A study shows that in people with mild COVID-19 cases, their antibodies against the coronavirus drop sharply over the first three months after infection.

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A research team at the University of California, Los Angeles, did an in-depth study of 34 people who had recovered from mild COVID-19 infections. They tested their blood two or three times over three months, Xinhua news agency reported on July 21. The researchers found a rapid drop in antibodies - the immune system proteins that help stop viruses from infecting cells in the body. On average, the antibody levels fell by half every 73 days, according to the study published in The New England Journal of Medicine.

The findings raise concern that humoral immunity against SARS-CoV-2 may not be long lasting in persons with mild illness, who compose the majority of persons with COVID-19, said the study. Further studies will be needed to define a quantitative protection threshold and rate of decline of antiviral antibodies beyond 90 days, according to the study.