"What is herd immunity and can it stop the coronavirus?"

"Two years later, in 2017, there was no longer nearly so much to worry about. A Brazilian study found by checking blood samples that 63% of the population in the northeastern beach city of Salvador had already had exposure to Zika; the researchers speculated that herd immunity had broken that outbreak."

source:
https://www.technologyreview.com/s/615375/what-is-herd-immunity-and-can-it-stop-the-coronavirus/