A recent study by the U.S. Centers for Disease Control and Prevention found that 6% of more than 3,000 health care workers who were tested had antibodies to coronavirus. Still, after being retested 60 days later, 28% of them had antibody levels so low they were no longer detected.

"These results suggest that serology testing at a single time point is likely to underestimate the number of persons with previous SARS-CoV-2 infection, and a negative serologic test result might not reliably exclude prior infection," the authors of the study said.

According to the CDC, blood samples were taken from 3,248 frontline health care personnel at 13 hospitals between April 3- June 19, 2020.
194 of those healthcare workers had detectable antibodies to COVID, the agency said.

Participants with higher initial antibody responses were more likely to have antibodies detected at the follow-up test than were those who had a lower initial antibody response, the study concluded.

The authors added that the study shows that the window is limited for collecting potentially useful "convalescent plasma" from the blood of patients who have fought off COVID-19.