The United Nations agencies joined forces with Gavi, the Vaccine Alliance, to warn that the pandemic has severely disrupted vaccination programmes in dozens of countries, paving the way for a deadly resurgence of preventable diseases.
" COVID-19 threatens to undermine life-saving immunisation services around the world," WHO director-general Tedros Adhanom Ghebreyesus told a virtual joint press conference.
"This risks putting tens of millions of children - in rich and poor countries - at risk of killer diseases like diphtheria, measles and pneumonia."
He said that while the world was seeking a safe and effective vaccine for the new coronavirus, vaccines that were already available to prevent other diseases still needed to be delivered.
"Initial analysis suggests the provision of routine immunisation services is substantially hindered in at least 68 countries and is likely to affect approximately 80 million children under the age of one living in these countries," Tedros said.
"Any suspension of childhood vaccination services is a major threat to life."
Meanwhile experts said it was vital to maintain the structure of routine vaccination programmes in poorer countries, because those networks would be the same ones used to distribute an eventual COVID-19 vaccine.
UNICEF, the UN children's agency, said measles vaccination campaigns had been suspended in 27 countries and polio campaigns in 38 states.
UNICEF executive director Henrietta Fore said countries have had to suspend campaigns due to the need to maintain physical distancing, while some health centres have been overwhelmed by coronavirus response efforts.
Meanwhile health workers have been redeployed to treat COVID-19 patients, and some parents have been unable to get their children to vaccination sites due to movement restrictions.
She said countries needed to step up efforts to track unvaccinated children, and find innovative solutions, as Laos has done by immunising children in supermarkets.
Gavi chief Seth Berkley said countries had to do everything they could to keep vaccinating.
"If we neglect the supply chains and immunisation infrastructure that keep these programmes running, we also risk harming our ability to roll our the COVID-19 vaccines that represent our best chance of defeating this pandemic, when they are ready," he said.
He appealed for countries to dig deep at a June 4 global vaccine donors' summit, hosted in London.
The novel coronavirus has killed at least 332,000 people since the outbreak first emerged in China last December, according to a tally from official sources compiled by AFP.
At least 5.1 million cases have been registered in 196 countries and territories.
"As the world passes 5 million recorded cases of COVID-19, we recognise the importance of building national unity and global solidarity to learn from each other and suppress the virus everywhere," Tedros said.