Just over 50 years ago, the World Health Organization launched its Essential Programme on Immunization. Since then, vaccination rates have improved dramatically and researchers estimate that 4.4 billion people have been reached and 154 million childhood deaths have been avoided.

These long-term trends obscure a less rosy picture of what’s happened in recent years, according to a study published on Tuesday in The Lancet. The researchers found that, since 2010, efforts to boost vaccination rates have stalled or reversed in many places.