The slowdown has prompted some countries, including Colombia, to significantly scale back their projections of the impact of the virus.
Epidemiologists tracking Zika warn that the downward trend appears to be limited to certain countries and should not be interpreted as a sign the epidemic has started to ebb everywhere in the Americas. It still appears to be expanding in Brazil, the epicenter of the virus, they caution.
“We think it has passed the peak of transmission in some countries, like Colombia, but the outbreak is continuing, and it’s possible there will be a second wave of infection,” Sylvain Aldighieri, head of epidemiological response at the Pan American Health Organization, said in an interview. “And Zika has not reached its peak yet in Central America and the Caribbean.”
Still, the latest figures raise hopes that the virus, which scientists think is linked to fetal malformations and the potentially deadly neurological disorder known as Guillain-Barre syndrome, may not produce as many infections as initially feared.
U.S. health experts convene Friday in Atlanta for a summit called by the White House to help state and local governments formulate Zika response plans ahead of summer, the most active season for the Aedes aegypti mosquito. The insect, the primary vector of the virus, is found in southern states along the Gulf Coast.
Officials say 312 Zika cases have been confirmed in the United States so far, but those infections were acquired by travelers who visited a Zika-afflicted area or were contracted by their partners through sexual contact. Another 352 cases have been registered in U.S. territories, primarily in Puerto Rico.
According to the most recent figures from the Pan American Health Organization, nearly 200,000 people have been infected with Zika across the Americas. Of those, 4,600 cases were confirmed with laboratory testing. The rest are based on clinical diagnoses, which are considered less precise.
In Colombia, the number of infections has slowed from more than 6,000 new cases a week in early February to fewer than 3,000 per week by late March.
“Our models confirm the epidemic has peaked,” said Marta Lucia Ospina, director of Colombia’s National Health Institute. She said the government has scrapped its initial projection of 600,000 infections in the country this year.
In El Salvador, the number of new Zika cases has fallen from more than 1,000 a week in early January to fewer than 100 per week in March. Honduras was tallying more than 2,000 cases weekly in early February; now the figure has dropped to fewer than 700 a week.
Health experts are not sure why Zika infections have declined in some areas. Sanitation campaigns to eliminate breeding sites may have helped in some urban hot spots. There may be increasing levels of local immunity in places where the virus has been circulating for months.
The virus continues to move into new areas, as individuals with Zika arrive from regions where it is more prevalent. Mosquitoes bite them, become infected, and then transmit the disease to others. Areas at high elevations, such as Bogotá in Colombia and Mexico City, are not considered at risk because they are beyond the 6,500-foot limit of the Aedes mosquito’s range.
See also: No new cases of Zika in Costa Rica in recent weeks
Even countries with a declining number of Zika cases may still face serious fallout from the disease. Colombian authorities are monitoring more than 1,000 pregnant women who have tested positive for Zika, and about 10,000 others who reported symptoms of the virus, to see whether their babies display signs of neurological damage. In Brazil, Zika is strongly suspected of causing a surge of infants born with underdeveloped brains and small heads.
Of 32 Colombian infants born since January with undersized heads — a condition known as microcephaly — eight have tested positive for Zika.
Counting Zika infections is hardly an exact science. There is no quick, commercially available test for the virus. It generally survives in the body only for a few days, and it produces few or no symptoms in the majority of patients. The quality of monitoring, information-gathering and reporting also varies widely from country to country.
Nor is there a cure for Zika, so many infected patients in poor areas prefer to lie in bed and take painkillers for a few days rather than coping with waits in crowded public hospitals.
Brazil has reported the biggest outbreak so far, with more than 72,000 suspected cases, but health officials think the actual number of infections is probably much higher, because Zika is often mistaken for dengue fever, and the country is in the grips of one of its worst dengue outbreaks in years.
Colombia, which experts say has much more rigorous standards for testing and reporting, has the second-highest Zika count, with nearly 56,000 cases to date. Next are Honduras, with 16,500 cases, and Venezuela, with 15,500. But WHO officials say reporting from Venezuela has been spotty because the health system has suffered enormously from the country’s economic crisis.
Predicting the impact of Zika has been extremely challenging for health officials. According to Aldighieri, the initial projections were based on the spread of other viruses, such as chikungunya, that are transmitted by the Aedes mosquito.
Chikungunya first appeared in the Americas in 2014, he noted, and it wasn’t until this year that it finally reached the southern limits of the Aedes mosquito’s range in northern Argentina.
Zika has yet to reach the full extent of the Aedes range in Brazil itself, Aldighieri notes, and the epidemic is still concentrated in the northern regions of the country where the outbreak began.
There are 33 countries or territories in the Americas that have reported local vector-borne transmission of Zika, meaning the infection was spread by mosquitoes.
“It is too early for us to determine if Zika virus disease cases and spread in the Americas peaked last month,” Marc Fischer, medical epidemiologist with the Division of Vector-Borne Diseases of the U.S. Centers for Disease Control and Prevention, said in a statement.
Several factors could be responsible for changes in the numbers, he said, including the adoption of different testing practices or reporting delays. “It will take more time to be confident about trends in numbers of disease cases,” he said.
Zika is not expected to proliferate as rapidly in the United States as it has in Latin America, where a scarcity of window screens, air conditioning and routine garbage collection in poor areas create optimal conditions for Aedes mosquitoes to thrive. The mosquitoes, which lay eggs in standing water, can breed rapidly in urban slums where residents lack running water and keep jugs and containers in their homes.