The committee that awarded the 2011 Nobel Medicine Prize was not aware that one of this year's laureates, Ralph Steinman of Canada, died just days before Monday's announcement, the head of the committee said.
Nobel prize regulations do not allow the award to be given posthumously, but Goeran Hansson said the committee stood by its choice.
Steinman, 68, died of pancreatic cancer on September 30, according to Rockefeller University in New York where he worked.
Hansson, the head of the Nobel assembly at Karolinska Institutet, said the committee had just found out about his passing.
"We just got the information. What we can do now is only to regret that he could not experience the joy," Hansson told Swedish news agency TT.
"We don't name new winners, that was our decision," he said.
"How it will be done in practice to hand out the prize is what we will have to investigate," he said, adding, "we are examining the rules."
Steinman was on Monday named the 2011 laureate together with Bruce Beutler of the United States and Luxembourg-born Frenchman Jules Hoffmann for their pioneering research on the immune system.
The prize is to be formally handed over at a ceremony in Stockholm on December 10.