About 25 million children worldwide missed out on routine immunizations against common diseases last year, making it the largest sustained decline in childhood vaccinations in approximately 30 years, said two UN agencies on Friday.

The 25 million children failed to get vaccinated against diphtheria, tetanus and pertussis, a marker for childhood immunization coverage within and across countries.

This is two million more children than in 2020 and six million more than in 2019, according to figures released by the World Health Organization (WHO) and the UN Children's Fund, UNICEF.

"This is a red alert for child health," said Catherine Russell, UNICEF's executive director. "We are witnessing the largest sustained drop in childhood immunization in a generation." Russell added that "the consequences will be measured in lives."

Data showed the vast majority of the children who failed to get vaccinated were living in developing countries include Ethiopia, India, Indonesia, Nigeria and the Philippines.

The coverage of human papillomavirus (HPV) vaccination, which has grave consequences for the health of women and girls, was only 75 percent of that of 2019. And 6.7 million more children missed the third dose of polio vaccine compared to 2019, according to the report.

The UN said the decline was due to many factors, including an increased number of children living in conflict and fragile settings and COVID-19 related issues, while "COVID-19 is not an excuse." 

WHO Director-General Tedros Adhanom Ghebreyesus said that "planning and tackling COVID-19 should also go hand-in-hand with vaccinating for killer diseases like measles, pneumonia and diarrhea," and "it's possible to do both."

(With input from agencies)