A one-square-mile area north of downtown Miami, marked by three streets and a highway, is a Zika hot zone that public health officials say pregnant women should avoid.

Many people don’t understand how those boundaries were picked as part of an unprecedented travel advisory from the Centers for Disease Control and Prevention. And they want to know why the advisory isn’t broader.

“We’ve been asked, ‘Why not all of Miami?’” CDC Director Tom Frieden said during a visit there last week. The answer, at least for now: “There’s no evidence that there’s any Zika spreading anywhere else in Miami.”

Active transmission currently is taking place only within a 500-square-foot section of that larger zone, which includes the trendy Wynwood neighborhood, according to Frieden. That’s where virtually all of the 17 people confirmed to have Zika have been bitten by infected mosquitoes. The latest was announced Monday afternoon, although state officials did not specify where the individual had been during a recent visit to Miami-Dade County.

Additional cases are likely to be identified as testing continues, and that could mean expanding the travel advisory. In deciding on the initial perimeter, public health officials wanted a margin of safety around any active transmission. The Aedes aegypti mosquito — the primary species to transmit Zika — flies only about 500 feet in its lifetime, and so once Florida officials identified the “hot zone,” they followed the CDC’s recommendation to expand it to an area about five times as big. They used roadways for the borders: NW Fifth Avenue on the west, U.S. 1 on the east, NW/NE 38th Street to the north and NW/NE 20th Street to the south.

“If we get new information, we will change that boundary,” Frieden said in an interview. “If we saw the area of infection expanding out or setting up toeholds contiguous to that area, that would be concerning.”

Florida officials also are investigating another Zika infection, announced last week, that occurred outside the advisory area in Miami.

The CDC’s Zika response plan, a 57-page document that federal, state and local authorities worked on for months, details the steps officials should follow when even a single case of local Zika transmission is suspected. The plan provides guidance on determining areas of infection, which reflects awareness of the economic impact and social disruption that can result from the kind of travel advisory now in place. “States should designate the smallest easily identifiable location that completely encompasses the geographic area for intervention,” the plan notes.

“It’s a stigma to the area,” Frieden said, acknowledging that a travel advisory can seem to penalize localities, like Miami, that have done a better job of identifying Zika cases. Shop owners in Wynwood are already reporting a sharp drop-off in business and foot traffic.

The advisory, aimed at pregnant women because Zika causes severe neurological damage to a fetus, could expand or shrink depending on several factors, such as:

• The number and location of additional Zika cases.

• The effectiveness of mosquito-control measures and how much of the breeding season remains.

• The presence of Aedes aegypti or Aedes albopictus, the two mosquito species that can transmit the virus, in an area and whether it has a history of dengue or chikungunya, viruses that also are spread by Aedes aegypti.

• The proportion of homes and offices with air conditioning and intact screen windows and doors.

The threat of Zika still exists in other communities across the United States because 40 million people travel to countries in the Americas and Caribbean that have been hard hit since the virus began spreading locally late last year. Infected travelers who return home to an area where either mosquito species is endemic raise the risk of local transmission should a mosquito then bite them.

About 1,800 travel-related Zika cases have been reported in the United States as of Aug. 3. Florida has by far the largest number of cases. But given that 80 percent of infected individuals don’t have symptoms, the true number of people with Zika nationwide is likely to be more like 7,200, according to the CDC.

It’s certainly possible that other parts of the country already have clusters of local Zika infections that have yet to be identified. Aedes aegypti is present in 30 U.S. states, and the South’s warm, humid climate is particularly fertile ground. Florida and Texas remain at highest risk because both previously had cases of dengue and chikungunya.

Texas officials say they’re on high alert. “It’s the perfect mix — local transmission in Florida, travel to Brazil [for the Olympics] and we’re at the height of mosquito season in Texas,” said John Hellerstedt, the state’s health services commissioner. “Local transmission here is likely at some point.”

Eight public health response teams are ready to deploy to help local authorities investigate possible cases, evaluate mosquito activity and provide door-to-door education, Hellerstedt said.

CDC officials warn that Miami’s hot zone could have sporadic Zika cases for months and that other advisories could be issued elsewhere if cases emerge. “Everything about this outbreak has been unprecedented,” Frieden said.

At what point will a travel advisory no longer be needed? The CDC points to three scenarios: when the Miami environment is no longer conducive for mosquito activity, when evidence shows that the risk of transmission has been sufficiently reduced, or when no new local-transmission cases have been identified for 45 days, which equals three mosquito incubation periods. (That’s the time from when a mosquito gets the virus from an infected human to the time it is capable of transmitting the virus to a new human host.)

“Ultimately, we have a responsibility to protect the public, and I think everyone in public health and government has accepted that,” Frieden said.

This story has been updated. 

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