The UN children's agency, Unicef, says a reduction in vaccination rates in the Democratic Republic of Congo could erase the gains made from immunisation over the past two years.
Unicef said vaccinations were already declining at the beginning of this year, and that the effects of coronavirus will make it worse.
Health workers lack equipment to protect themselves or the children from Covid-19, and parents are afraid to bring them to vaccination centres.
Hundreds of thousands of children have not received polio, measles, yellow fever and other vaccines.
DR Congo might lose its polio-free status and there could be a resurgence of other deadly diseases.
The country has been affected by decades of conflict and an Ebola epidemic.
Unicef is worried that the effects of coronavirus could now push it over the edge.