(NEXSTAR) – When it comes to measles, there’s good news and there’s bad news.

Between 2019 and 2020, there was an 80% drop in reported measles cases, the Centers for Disease Control and Prevention and the World Health Organization said Wednesday.

But here comes the bad news: While many parts of the world were on lockdown and COVID-19 became the No. 1 health concern, 22 million children missed their first dose of the measles vaccine, the agencies estimate in a new report.

That’s 3 million more unvaccinated infants than the year prior, “marking the largest increase in two decades and creating dangerous conditions for outbreaks to occur,” the CDC and WHO said in a statement.

Also, only 70% of children received their second dose of the measles vaccine. The CDC and WHO warn a 95% completion rate is necessary to keep the spread of the dangerous disease under control.

“While reported measles cases dropped in 2020, evidence suggests we are likely seeing the calm before the storm as the risk of outbreaks continues to grow around the world,” said Dr. Kate O’Brien, director of the WHO’s Department of Immunization, Vaccines and Biologicals, in a statement.

WHO is also concerned that measles cases were underreported in 2020. Fewer reported cases means fewer specimens collected and studied – a key part of monitoring and preventing the spread of the disease.

“While we have not seen an increase in cases yet, measles is simply too contagious. If we do not act, gaps will become outbreaks, and many children will be exposed to a preventable but potentially deadly disease,” said Ephrem Tekle Lemango, UNICEF’s associate director for immunization.

Measles is a highly contagious and serious disease, especially for babies and young children. The CDC advises children get their first measles, mumps and rubella shot between 12 and 15 months old, followed by a second dose between 4 and 6 years old.

Two doses are 97% effective at preventing measles, the CDC says.

“As millions of kids are missing their vaccine, the herd immunity rate really goes down, and that’s what we count on for MMR. MMR vaccines usually start at age 1; they can be given as early as 6 months. But generally from age 0 to 12 months, these kids are not vaccinated. So we need about 95% of people to have their vaccination to get the herd immunity to protect all the unimmunized people,” Dr. Payal Adhikari, a pediatrician with Child and Adolescent Health Associates in Chicago, said on NewsNation’s “Morning in America” Thursday.

The WHO estimates that the measles vaccine has prevented 30 million deaths worldwide over the past 20 years.