Update (January 20): In a first, University of Alabama at Birmingham surgeons have successfully implanted a pig kidney into a person—in this case, a brain dead man—with no signs of rejection, and the organ produced urine for several days, according to a university news release.

According to the National Kidney Foundation, the wait time for a kidney transplant spans several years due to a shortage of available organs. This scarcity could be rectified by organs from nonhuman animals, if such xenotransplant organs could prove viable. Now, research has taken a big step in that direction, experts say, as doctors at NYU Langone Transplant Institute claim they’ve performed the first-ever successful pig-to-human kidney transplant.

The surgery, which was into a person on life support with no detectable brain activity and occurred in September, attached a single kidney to a pair of blood vessels external to the patient’s body to enable observation, reports Reuters (the procedure was approved by the patient’s family). The case has not yet been published in a peer-reviewed journal. After 54 hours, there were no signs of rejection, and the kidney was functioning well, Robert Montgomery, the director of the NYU Langone Transplant Institute and who performed the surgery, tells The New York Times. “A lot of kidneys from deceased people don’t work right away and take days or weeks to start,” he notes. “This worked immediately.”

Johns Hopkins transplant surgery professor Dorry Segev, who was not involved in the operation, tells the Times the xenotransplant is a “huge breakthrough,” adding that “It’s a big, big deal.”

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Animals such as pigs that can be bred on a large scale and whose organs are appropriately sized for human use have long been eyed as a potential solution to the dearth of transplantable organs. Unfortunately, the human immune system usually attacks the foreign tissues, causing the xenotransplant to fail. That’s why, for this surgery, the donor pig was a “GalSafe” pig: an animal genetically altered to lack a gene that plays a role in the production of alpha-gal, a carbohydrate that triggers rejection by the human recipient, according to Reuters. GalSafe pigs were approved by the FDA in December 2020 for consumption and medical uses.

The successful surgery signals that genetically engineered pigs “could potentially be a sustainable, renewable source of organs—the solar and wind of organ availability,” Montgomery tells the Times

However, further work is needed, other experts say. Segev notes to the Times that “We need to know more about the longevity of the organ,” as it was only observed for about two days. Jay A. Fishman of the transplantation center in Massachusetts General Hospital echoes Segev’s sentiments: “Whether this particular study advances the field will depend on what data they collected and whether they share it, or whether it is a step just to show they can do it.”

There are also ethical concerns around xenotransplantation, the Associated Press reports. Karen Maschke, a Hastings Center research scholar tasked with developing ethics and policy recommendations for the first NIH-funded clinical trials of pig to human organ transplants, notes that animal welfare needs to factor into any future plans for GalSafe pigs. “The other issue is going to be: Should we be doing this just because we can?” Maschke says.