July 22 (UPI) -- Immunity against COVID-19 in people who have recovered from the disease may fade after about 90 days, according to a small study published this week by the New England Journal of Medicine.
Thirty-four patients included in the analysis produced antibodies against the new coronavirus, SARS-CoV-2, from as short as 44 days to as long as 119 days after symptom onset, the researchers said.
It is still unclear whether antibodies against the virus protect people from future re-infection, they said.
"If our measurements accurately reflect antibodies that protect people from infection, our results suggest that they may be relatively short-lived and immunity may wane," co-author Dr. Otto Yang, an infectious disease specialist at UCLA's Geffen School of Medicine, told UPI.
"We don't know how much antibody is needed, but these results imply that protective amounts of antibody may not last long," he said.
The findings need to be confirmed in a larger study, with more patients, the researchers said.
If accurate, however, the short life span of antibodies against the new coronavirus "would be a big barrier to herd immunity," Yang said.
"If people become susceptible again after some period of time ranging from months to a couple of years, then letting people get infected to achieve herd immunity may be ineffective if peoples' immunity is only temporary," he said.
The analysis evaluated 34 patients ranging in age from 21 to 68 years who had recovered from COVID-19, the researchers said.
Blood tests first detected antibodies -- or cells produced by the immune system to fight off a virus -- as early as 18 days after symptom onset in some patients but as late as 65 days in others, they said.
In some patients, antibodies were undetectable in as little as 44 days after symptom onset, the researchers said.
In general, these findings mirror the results of a similar study of 65 recovered COVID-19 patients in England, which found that only 17% still had antibodies against the virus 90 days after symptom onset.
Still, given that it's still unclear how "protective" antibodies are against the virus, more research is needed to get a true sense of immunity, Yang said.
"If antibodies are important, it's not clear what measurements are most relevant, and whether simple quantitative measurements are sufficiently reflective versus qualitative analysis of some type," he said.