(CNN) The U.S. Centers for Disease Control and Prevention screened 4,534 people for the Zika virus between January and March, according to a report published Friday. All of those screened either traveled to or moved from areas where the mosquito-borne disease is circulating. Just over 4% tested positive for the virus. More than 94% tested negative and another 1% had an unspecified virus in the same family as Zika, such as dengue or yellow fever, or may have been vaccinated against a related virus.
The report, published by the CDC's Morbidity and Mortality Weekly Report, sheds some light about the risk for travelers to the more than 40 countries and territories where the virus is circulating. An estimated 40 million people travel between the continental United States and areas infected with the Zika virus, according to Health and Human Services Secretary Sylvia Burwell.
It's not definitive, said Stephen Morse, professor of epidemiology at the Columbia University Mailman School of Public Health and director of the Infectious Disease Epidemiology Certificate Program. It does, however, offer a rough estimate for a returning traveler to know what the chances are of contracting the virus. "The fact that the risk of contracting it is less than you might think ... it might be somewhat reassuring," he said.
Of those who were screened, 3,335 (73.6%) were pregnant women. Twenty-eight (0.8%) tested positive for the virus. The CDC recommends all pregnant women returning from travel to areas where the virus is being transmitted be screened for the virus within two weeks, regardless of whether they are experiencing symptoms. (Before February 12 that recommendation only applied to returning pregnant travelers with symptoms.)
Symptoms include rash, fever, joint pain and conjunctivitis, or red eyes, and are present in 20% of those infected with the virus. Of the pregnant women with no symptoms who were screened as recommended, 99% of them were negative for the virus.
While this indicates the risk may be lower than thought, the effects of the disease are still devastating. It can cause severe birth defects in babies born to mothers who had the virus while pregnant.
"It is reassuring that the proportion of asymptomatic pregnant women with confirmed Zika virus infection in this report was low. However, because of the potential serious adverse pregnancy and neonatal outcomes associated with maternal Zika virus infection, health care providers should continue to offer testing to pregnant women with potential exposure to Zika virus, even if they do not have symptoms," the report said
The report has several limitations. It does not account for where the travelers visited, how long they were in the affected area, or details of their potential exposure to the virus, such as if windows were screened or if they were using mosquito repellant. Only results from tests performed at the CDC are included, not those performed by state health departments. In addition, testing may have been delayed and therefore the individual screened may have forgotten that they experienced symptoms, an important consideration since those with symptoms were more likely to test positive than those who do not have any. The authors also caution that their findings do not apply to areas where there is active virus transmission.
Dr. Scott Lillibridge, an epidemiologist and professor at the Texas A&M Health Science Center School of Public Health, agreed with Morse that it's reassuring that the numbers are not higher. However, "this isn't anything to be taken lightly," he said, adding that it signals the need to worry about travelers who do not have symptoms.