Gloria Golú, 41, holds her belly at University Hospital of Valle, in Cali, Colombia, last month. Golú is carrying her third child and is infected with the Zika virus. (Eduardo Leal/For The Washington Post)

The number of new Zika infections in Colombia is falling so fast that government health officials declared an end to the outbreak Monday, saying the epidemic phase of the virus’s spread was over.

Nearly 100,000 Colombians have been diagnosed with Zika since the first cases were confirmed last October. But Colombian health officials say the number of new infections in their country is falling by more than 600 a week, meaning that the virus has moved into an “endemic” phase in which it continues to circulate but is no longer spreading pervasively.

Only Brazil, the epicenter of the Zika outbreak, has reported more cases, but Colombia’s total is far below the projected 700,000 infections that health officials were bracing for earlier this year.

“Colombia is the first country in the world to declare an end to the Zika epidemic,” Vice Minister of Health Fernando Ruiz told reporters.

In this April 26, 2016, file photo, an Aedes aegypti mosquito is seen through a microscope at Colombia's National Institute of Health in Bogota. Officials have declared an end to the Zika epidemic in Colombia, the second-hardest hit nation in the region from the mosquito-borne virus. (AP Photo/Fernando Vergara, File) (Fernando Vergara/AP)

The announcement is especially significant because Colombia has had a well-regarded and robust monitoring system in place to track the epidemic and its gravest health risks, including fetal birth defects and the potentially fatal neurological disorder known as Guillain-Barré syndrome.

Colombian officials said they have confirmed 21 cases of Zika-related microcephaly, in which the virus attacks developing fetal brain cells to cause congenital birth defects. Another 160 cases remain under investigation, they said.

More than 16,000 pregnant women have been infected with the virus in Colombia, according to the latest government data.

The Zika epidemic peaked in Colombia in February, and many of the mothers who were infected during the late first trimester — when the developing fetal brain is most vulnerable — are due to give birth in the coming weeks. Ruiz said the number of microcephaly cases is likely to rise, and health officials continue to expect up to 300 cases this year.

That would be roughly double the average annual number of babies born each year in Colombia with microcephaly, which can be caused by a number of other factors. But it is far below the more than 1,600 cases of Zika-related birth defects reported in Brazil.

The U.S. Centers for Disease Control and Prevention (CDC) has teamed with Colombian doctors to set up long-term monitoring programs in three major cities that will spend a year tracking the development of babies born to mothers infected with Zika.

Colombian officials also blame Zika for at least 350 cases of Guillain-Barré syndrome.

Martha Ospina, director of Colombia’s National Health Institute, said declaring an end to the epidemic did not mean there would be no new infections, only that the virus’s spread was clearly in decline. It will remain endemic in Colombia, she said, meaning that “it will continue to be with us” and could come roaring back with a new outbreak at some point.

Zika has infected more than 1,400 people on the U.S. mainland, including 400 pregnant women, according to the latest CDC figures. The virus is not yet being spread by mosquitoes in the United States, so all of those infections were probably acquired abroad or through sexual transmission.

A recent study by British modeling experts predicted that the outbreak is likely to end within three years because of a phenomenon known as “herd immunity,” in which so many people develop antibodies to Zika after catching the virus that it runs out of new patients to infect.