Millions of children risk missing "life-saving" vaccines, the UN has warned, after a "massive backlog" of shipments built up due to the coronavirus pandemic.
The outbreak has had a huge impact on the air industry, drastically reducing commercial and charter flights.
Dozens of countries are at risk of running out of vital vaccines, the UN children's agency Unicef says.
It wants governments and the private sector to free up freight space.
Immunisation programmes are one of Unicef's key activities. The organisation estimates that vaccinations for serious diseases like measles, polio and tetanus save the lives of up to three million children a year.
With medical researchers hard at work on a coronavirus vaccine, Unicef says the outbreak is disrupting active efforts against other illnesses.
"Unicef is calling for support to unlock a massive backlog in vaccine shipments due to unprecedented logistical constraints related to Covid-19 mitigation measures including lockdowns in some countries," said spokesperson Marixie Mercado.
Warning of a "dramatic decline" in commercial flights and the "exorbitant" cost of securing them, she said: "Countries with limited resources will struggle to pay these higher prices, leaving children vulnerable to vaccine-preventable diseases.
"Unicef is appealing to governments, the private sector, the airline industry, and others to free up freight space at an affordable cost for these life-saving vaccines."
Last month the organisation warned measles outbreaks might occur as a result of vaccine programmes being delayed by the coronavirus outbreak.
Even before coronavirus emerged Unicef estimated that more than 20 million children a year were missing out on a measles vaccine, with the organisation citing scepticism of vaccines as a factor.
On Thursday, teenage climate activist Greta Thunberg donated $100,000 (£80,000) she won from a Danish charity to Unicef to help its fight against coronavirus.
Launching a campaign to help protect children's lives in the outbreak, she said: "Like the climate crisis, the coronavirus pandemic is a child-rights crisis. It will affect all children, now and in the long-term, but vulnerable groups will be impacted the most."