The World Health Organization said Monday that even with the widespread rollout of vaccination programs, herd immunity will not be achieved in 2021.

Dr. Soumya Swaminathan, the health body's chief scientist, spoke at a press conference and urged people to continue strict social distancing measures for the rest of the year because it will take a while to build up that immunity.

"Even as vaccines start protecting the most vulnerable, we’re not going to achieve any levels of population immunity or herd immunity in 2021," Swaminathan said. "Even if it happens in a couple of pockets, in a few countries, it’s not going to protect people across the world."

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"The vaccines are going to come," she continued. "They are going to go to all countries ... but meanwhile we mustn't forget that there are [public health] measures that work."

Those measures include social distancing, wearing masks, and proper hygiene.

Herd immunity -- which is usually achieved through vaccination -- occurs when enough people in a population have the antibodies needed to ward off infection, thus preventing its spread. Experts estimate that it will take about 75% of the population to achieve herd immunity.

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Dr. Bruce Aylward, an adviser to WHO’s director-general, called on the global community to ensure all countries have access to vaccines.

He added that the U.N. health agency was hoping coronavirus vaccinations could start either this or next month in some of the world’s poorer countries. 

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"We cannot do that on our own," Aylward said.

Fox News' Madeline Farber and the Associated Press contributed to this report