'Herd immunity' may be curbing U.S. Zika numbers
In practical terms, herd immunity means that people traveling to the United States from South America and the Caribbean may have been infected with Zika in the past, but they can no longer pass the virus on to mosquitoes that might pass it on to local residents.
"People that were infected before can't be infected again. That's our understanding," Dr. Henry Walke, incident manager for Zika response at the U.S. Centers for Disease Control and Prevention, told the Miami Herald.
The latest statistics in Florida seem to bear that theory out: The number of Zika cases reported in the state this year has reached 140, a drastic decrease from nearly 1,500 in 2016, Florida health officials report.
At the time of the Florida outbreak, the CDC issued a domestic travel advisory warning pregnant women to avoid Miami-Dade County because the virus can cause severe birth defects. All told, 384 pregnant women have tested positive for Zika in Florida since January 2016, with nine delivering children with Zika-related birth defects, the Herald reported.
Luckily, the same drop in Zika cases has been seen in other areas that were hard hit by the virus last year, health officials added.
"You don't have as much of the virus circulating.
That's true not only in Puerto Rico, but throughout the Caribbean and throughout South America," Walke said.
Another expert explained the phenomenon this way: "If a large enough proportion of the herd - be it cows or mice or people - are resistant to a disease, it's very difficult for the disease to spread," said Uriel Kitron. He is chair of the department of environmental sciences at Emory University, in Atlanta.
Kitron, who studies Zika in Brazil, said that very few Zika infections have been seen there since a major outbreak occurred in that country back in 2015. ■