The World Health Organization warned of a major measles outbreak in a number of African countries which delayed their vaccination campaigns last year due to the COVID-19 pandemic.

According to the W.H.O., 15 countries delayed their campaigns last year, and while seven of them went on to complete them, eight are yet to do so.

The W.H.O. said that initial statistics indicated that around 16.6 million children in Africa failed to get planned supplemental measles vaccine doses between January 2020 and April 2021. As a result, eight countries reported major measles outbreaks that affected tens of thousands of people during that period.

"The outbreaks were largely due to low routine immunization coverage or delayed vaccination drives. In addition, the quality of measles surveillance in Africa fell to the lowest level in seven years in 2020, with just 11 countries meeting their target," the W.H.O. said in a statement of Thursday.

At least 95 percent immunization coverage is required in the population to prevent outbreaks.

Around nine million children in the African region miss life-saving vaccines every year and one in five children remains unprotected from vaccine preventable diseases, according to the W.H.O.

W.H.O. Regional Director for Africa Dr. Matshidiso Moeti said the recent outbreaks of measles and other diseases, such as yellow fever, cholera and meningitis, were indications of "worrying gaps" in immunization coverage and surveillance in the continent.

"As we fight COVID-19, we cannot leave anyone dangerously exposed to preventable diseases. I urge all countries to double down on essential health services, including life-saving vaccination campaigns," Moeti said.

Measles is a highly contagious disease which claims the lives of more than 500,000 children under the age of five in Africa annually.

(With input from World Health Organization)