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Asymptomatic Zika Infections

photo by Muhammad Mahdi Karim

photo by Muhammad Mahdi Karim

One of the biggest unknowns about Zika viral illnesses is the number of patients who contract the disease but have a sub-clinical or asymptomatic illness. The incidence of asymptomatic Zika is critical to understanding how the illness is spread. Can an asymptomatic patient pass the virus to another person through sex? Can an asymptomatic patient infect a mosquito that bites him and infect the mosquito enough that the mosquito can transmit the virus to another human?

There is some data that can be useful, and it uses only laboratory testing done in the United States. Three sources provide a total of 14,079 patients tested for Zika. 883 tested as positive. 252 of those patients had no symptoms of a Zika illness, 28.5% of that total.

Patterns in Zika Virus Testing

The first source detailed the results of testing by the Centers for Disease Control and Prevention. From January 3 to March 5, 2016, 4,534 patients were tested. 197 were positive for Zika. 15 of those, 7.6%, were asymptomatic.

In that total are 3,335 pregnant women. 18 had Zika symptoms. 3 had other symptoms and 7 had no symptoms. 25% of the pregnant women found infected with Zika had no symptoms, and 36% had no symptoms or non-Zika specific symptoms.

All of these patients were tested because they had reason to believe that they could have contracted a Zika illness. They had lived in or traveled to a region where Zika was being locally transmitted. Over 94% of those tested had no sign of an infection with Zika or another unspecified flavivirus.

Update: Ongoing Zika Virus Transmission — Puerto Rico

This data is part of a continuing series of reports from Puerto Rico, where the mosquito borne virus is being locally transmitted.

9,343 pregnant women were tested. 672 were positive, 7.2%. Of that total, 231 were asymptomatic, 34.4% of all infected patients.

A total of 23,487 patients were evaluated, by clinical exam and / or laboratory testing. 5,582 were confirmed positive for Zika, 23.8%.

Department of Health Daily Zika Update

The Florida Department of Health has also been doing some testing. From Jul 7 to Aug 1, 2016, the DoH tested a total of 202 individuals in Miami-Dade and Broward counties. The testing was occasioned by local transmission of the Zika virus.

14 patients were confirmed, through laboratory tests, to be positive for Zika. That represents 6.9% of those tested. 8 of those patients had symptoms and 6 were asymptomatic. Testing was conducted on people who live or work near the individuals known to have contracted Zika locally.


  • Three sources provides Zika testing data that includes the number of asymptomatic illnesses verified through lab testing.
  • 14,079 people with reason to believe they had been exposed to Zika were tested. 13,196 were not positive for the virus. 883, 6.3%, were positive.
  • Of those testing positive, 631 had symptoms.
  • 252 patients had no symptoms or did not have Zika symptoms, 28.5% of all positive patients.
  • The three sources report on 28,223 patients tested or evaluated. 20.5%, 5,793 patients, are considered positive for Zika virus.

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