Transcript for Contracting COVID-19 may not confer immunity: WHO
Now to what most Americans at home are most concerned with, how do we get back to some sort of normalcy in the country? And are antibody tests part of the solution? Do you have some sort of immunity? Tonight, the world health organization and what they said, saying they do not know yet whether having this once means you won't have it again. Here's Matt Gutman tonight. Reporter: The warning from the world health organization acknowleding it does not know if surviving the coronavirus means you have immunity. We need much more information from recovered patients. Reporter: In South Korea, reports of people testing positive twice. Here in the U.S., authorities remain hopeful that you may have some immunity. Here in California, multiple trials are under way to test for those antibodies -- telltale proteins in the blood that show exposure to the virus. We were there as 50 firefighters were being tested. But the CDC is concerned about accuracy and reliability of the new tests. And one said it's possible immunity could weaken over time. We don't know. Reporter: If the antibodies disappear, that means theoretically you lose the immunity. Meantime, the race is on to help the most severe patients still fighting for survival. We have reported here, the fda has fast-tracked an experimented treat that allows people that have survived the coronavirus to donate their blood. Jason Garcia from San Diego recovered from the virus. Hopeful an age-old technique will work again. It's such a small thing to have such a huge impact. Reporter: He's donated to three patients, one of whom doctors say has shown remarkable improvement. In Florida, Kevin Rathel was on a ventilator, in a coma, and near death. Four days ago, he started receiving donor plasma. His wife got a surprise facetime call with her husband. And we just started talking I told him, I said, "Kevin, you've been asleep for a week. It's time to wake up. It's Easter morning and god performed a miracle. You don't need to come down to us, and we love you, we're waiting for you." Reporter: At this point, doctors are unwilling to draw a correlation between the plasma treatment and his apparent recovery. They say while it's worked against other viruses in the past, with covid, the plasma treatment is uncharted territory. Matt, thank you. We'll have much more of his reporting later tonight on "Nightline."
This transcript has been automatically generated and may not be 100% accurate.