Colombian women listen as a health worker distributes information on preventing the spread of the Zika virus, in Bogota, Colombia, on Sunday. (John Vizcaino/Reuters)

Local officials say two people in Brazil contracted the Zika virus through blood transfusions, the first reports of that sort of transmission in an outbreak that is projected to infect millions in the coming months.

Cármino Antonio de Souza, health secretary of Campinas, a city northwest of São Paulo, said both transfusions occurred during the first four months of 2015 but that the transmission wasn't confirmed until recently, according to the Wall Street Journal. One patient was a liver-transplant recipient and the other a gunshot victim.

Marcelo Addas Carvalho, director of the Blood Center at the Sao Paulo state University of Campinas, told Reuters in an interview that genetic testing confirmed that a man who received a blood transfusion using blood from a donor with Zika in March 2015 became infected with the virus. The patient later died from his gunshot wounds and not the Zika infection, local health officials and Carvalho told the news service.

Worries about Zika in the blood supply have been a growing concern for health officials around the world. While there are no scientifically confirmed cases of transmission through the blood supply, it is theoretically possible and has been shown to occur with viruses in the same family.

The American Red Cross, as well as health officials in Canada and Britain, have been urging people who have traveled to regions affected by the Zika virus to wait at least 28 days before giving blood.

Susan Stramer, the vice president of scientific affairs for the Red Cross, said this week that the organization it is also asking donors to immediately notify the organization if they subsequently develop symptoms consistent with the Zika virus within 14 days of donating blood "so that we can quarantine the product."

On Thursday, a Red Cross spokeswoman said that the two reported cases in Brazil "are under investigation and will provide further information regarding the risk of transmission of Zika by blood transfusion." She added that the 28-day "self-deferral" policy in the United States should further safeguard the nation's blood supply, and that "it is important to note that due to the absence of local mosquito transmission, the risk of contracting Zika virus by blood transfusion in the continental U.S. at this time is believed to be extremely low."

Officials at the Food and Drug Administration said earlier this week they are assessing whether travelers who have visited places with local Zika transmission should defer donating blood. The FDA did not immediately respond to a request for comment Thursday.

The American Association of Blood Banks, the professional standards group, said its 28-day self-deferral recommendation also applies to other tropical viruses, such as dengue and Chikungunya, and that it should result in only minor decreases — about 2.25 percent — in overall donations to the blood supply.

"It wouldn’t be surprising to see transmissions through blood on rare occasions,” said Tom Skinner, a spokesman for the Centers for Disease Control and Prevention, saying it depends on how sick the donor is. The Zika virus usually clears the bloodstream within a week, researchers have said.

Lena H. Sun contributed to this report.

This post has been updated. 

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