Thanks to COVID vaccines, more people were immunized in 2021 than in any other year in history. Yet that same year, with tragic irony, more children ended up at risk of highly preventable infectious diseases than before the pandemic began.

This is because of what the World Health Organization and UNICEF have described as the largest backslide in childhood vaccinations in three decades. It means that, for the second year in a row after 2019, at a time when the COVID pandemic has focused the world’s attention on the need for vaccination, the number of children missing out on basic vaccines has increased. This is particularly the case in lower-income countries.

Given the enormous toll COVID is having on the world’s poorest economies, an overall decline in childhood vaccinations is not unexpected. But we can’t let these numbers slip further. We urgently need more global partners to help lower-income countries, often working with limited resources and challenging conditions, to get back up to those prepandemic levels and beyond. The lives of children are at stake, and no parent should lose a child to a preventable illness.

In a typical year Gavi, the Vaccine Alliance—an international immunization organization for which I am CEO—provides the vaccines to immunize nearly half the world’s children. The gradual increase in vaccination has meant a 70 percent reduction in vaccine-preventable diseases, which, in turn, has contributed to a more than 50 percent reduction in the mortality of children under age five. This is part of a coordinated global effort that involves the WHO, UNICEF, the World Bank, thousands of civil-society organizations around the world and ministries of health in the countries where we work.

The global benchmark used to measure vaccination coverage is the percentage of a country’s children who receive three doses of a diphtheria-tetanus-pertussis, or DTP3, vaccine. In 2020, in the 57 lower-income countries supported by Gavi, vaccination coverage as measured by DTP3 fell four percentage points from the previous year to 78 percent. This may still seem like a high number of children vaccinated, but those four percentage points correspond to many children now at risk of dying from a preventable infection. The following year, vaccination coverage fell by an additional one percentage point. In addition, the number of children in countries where we work who did not receive even a single dose of a basic vaccine rose for a second year.

In contrast, 94 percent of children in the U.S. have received their DTP3 vaccines, and new data show that between 2019 and 2021, coverage in the country dropped by one percentage point. This mirrored the small-but-consequential average decline in all high-income countries.

This decline in all countries, particularly poorer nations, is pretty grim news, especially considering that in the decades preceding the pandemic, there was huge progress in reducing the number of children at risk from infectious disease. In 1985, for example, DTP3 coverage for all children hovered just below 50 percent; it peaked at 86 percent in 2017. Likewise just 47 percent received a first dose of measles vaccines (MCV1) among this age group in 1985, compared with a high of 86 percent among all children in 2019 and 81 percent by 2021. All this means that, in addition to dealing with recurring waves of COVID, countries that have experienced declines in coverage during the pandemic could now also find themselves tackling outbreaks of other deadly and debilitating diseases. Indeed, we are already seeing drops in vaccination coverage followed by rises in cases of vaccine-preventable infectious diseases, such as measles and polio.

This is not just in lower-income countries: the U.S. has recently reported a case of paralytic polio, and there is fear of an outbreak in the U. K.

This highlights the urgency with which we need to help all countries recover, but it also demonstrates the fragile nature of the progress we and others have made. No country has escaped this pandemic unscathed, but the poorest have been hit the hardest. This is despite efforts by global health organizations to help countries recover their health systems, which have been devastated by the occurrence of COVID cases and challenged by the need to rapidly scale up COVID vaccines.

When the COVID pandemic first struck, it was clear that lockdowns and other disruptions would have a short-term effect on immunization levels. The new data reveal that that effect has been more sustained and deep-rooted than anyone anticipated, partly because health systems were so weak to begin with but also because of other compounding factors, such as the rise in vaccine hesitancy and the effect the pandemic has had on economies.

The overall drop in coverage has primarily been driven by larger countries, such as the Democratic Republic of the Congo (DRC) and India. DRC’s vaccination coverage in children fell from 73 percent in 2019 to 65 percent in 2021, while India’s dropped from 91 percent to 85 percent. Similarly, North Korea, Myanmar and Mozambique all experienced large declines in coverage in 2021.

The good news is that, despite the extremely challenging circumstances that have led to the overall decline in vaccination rates, many countries have done an incredible job getting back on track. More than 30 lower-income nations saw coverage actually increase or remain stable after the initial COVID effects. In particular, Chad—a notably fragile country as the third lowest nation in the Human Development Index—increased coverage by 8 percent from 2019 to 2021, while Pakistan reduced the number of zero-dose children by more than 400,000, bringing their numbers close to prepandemic levels. To put these statistics into perspective, because global population is increasing, each year we have to vaccinate more than 600,000 children in the countries where we work just to maintain coverage levels.

The fact that coverage didn’t fall lower in these countries is a testament to the investments they have made in relatively resilient immunization systems in recent years—and that they have continued to prioritize since the pandemic began. This allowed them to perform better than most other health and social service interventions.

Also worth mentioning are the incredible recovery efforts underway in all the other lower-income nations. Even during prepandemic conditions, achieving an increase in vaccination in these countries involved considerable resources, problem-solving and political commitment—often in the face of a multitude of complex problems, such as health-worker strikes, political unrest, conflict and vaccine supply. And now it’s an even bigger challenge.

Restoring vaccination coverage represents an epic challenge and one that will require the same kind of effort that these countries have already put into tackling COVID. These massive recovery efforts could last for some time. They will also require continued collaboration with Gavi, WHO and UNICEF, as well as COVAX—the facility we manage in collaboration with our partners that aims to distribute COVID vaccines to hard-to-reach populations in lower-income countries.

In order to help lower-income nations recover, we need to better understand why they are struggling and provide them with tailored support. One way to do that is to learn from countries such as Chad.

In the past 18 months, low and lower middle income countries assisted by Gavi and our partners have not only continued to provide routine vaccines for their children but also managed to reach an unprecedented 50 percent of their population with a complete course of COVID vaccines.

We know we can immunize those who need it. We just need to make sure it happens.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.