WELLINGTON, New Zealand (AFP) — The World Health Organization (WHO) warned of a “slide back” in global efforts to eliminate measles Tuesday, as the death toll from an outbreak that has killed dozens of children in Samoa continued to climb.

A total of 55 people has died since the epidemic began in mid-October, 50 of them children aged four or under, officials in the Pacific nation said Tuesday.

Another 18 infants are critically ill in hospital and the crisis shows no sign of slowing, with 153 new cases in the past 24 hours, taking the national total to 3,881 in a population of 200,000.

Emergency measures including compulsory mass immunizations and school closures have so far done little to stop the virus spreading in a country that was particularly vulnerable to measles due to low vaccination rates of about 31 percent.