COVID antibodies in patients with mild symptoms fade quickly, raising concerns that their immunity from a future infection may not last very long, researchers said in the New England Journal of Medicine.

The first analysis was done on antibodies taken an average of 37 days after symptoms began, with a second after about 86 days, or less than three months.

The researchers determined that antibody levels had fallen precipitously, with a half-life of about 73 days between the two time frames.

That raises concern that immunity may not last long in people who develop a mild infection, which accounts for the majority of cases.

The results call for caution regarding antibody-based “immunity passports,” herd immunity and perhaps vaccine durability, especially in light of short-lived immunity against common human coronaviruses, according to the researchers led by F. Javier Ibarrondo, from the David Geffen School of Medicine at the University of California Los Angeles.