Geneva, Switzerland - Measles infections soared by a fifth last year to over 10 million cases globally, revealing alarming gaps in vaccine coverage, a study showed Thursday.

Worldwide, there were an estimated 10.3 million measles cases in 2023, according to a joint publication by the World Health Organization and the US Centers for Disease Control and Prevention (CDC).

That marked a 20% increase from 2022, the study showed, saying that "inadequate immunization coverage globally is driving the surge in cases."

Measles is one of the world's most infectious diseases. At least 95% coverage with two doses of the measles/rubella vaccine is needed to prevent outbreaks.

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But in 2023, only 83% of children worldwide received their first dose of the measles vaccine through routine health services – the same level as in 2022 but down from 86% before the pandemic.

Only 74% received their second dose last year, the study showed.

"Measles vaccine has saved more lives than any other vaccine in the past 50 years," WHO chief Tedros Adhanom Ghebreyesus said in a joint statement.

"To save even more lives and stop this deadly virus from harming the most vulnerable, we must invest in immunization for every person, no matter where they live."

CDC director Mandy Cohen said, "The measles vaccine is our best protection against the virus, and we must continue to invest in efforts to increase access."

As a result of global gaps in vaccination coverage, 57 countries experienced large and disruptive measles outbreaks in 2023, up from 36 countries a year earlier, the study showed.

All regions except the Americas were impacted, it said, with nearly half of all large and disruptive outbreaks occurring in the African region.

The virus that can cause a rash, fever, and flu-like symptoms but also particularly severe complications in young children is estimated to have killed 107,500 people in 2023, most of them under the age of five. This marks an 8% decrease from the previous year.

The agencies explained that the decline was mainly due to the fact that the surge in cases occurred in countries and regions where children with measles were less likely to die due to better nutritional status and access to health services.

"Far too many children are still dying from this preventable disease," they said.

The agencies cautioned that a global target of eliminating measles as an endemic threat by 2030 was "under threat."

By the end of last year, 82 countries had achieved or maintained measles elimination.

After Brazil this week reverified having eliminated measles, WHO's Americas region is once again considered free of endemic measles.

All regions, with the exception of Africa, count at least one country that has eliminated the disease.