Progress against malaria has stalled, and the disease remains a significant threat to billions of people despite the expensive, decades-long efforts to contain it , the World Health Organization reported on Monday.

According to the W.H.O.’s latest annual assessment, there were an estimated 220 million cases of malaria last year, and about 435,000 deaths from the disease. Of the dead, 262,000 were children under age 5 .

The disease is caused by parasites transmitted to humans by mosquitoes. The world’s malaria rate — which began dropping sharply in the early 2000s as insecticide-impregnated nets, new drugs and much more money were poured into the fight — has been roughly the same since 2013, the report said.

Without openly criticizing any particular country, officials made it clear that some are doing too little to protect their citizens. “The past year has seen lights and shadows,” said Dr. Pedro L. Alonso, the W.H.O.’s malaria program director.