More than 100 million children could be at risk for measles because countries around the world are suspending national immunization programs in order to reduce the risk of coronavirus infection, international public health leaders warned on Monday.

So far, 24 low- and middle-income countries, including Mexico, Nigeria and Cambodia, have paused or postponed such programs, according to the Measles and Rubella Initiative, a consortium whose members include UNICEF, the American Red Cross, the World Health Organization, the United Nations Foundation and the Centers for Disease Control and Prevention.

Unlike wealthier countries, where parents typically make appointments to follow a routine vaccine schedule at clinics or private pediatric offices, these countries inoculate large numbers of infants and children in communal settings, like marketplaces, schools, churches and mosques.

Dr. Robin Nandy, the chief of immunization for UNICEF, acknowledged that finding the balance between guarding against the spread of Covid-19, the illness caused by the coronavirus, and preventable diseases like measles was delicate and difficult.