Vaccinating children who missed their measles shots during the COVID-19 pandemic is critical, a senior World Health Organization official said on Tuesday, as outbreaks of the infectious disease increase worldwide.

More than 50 countries have experienced “large and disruptive” measles outbreaks in the last year, twice as many as in 2022, said Kate O’Brien, WHO director of immunization, at a virtual press conference.

Measles is a very contagious viral illness that causes flu-like symptoms and a rash. It can be fatal but is preventable with two doses of vaccine.

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COVID-19 massively disrupted routine vaccination efforts worldwide, and around 60 million children missed their doses over that period, O’Brien said.

She said catch-up efforts were “really critical.”

“It’s now a race between whether the catch-up activities can happen quickly enough or whether the outbreaks will continue to scale,” she said.

On Monday, the United States Centers for Disease Control and Prevention also urged people to get vaccinated against measles amid rising cases globally.

WHO experts also backed new ways of using existing vaccines to tackle other disease outbreaks, including the use of the mpox vaccine made by Bavarian Nordic for at-risk children in African countries.

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They also recommended the use of the hepatitis E vaccine for all women of childbearing age in conflicts and other emergency settings. The infection, mostly transmitted through contaminated water, can be particularly dangerous for pregnant women.

The vaccine, developed by China’s Xiamen Innovax Biotech, has not been widely used outside China although it has been backed by WHO for use in outbreaks since 2015.

The chair of the WHO’s expert vaccine committee, Hanna Nohynek, told the press conference that the focus on outbreaks was a sign that “the normalcy is starting to be living with outbreaks… that’s kind of alarming.”