The pandemic has led to a sharp fall in the number of children around the world being vaccinated, the UN says.
The decline in immunisation against diphtheria, tetanus and whooping cough over the first four months of the year is the first in nearly three decades.
World Health Organization head Dr Tedros Ghebreyesus said vaccines were a hugely powerful public health tool.
He said the suffering and death caused by children missing out on vaccines could dwarf that caused by the virus.
Immunisation programmes in three-quarters of the more than 80 countries that responded to a UN survey have been disrupted, Unicef and the WHO said.
They said the disruptions were linked to a lack of personal protective equipment for health workers, travel restrictions, low health worker staffing levels and a reluctance to leave home, all of which saw programmes curbed or shut down.
By May this year at least 30 measles vaccinations campaigns had been cancelled or were at risk.
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Measles outbreaks were already rising before the pandemic struck, with 10 million people infected in 2018 and 140,000 deaths, most of whom were children, according to UN data.
Unicef head Henriette Fore said the coronavirus had made routine vaccinations a “daunting challenge”.
“We must prevent a further deterioration in vaccine coverage… before children’s lives are threatened by other diseases, she said, adding: “We cannot trade one health crisis for another.”
