Experts are warning of a “child health crisis” as new research reveals one of the largest ever sustained reversals in global vaccination coverage. Tens of millions of children are vulnerable to preventable diseases as immunisation rates have continued to fall since Covid-19, according to UN figures. About 25 million children risk contracting diphtheria, tetanus, whooping cough and polio, 6 million more than in 2019 and the highest number since 2008, according to estimates by the World Health Organization and Unicef, the UN children’s agency, published on Friday. Almost two-thirds (62%) of children who received no vaccines in 2021 lived in 10 countries, all low or lower-middle-income countries, except Brazil. Niklas Danielson, senior immunisation specialist at Unicef, said the situation was “extremely serious”, and made worse by drought and increasing malnutrition rates. “I see this as a child health crisis,” he said. Almost 25 million children missed their first measles shot in 2021, 5.3 million more than in 2019. A further 14.7 million did not receive the required second dose. And 3.5 million more children missed the first dose of the human papillomavirus (HPV) vaccine, which protects against cervical cancer later in life, last year compared with 2019. Inadequate immunisation coverage has already led to deaths and resulted in avoidable outbreaks of measles and polio in the past 12 months, said the UN agencies. Dr Kate O’Brien, director of the department of immunisation, vaccines and biologicals at the WHO, said 2021 had not been a year of “recovery” but of “further backsliding” for immunisation programmes after the lockdowns in place around the world in 2020. She said: “When [vaccinations] are not given, there are tens of millions of children and adolescents, as well as pregnant women and adults, not receiving vaccines … and there are people suffering severe health consequences. People have lost their lives as a result of vaccine-preventable diseases.” There were lifelong consequences for those recovering from some illnesses, she added. Children who get measles are more susceptible to other diseases, including pneumonia, for up to a year afterwards. Meningitis can cause deafness, blindness and can have long-term neurological consequences. Reasons behind the drop in vaccine coverage include an increase in the number of displaced people, making it difficult to deploy immunisation services. Misinformation on the internet and conspiracy theories about vaccines also contributed, said researchers, as well as the enormous strain health services have been under since the Covid pandemic. Jennifer Requejo, global health data lead at Unicef, said: “The backslides are alarming.” In Africa, she said: “There’s the risk of famine in countries like Ethiopia, food insecurity levels are increasing because of the war in Ukraine, and also with Covid there are service disruptions, as well as economic challenges that a lot of countries are facing. Those are worrying trends put together.”