MIAMI — Aerial spraying of insecticide began Thursday in the one-mile-square area of Miami where mosquitoes have infected people with the Zika virus, and officials reported some glimmers of progress.
“We are very encouraged by the initial results, which showed a large proportion of the mosquitoes killed,” Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention, said at a news conference here. But Dr. Frieden added, “This is going to take an intense effort.”
On Monday, faced with 14 locally transmitted infections, the C.D.C. took the unprecedented step of advising people to stay away from a location in the continental United States, urging pregnant women not to travel to the one-square-mile area where 12 of the cases are linked.
That area is part of the Wynwood section of Miami, north of downtown. Since then, mosquito-control efforts have been accelerated and teams from the Florida Department of Health have been testing people in the neighborhood, a mix of industrial and residential buildings.
On Thursday, the health department said it had completed testing in a 10-block section of the neighborhood and found no additional cases.
“We feel comfortable now that in that one-mile radius, we can take 10 blocks in that northwest corner and say we don’t believe there is any active transmission of Zika,” Gov. Rick Scott said at the news conference.
Nevertheless, Dr. Frieden said, “we would not be surprised to see additional infections diagnosed” in a core area that measures 500 square feet.
“That’s the way Zika works,” he said. “But what we want to see is the mosquito counts coming down.”
This week, the department has reported only one other case of local transmission of Zika, bringing the total to 15, but that was in another part of Miami-Dade County with no connection to Wynwood. Officials said they were testing people in the community to see if the virus had spread but so far had found no evidence.