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Two asymptomatic travelers returning from China tested positive for novel coronavirus, or SARS-CoV2, despite being afebrile and reporting no symptoms, German researchers found.

In a group of 114 mostly German travelers evacuated out of Hubei province, all of whom reported no symptoms and had no fever, two tested positive for SARS-CoV2, reported Sandra Ciesek, MD, of University Hospital Frankfurt in Germany, and colleagues.

However, there was no conclusive evidence that the two individuals who tested positive were in danger of passing the virus to others, they stated in a Correspondence in the .

In fact, the researchers noted "epidemiologic uncertainty regarding possible transmission of the virus by asymptomatic or subclinically symptomatic infected persons" and that it is also not clear "whether persons who show no signs or symptoms of respiratory infections shed SARS-CoV2."

In the cases they describe, however, throat swabs from the asymptomatic individuals were positive for the virus, raising the possibility that they could have spread virus particles through normal exhalation.

The cases came from a situation familiar to many countries recently: the evacuation of citizens from Hubei province, to be placed in quarantine for 14 days. And they further describe procedures taken to assess infection of quarantined passengers, including those currently on cruise ships: symptom questionnaires and temperature checks.

Ciesek and colleagues found 126 passengers were screened for symptoms and clinical signs of infection prior to boarding, but on the flight, 10 passengers were isolated -- six with reported symptoms, two who had contact with a confirmed case, and two accompanying family members with suspected infection or other symptoms. All tested negative for SARS-CoV2 via real-time reverse-transcription-polymerase-chain-reaction (RT-PCR) assays of throat swabs and sputum, they said.

The remaining 116 passengers were evaluated by a team of physicians in Frankfurt, who gave them symptom questionnaires and took their temperatures. One passenger was febrile and reported dyspnea and a cough, but ultimately tested negative.

In addition to this "preplanned multi-step process of screening for signs and symptoms of infection and observing the asymptomatic cohort in quarantine," researchers decided to offer passengers a throat swab for SARS-CoV2. Of the 114 of 116 who consented, two tested positive.

While there was no conclusive evidence these two passengers were infectious, researchers noted "the isolation of SARS-CoV2 from both samples in cell culture of Caco-2 cells indicated potential infectivity."

The two cases were described in more detail in a supplemental appendix: a man, age 58, was asymptomatic and afebrile, even after 7 days of observation, with no lab abnormalities except for anemia.

A woman, age 44, reported various symptoms in the prior weeks, such as a whole body rash, a dry cough, an itchy throat, otalgia (ear pain), and one instance of an elevated temperature. Upon admission, she presented with a slight rash on her chest and legs, and "minimal inflammation" of the throat, though she "remains well" after 7 days, with no laboratory abnormalities reported.

Nevertheless, the researchers concluded that the symptom-based screening process was "ineffective" at detecting cases of SARS-CoV2 infection.

"We discovered that shedding of potentially infectious virus may occur in persons who have no fever and no signs or only minor signs of infection," they wrote.