Semen samples from 35 men taken within a month of their recovery showed a 60 per cent drop in their sperm's ability to move and a 37 per cent reduction in sperm count.
The study, published on Monday in the Fertility and Sterility journal, took samples from 120 men in Belgium with an average age of 35 years and at an average of 52 days after their COVID-19 symptoms had cleared.
As time since recovery from COVID-19 increased, the quality of sperm also improved. Samples from 51 patients taken between one and two months after recovery showed 37 per cent had reduced sperm motility and 29 per cent had low sperm counts, falling further to 28 per cent and 6 per cent after at least two months had passed.
The researchers also said they had found "strong evidence" that COVID-19 could not be sexually transmitted through semen after a person had recovered from illness.
Avoid COVID if you want to get pregnant
However, the Belgian researchers behind the study warned that further work was needed to establish whether or not COVID-19 could have a longer-term impact on fertility.
"Couples with a desire for pregnancy should be warned that sperm quality after COVID-19 infection can be suboptimal," the researchers concluded.
"The estimated recovery time is three months, but further follow-up studies are under way to confirm this and to determine if permanent damage occurred in a minority of men".
Patients with more severe cases of COVID-19 were not more likely to experience falls in sperm count and motility, the researchers said, noting that they "found no differences" in the sperm quality of those who were hospitalised with the virus and those who stayed at home with milder symptoms.
How does COVID affect sperm?
Some viruses like influenza are already known to damage sperm. In the case of flu, the higher body temperatures experienced as a result of fever are to blame.
But in the case of COVID-19, the researchers found no link between the presence or severity of fever and sperm quality. Instead, they believe the cause could be linked to the body's immune response to the virus.
Tests showed that higher concentrations of specific COVID-19 antibodies in patients' blood serum were strongly correlated with reduced sperm function, the researchers said, which indicated an "immunologic rather than a fever-induced causality of the temporary sperm dysfunction".
While the study showed that there was no COVID-19 RNA present in the semen of patients who had got over the virus, the fact that antibodies were attacking sperm suggests the virus may "breach…the blood-testis barrier" during the peak of an infection, the researchers said.
This supports an earlier study from Wuhan in China, where PCR tests on semen samples from infected COVID patients tested positive for the virus, they added.
(Except for the headline, this story, originally published by euronews.com has not been edited by SLM staff)