We've heard a lot about women getting blood clots after receiving the Johnson & Johnson vaccine.
Now officials from the University of California, San Francisco say a man is recovering after developing one.
It popped up in his leg within two weeks of receiving the Johnson & Johnson vaccine.
There's now a call for researchers to pay attention to the different responses men and women may experience during medical trials.
"There is a growing call to look at these differences as new vaccines and new drugs are brought out. It is something that has not been done on a widespread basis in the past. But I think that it is time and people have seen and we will probably be continuing to see differences in the way women and men respond," said Dr. John Hammer.
During the Johnson & Johnson clinical trial only one man experienced a blood clot.