Measles, the preventable but highly infectious disease, could be on the verge of a comeback after a lull in the immediate months following the emergence of the coronavirus, the World Health Organization and the U.S. Centers for Disease Control and Prevention said Wednesday.

Calling measles an “imminent threat in every region of the world,” the two public health bodies said in a report that almost 40 million children missed their vaccine doses last year. They said 25 million children did not receive their first dose, while an additional 14.7 million children missed their second shot, marking a record high in missed vaccinations.

The number of measles infections has declined over the past two decades, though it remains a mortal threat, particularly for unvaccinated young children in the developing world. But there were an estimated 9 million cases and 128,000 deaths globally last year, up from 7.5 million cases and 60,700 in 2020. That increase came amid poorer disease surveillance and vaccine campaigns that were delayed by the pandemic, the WHO and CDC said.

Vaccination can also confer benefits to one’s community, a concept known as herd immunity. About 95% of a population needs to be vaccinated with two doses for herd immunity to occur, but only around 81% of children globally have received their first dose, and 71% their second, the two bodies said.

Measles, which starts with cold-like symptoms, undermines the immune system, making those infected more susceptible to other diseases. Seizures and blindness are possible in some instances, according to Britain’s National Health Service.

The WHO has previously warned that the dip in measles infections early in the pandemic was the “calm before the storm.”

“Routine immunization must be protected and strengthened” despite the coronavirus, said Kate O’Brien, WHO’s director of immunization, vaccines and biologicals, last year. Otherwise, “we risk trading one deadly disease for another.”

Hur Jian, an infectious-disease expert at South Korea’s Yeungnam University Medical Center, said the recent rebound in global travel portends a probable return of measles even in wealthy countries with higher vaccine coverage. Younger generations who have had less exposure to the disease may have weaker defenses, she added.

The United States declared that it had eradicated measles – defined as no transmission for a year and a well-performing surveillance system – in 2000, but occasional outbreaks still occur. This year 50-plus cases have been detected in the United States, according to the CDC.