During the coronavirus pandemic, public health authorities have urged people to exercise caution before having partner sex, due to unknowns about how the virus is transmitted. One small study published on Thursday appears to give some credence to this concern, indicating that the novel coronavirus may be detectable in semen.

Published in JAMA Network Open, the study assessed semen samples from 38 patients at Shangqui Municipal Hospital in Shangqui, China, all of whom had been diagnosed with COVID-19. The authors found that a small number of the patients (about 16%), had detectable levels of the virus in their samples.

Significantly, the researchers note, the progression of the disease did not appear to have any impact on the levels of the virus in their semen; four of the men were at the “acute” stage of the virus, while two of the men were classified as having already recovered.

The study is small, and its findings do not necessarily mean that COVID-19 is transmissible via sex, just that the virus may be present in small amounts in some patients’ reproductive tracts. The study also did not follow up with its subjects after recovery, so it’s unclear exactly how long the virus remained in some of the men’s semen.

Yet the results contrast with those from a previous (and even smaller) study from February and March, which found no evidence of the virus in men who had tested positive for COVID-19.

“If it could be proved that SARS-CoV-2 can be transmitted sexually in future studies, sexual transmission might be a critical part of the prevention of transmission, especially considering the fact that SARS-CoV-2 was detected in the semen of recovering patients,” the study authors note, recommending “abstinence or condom use” as possible preventive measures.

COVID-19 is transmitted via respiratory droplets, such as those that are emitted when a person coughs or sneezes. Traces of the virus have also been found in blood and feces.

Because the virus can be transmitted via sustained interpersonal contact, many public health officials have urged people to exercise caution in their sex lives. New York City Department of Health, for instance, issued safe sex guidelines stating “you should avoid close contact — including sex — with anyone outside your household” and that “you are your safest sex partner.”