Regardless of how the current ceasefire agreement in Syria came about, it has—to a large extent—brought a welcome halt to hostilities in many parts of the country. But as one humanitarian crisis is suspended, another potentially hangs in the balance—the growing threat of epidemics. With no way of knowing how long the ceasefire will last, there is an urgent need to get vaccines into Syria before this much-needed opportunity slips away and before the suffering of people in Syria is made even worse by the growing threat of deadly infectious disease.

The UNHCR, the World Health Organization, UNICEF, and humanitarian organizations have called for an immediate and unconditional ceasefire to allow access across the country. With more than 8 million people reported to be displaced and still living within Syria’s borders, and nearly 5 million more people thought to be trapped in besieged cities and hard to reach areas, including two million children, an ever-growing number of Syrian children are missing out on basic, yet vital vaccinations.

Immunisation coverage rates have almost halved since the conflict began, falling from 80% to just 41%, the fourth lowest in the world. We have already seen outbreaks of polio, measles, and meningitis. Now, as conditions on the ground worsen with extremely limited access to clean water, the risk of epidemics of vaccine-preventable disease increases with each day, threatening to compound the wider humanitarian emergency across the region.

It’s not just a matter of access. While it might be common for civilians to become caught between hostile forces and cut off from the rest of the world and humanitarian support, in fact even in the most bitter conflicts there has been a tradition in the last century of opposing military forces allowing essential medical supplies, such as childhood vaccinations, to cross front lines. We saw this at the height of Sri Lanka’s brutal civil war, thanks to an agreement between the Sri Lankan government and the Tamil Tiger leaders.

Thanks to the Syrian Arab Republic Humanitarian Response Plan, there has been some success in getting vaccines to vulnerable children across the war-torn country. Even in areas controlled by ISIS, cross-border operations have enabled large numbers of under-fives to receive the vaccinations they need.

So why then have vaccination levels dropped so much? One factor is a lack of funding. Before the conflict Syria was a country funding its own vaccination programmes. It maintained relatively good immunisation coverage. There were even plans to introduce new vaccines, such as those that protect against the two biggest killers of under-fives—pneumonia and diarrhoea.

In addition to this, the conflict in Syria has given rise to a new kind of fragility, both in terms of scale and nature. More than a third of the entire population has been uprooted from their homes, and yet remains within Syria. While many of these have reached humanitarian camps or shelters, a very large number of them remain within urban areas, taking refuge where they can.

This is worrying because, with people living off-the-grid they effectively become invisible to the humanitarian agencies that are trying to help them. Life in sheltered camps is by no means ideal, but it at least means access to food, water, and health facilities. For those living in what is known as “individual accommodation,” the combination of large numbers of unvaccinated people living in close-proximity in urban dense areas, with limited access to clean water and sanitation, represents fertile breeding ground for infectious disease.

In fact, this trend is not limited to Syria. In 2008, roughly 60% of internally displaced people ended up in rural areas, the majority in humanitarian camps. By 2015, this had reversed. Of the roughly 6 out of 10 internally displaced people that now end up in urban settings, just 1% reside in camps. That means, with 65 million people displaced across the globe in 2015, instead of fleeing the cities, the majority now appear to be hiding in them.

All this has huge implications for the global health landscape, and not just in terms of the immediate threat of outbreaks. Even after the war in Syria is over, low immunisation rates will severely impair efforts to rebuild Syria’s shattered infrastructure and economy, and protect the next generation. Like war, the impact of infectious disease can be felt for generations to come, not only in lives lost but long-term disability too.

In the absence of a political solution, Gavi, the Vaccine Alliance, is attempting to help address this by committing US$ 25 million per year for the next two years to help support the purchase of vaccines and the replacement of cold chain equipment required to get the vaccines to children in Syria. However, figuring out how to best reach these incredibly vulnerable people will not just require funding, but also new thinking, innovation, and urgent action. With so many unvaccinated children missing out, it is just a matter of time before we see a major epidemic sweep its way through a people who have already suffered so much.

Seth Berkley, CEO of Gavi, the Vaccine Alliance.

Competing interests: None declared.