Public health agencies warned the public on Wednesday not to take their eye off an old threat: measles. 

The World Health Organization and Centers for Disease Control and Prevention released a report that warns a serious measles outbreak remains a possibility. After two years of battling the COVID-19 pandemic, the two agencies caution that monitoring for measles and other infectious diseases has fallen off. They warn that it may leave public health authorities in the dark about the true number of possible measles cases that are active worldwide.

A central point of the joint report was that vaccinations against measles for children fell dramatically as COVID-19 reduced the number of medical appointments in 2020. Vaccination efforts similarly stalled last year. 

The health groups note that the slowdown in measles vaccinations left more than 93 million people at risk for the disease, including 70% of children who received only a single dose of a measles vaccine. The report notes that there was the largest increase in measles cases in the last 20 years. It added that usually a 95% vaccination rate is expected to protect communities from measles outbreaks. 

Dr. Kate O'Brien, director of the Department of Immunization, Vaccines and Biologicals at WHO, warns that the worst could be yet to come.

"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 O’Brien in a press statement accompanying the report.

According to CDC data, the number of measles cases reflected this uptick. In 2019, there were 1,282 confirmed across 31 states but this number fell sharply to only 13 detected in 2020 before going up to 47 in 2021. Globally, WHO estimated that measles killed an estimated 207,500 people last year, which it blamed on a failure to increase vaccination coverage against disease despite years of decline up until that point.

Kevin Cain, the CDC's Global Immunization Director, warned that it was urgent to strengthen monitoring of measles and other diseases as communities begin to slowly recover from the pandemic.

“We must act now to strengthen disease surveillance systems and close immunity gaps, before travel and trade return to pre-pandemic levels, to prevent­­ deadly measles outbreaks and mitigate the risk of other vaccine-preventable diseases,” said Cain.