The situation in lower- and middle-income countries where child labor is more prevalent is even more concerning, he said.
If parents lose their job because of the pandemic, students whose schools close may feel pressure to look for work and leave education altogether, said Posso. "We already see a big jump in child labor when kids are around 14 or so. If you close schools, it's going to make it very difficult for many children to go back."
Recent history suggests school closures can also have a particularly negative impact on girls, Posso said, especially those from poorer families and communities where girls' rights to education are less entrenched.
During the 2014-2015 Ebola epidemic in Guinea, Liberia, and Sierra Leone, West Africa, schools were closed and few children were able to access remote learning.
A subsequent United Nations Development Programme report into the epidemic found that "gender gaps in education have widened with school closures and because of girls' increased dropout rates, owing to teenage pregnancies and early marriages."
With more people at home because of the pandemic, girls may also be expected to stay out of school in order to take on more household chores, said Posso.
The World Bank warned last month that the pandemic risks pushing tens of millions more children worldwide into "learning poverty" -- meaning that they are unable to read and understand a simple text by age 10 -- with potentially lifelong effects on their earnings.
Faced with this unprecedented situation, teachers and education leaders around the world should take action now to try to ease the effects of school closures when children return, said Michelle Kaffenberger, a research fellow on the RISE Programme at the University of Oxford.
Her simulations indicate that a three-month school closure could reduce long-term learning by a full year's worth of learning.
But if education leaders and teachers take steps to tailor learning to where the children are, rather than picking up where they ought to be, and focus on foundational skills in maths and reading, the impact can be lessened or even overturned, Kaffenberger told CNN.
"The crisis doesn't end when schools reopen," she said. "The crisis is going to keep going, if adequate remediation is not taken when children come back."
Kaffenberger cites research into the impact on schooling of the devastating 2005 earthquake in Pakistani-administered Kashmir as evidence. A study by Tahir Andrabi, Benjamin Daniels and Jishnu Das, published last May, uses survey data collected in 2009 to compare households close to the fault line with similar households that were not affected by the quake.
"Schools in the affected area were closed for an average of 14 weeks, a little more than three months. However, four years later children in the affected areas were not just three months behind, they were the learning equivalent of 1.5 years of schooling behind," Kaffenberger wrote in her own paper.
Households affected by the quake received financial compensation, the paper's authors said, and the data did not indicate higher drop-out rates. But independently measured test scores provide "evidence across the entire age range that persistent developmental deficits can arise in young children due to a large, albeit 'temporary,' shock," they concluded.
Catching up whole cohorts of children will be really hard, said Kaffenberger, but existing programs in lower and middle income countries show that the approach she espouses does work.
Buljusmic-Kustura sounds an optimistic note. Most of her classmates -- particularly among the refugee diaspora but also those who remained in Bosnia and Herzegovina -- managed to complete their schooling successfully and are doing well in life, despite their traumatic early childhood, she said.
And while she was initially "immensely concerned" for her daughter, now nine, as UK schools closed, Buljusmic-Kustura has since been encouraged by seeing her keep in touch with classmates online and engage well with home schooling.
Last week, she posted words of encouragement on Twitter for worried parents in the UK, saying the "kids will be fine" and will likely catch up on lost learning, given the right emotional and mental support.
"My education, my learning, even though it stopped, even though there was these huge impediments, there were times I couldn't go to school at all ... it didn't prevent me from continuing to learn, and continuing to get my education and catching up," she said.