Vaccine coverage continued to decline worldwide in 2021, with 25 million children missing out on lifesaving vaccines, according to data published Thursday by the World Health Organization and UNICEF.

"The largest sustained decline in childhood vaccinations in approximately 30 years has been recorded," the organizations said in a news release.

Between 2019 and 2021, there was a 5-point drop in the percentage of children who got three doses of DTP3, the vaccine against diphtheria, tetanus and pertussis. This took the coverage down to 81%.

DTP3 coverage is used as a marker for broader immunization coverage, WHO and UNICEF said.

"As a result, 25 million children missed out on one or more doses of DTP through routine immunization services in 2021 alone. This is 2 million more than those who missed out in 2020 and 6 million more than in 2019, highlighting the growing number of children at risk from devastating but preventable diseases," they said. Eighteen million of these children didn't get a single dose of the vaccine, the majority of whom lived in low- and middle-income countries.

Other decreases were seen in HPV, with which over a quarter of the coverage achieved in 2019 was lost, and measles, with which first-dose coverage dropped to 81% in 2021. WHO notes that this is the lowest level since 2008 and means 24.7 million children missed their first dose in 2021.

All regions experienced a drop in vaccine coverage, with the steepest decline for DTP3 coverage in the East Asia and Pacific region. Some countries were able to hold off declines, though, including Uganda and Pakistan.

There were several factors behind the declines, including increased numbers of children living in conflict and other fragile settings, increased misinformation and Covid-19-related issues.

"This is a red alert for child health. We are witnessing the largest sustained drop in childhood immunization in a generation. The consequences will be measured in lives," UNICEF Executive Director Catherine Russell said. "While a pandemic hangover was expected last year as a result of COVID-19 disruptions and lockdowns, what we are seeing now is a continued decline. COVID-19 is not an excuse. We need immunization catch-ups for the missing millions or we will inevitably witness more outbreaks, more sick children and greater pressure on already strained health systems."

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. It's not a question of either/or, it's possible to do both."

The members of the global Immunization Agenda 2030, an effort to maximize the lifesaving impact of vaccines, ask that governments and other groups help address the backslide in vaccination, including intensifying efforts for catch-up vaccination; implementing evidence-based, people-centered, tailored strategies to build trust; and prioritizing strengthening health information and disease surveillance systems to provide the data and monitoring needed.