Puerto Rico’s Zika epidemic has ended, officials said Monday, noting substantially fewer new cases this spring.

Only 10 cases have been reported in each four-week period since April, a dramatic decrease from the more than 8,000 cases reported in a four-week period at the peak of the epidemic last August, according to a health ministry statement.

The island has been the part of the United States hardest hit by the mosquito-borne virus, with authorities counting more than 40,000 confirmed cases of Zika infection as of May 20, including 3,678 pregnant women. There have been 35 cases of Zika-related birth defects.

Health officials and experts have expressed concern about underreporting of birth defects in Puerto Rico. In the continental United States, with 1,579 Zika-infected pregnant women, there have been 80 pregnancy losses or babies born with birth defects, according to the Centers for Disease Control and Prevention.

In the Puerto Rico ministry’s statement, CDC acting director Anne Schuchat said officials “cannot let our guard down.” The agency will continue to focus on protecting pregnant women and to work closely with local health authorities to support comprehensive Zika surveillance and prevention efforts, she said.

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