The WHO and partners called Monday for action to boost vaccination against measles and other diseases worldwide after the pandemic severely disrupted access to routine jabs.
The World Health Organization, along with the UN children's agency and the vaccine alliance Gavi, said their new global strategy had the potential to save 50 million lives within less than a decade.
"If we're to avoid multiple outbreaks of life-threatening diseases like measles, yellow fever and diphtheria, we must ensure routine vaccination services are protected in every country in the world," WHO chief Tedros Adhanom Ghebreyesus said in a statement.
The push comes as the Covid-19 pandemic continues to disrupt essential health services worldwide.
While the situation has improved somewhat from last year, a WHO survey showed more than one third of countries were still seeing disruptions to their routine immunisation services.
And around 60 mass vaccination campaigns were currently postponed in 50 countries, putting around 228 million people, mostly children, at risk for diseases such as measles and polio, the joint statement said.
"Millions of children across the world are likely to miss out on basic vaccines as the current pandemic threatens to unravel two decades of progress in routine immunisation", Gavi chief Seth Berkley warned.
The supply of vaccines and other equipment is also essential for child vaccinations.
Unicef said disruptions due to Covid-19 had dramatically reduced the vaccine doses it delivered last year to 2.01 billion, down from 2.29 billion in 2019.
UNICEF head Henrietta Fore agreed, stressing that "even before the pandemic, there were worrying signs that we were beginning to lose ground in the fight against preventable child illness."
Prior to the Covid-19 crisis, she said, some 20 million children were already missing out on critical vaccines, and "the pandemic has made a bad situation worse".
"Now that vaccines are at the forefront of everyone's minds, we must sustain this energy to help every child catch up on their measles, polio and other vaccines," she said.
"We have no time to waste. Lost ground means lost lives."
Gaps in vaccination coverage had already led to serious measle outbreaks in a range of countries, including the Democratic Republic of Congo, Pakistan and Yemen, the organisations said, warning that other outbreaks should be expected as growing numbers of children miss their shots.
The new vaccination strategy to boost immunisations services globally, with the aim of achieving 90-percent coverage for essential vaccines given in childhood and adolescence by 2030.
By that time, it also aims to slash in half the number of children in the world who receive no vaccines at all.
And it strives to introduce or scale up the use of new or under-utilised vaccines for diseases like Covid-19, rotavirus and human papillomavirus (HPV).
The push, the organisations said, had the potential of saving 50 million lives -- 75 percent of them in poorer countries.
"To support the recovery from Covid-19 and to fight future pandemics, we will need to ensure routine immunisation is prioritised," Berkley said.
"To do this, we need to work together -- across development agencies, governments and civil society -- to ensure that no child is left behind."