Zika Infection Rate in US Is Low Among Asymptomatic, Potentially Exposed

Diana Phillips

April 15, 2016

Fewer than 1% of asymptomatic, potentially exposed pregnant women tested positive for Zika in the United States this year, according to new data.

"In the current U.S. setting, where most exposure is travel-associated, the likelihood of Zika virus infection among asymptomatic persons is low," write Sharoda Dasgupta, PhD, from the Epidemic Intelligence Service, Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, and colleagues.

However, the CDC continues to recommend that asymptomatic pregnant women within 12 weeks of potential exposure be tested for the virus, given the potential for adverse pregnancy and infant outcomes associated with infection.

In addition, individuals with potential exposure to the virus who show signs or symptoms consistent with Zika virus disease should also be tested, the CDC recommends.

The renewed recommendations are based on a review of Zika virus testing and infection patterns in the United States from January 3, 2016, through March 5, 2016, published in an early release April 15 in Morbidity and Mortality Weekly Report.

During this period, the CDC tested specimens from 4534 individuals who had traveled to or moved from areas with active Zika virus transmission, including 3335 (73.6%) pregnant women. Of the 4534 specimens, 197 (4.3%) tested positive for Zika virus, 55 (1.2%) tested positive for recent unspecified flavivirus, and 4282 (94.4%) showed no evidence of recent Zika virus infection.

However, the researchers note that nearly all the positive tests were among individuals with one or more symptom. Specifically, 1541 individuals tested, including 620 pregnant women, reported one or more Zika virus-associated symptom (eg, fever, rash, arthralgia, or conjunctivitis). Infection with the virus was confirmed in 182 (11.8%) of these individuals, including 18 of the pregnant women.

Of 436 (9.6%) individuals, including 290 pregnant women, who underwent testing and reported at least one other symptom, eight overall (1.8%) and three of the pregnant women had confirmed Zika virus.

Finally, of 2557 asymptomatic individuals, including 2425 pregnant women, who underwent testing, seven women (0.3%) — all of whom were pregnant — tested positive for the Zika virus.

Infection with the Zika virus — a flavivirus primarily transmitted by Aedes species mosquitoes — is often asymptomatic or associated with mild clinical disease, the authors write. Signs and symptoms of infection can include fever, rash, arthralgia, or conjunctivitis. For the period of January 1, 2015, to April 13, 2016, the CDC has confirmed 358 travel-associated cases of Zika virus disease, 351 of which were in individuals who traveled to or moved from areas with active Zika virus transmission.

"Overall, relatively few persons receiving testing for Zika virus at CDC had confirmed Zika virus infection, and the proportion with confirmed Zika virus infection was higher among persons who reported at least one Zika virus-associated symptom than among persons with other symptoms only or asymptomatic persons," the authors write. They stress that these results reflect the current situation in the United States and District of Columbia, where most exposure is travel-related and "where there is no local mosquito-borne transmission." Testing results might differ in areas where transmission of the virus is active, they acknowledge.

Although Zika virus infection was confirmed in seven asymptomatic pregnant women who received testing, "it is reassuring that the proportion of asymptomatic pregnant women with confirmed Zika virus infection in this report was low," the authors write.

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. Published online April 15, 2016. Full text

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