“On Wednesday afternoon, I received the results of a COVID-19 ELISA IgG Antibody Test, which were positive," Casey said in a statement. "This positive test means that I likely had COVID-19 at some point over the last several months and have since developed an antibody response to the virus."
Sen. Tim Kaine, D-Va., also announced this week that he and his wife also had a positive antibody test. Sen. Rand Paul, R-Ky., was the first senator who was diagnosed with COVID-19 in real-time by taking a diagnostic test.
Casey was sick earlier this spring with a low-grade fever and some mild flu-like symptoms. After talking to his doctor, he quarantined at home in Scranton for two weeks and didn't seek medical care since his symptoms were manageable, the senator said.
"My fever went away on its own by mid-April, and it was never recommended that I be tested for the virus," Casey said.
But recently, Casey consulted Dr. Brian Monahan, the Capitol’s attending physician, and received the blood test to see if his mild sickness earlier this spring could have been COVID-19.
The results showed Casey had "substantial levels of COVID-19 antibody in my blood, significantly more than the amount required to qualify me as a plasma donor," Casey said.
Casey said he's making his first plasma donation Friday in Taylor, Penn., to help others fighting the virus.
COVID-19 has now killed more than 100,000 Americans.
The presence of antibodies for a certain disease in someone's system is a reliable indicator that he or she has had that illness in the past. Antibodies, according to a CDC page on the coronavirus, typically "help fight off infections and usually provide protection against getting that disease again (immunity)." But it remains unclear how much immunity they provide from getting the coronavirus again.
Given the uncertainly, Casey and Kaine both said they'll continue to follow the advice of public health experts by wearing masks and follow social distancing guidelines. Meanwhile, Paul has chosen not to wear a mask, claiming immunity.
Fox News' Tyler Olson contributed to this report.