Antibody levels in patients with mild COVID-19, the level of disease most people have, appear to drop by half within 36 days, new research suggests. The research was conducted by F. Javier Ibarrondo, PhD, and colleagues and was published online on July 21 in a letter to the editor of the New England Journal of Medicine. Ibarrondo is associate researcher at the University of California, Los Angeles. (The original letter incorrectly calculated the half-life at 73 days.)

Coauthor Otto Yang, MD, professor of medicine in the division of infectious diseases at UCLA, told Medscape Medical News that the rapidity in the antibody drop at 5 weeks “is striking compared to other infections.”

The phenomenon has been suspected and has been observed before but had not been quantified.

“Our paper is the first to put firm numbers on the dropping of antibodies after early infection,” he said.

The researchers evaluated 34 people (average age, 43 years) who had recovered from mild COVID-19 and had referred themselves to UCLA for observational research.

Previous report also found a quick fade

As Medscape Medical News reported, a previous study from China that was published in Nature Medicine also found that the antibodies fade quickly.

Interpreting the meaning of the current research comes with a few caveats, Dr. Yang said.

“One is that we don’t know for sure that antibodies are what protect people from getting infected,” he said. Although it’s a reasonable assumption, he said, that’s not always the case.

Another caveat is that even if antibodies do protect, the tests being used to measure them – including the test that was used in this study – may not measure them the right way, and it is not yet known how many antibodies are needed for protection, he explained.

The UCLA researchers used an enzyme-linked immunosorbent assay to detect anti–SARS-CoV-2 spike receptor–binding domain immunoglobulin G concentrations.

“No reason for anybody to be getting an antibody test medically”

The study provides further proof that “[t]here’s no reason for anybody to be getting an antibody test medically right now,” Dr. Yang said.

Additionally, “FDA-approved tests are not approved for quantitative measures, only qualitative,” he continued. He noted that the findings may have implications with respect to herd immunity.

“Herd immunity depends on a lot of people having immunity to the infection all at the same time. If infection is followed by only brief protection from infection, the natural infection is not going to reach herd immunity,” he explained.

Buddy Creech, MD, MPH, associate professor of pediatrics and director of the Vanderbilt Vaccine Research Program in Nashville, Tenn., pointed out that antibodies “are just part of the story.”

“When we make an immune response to any germ,” he said, “we not only make an immune response for the time being but for the future. The next time we’re exposed, we can call into action B cells and T cells who have been there and done that.”

So even though the antibodies fade over time, other arms of the immune system are being trained for future action, he said.

Herd immunity does not require that populations have a huge level of antibodies that remains forever, he explained.

“It requires that in general, we’re not going to get infected as easily, and we’re not going to have disease as easily, and we’re not going to transmit the virus for as long,” he said.

Dr. Creech said he and others researching COVID-19 find that studies that show that antibodies fade quickly provide more proof “that this coronavirus is going to be here to stay unless we can take care of it through very effective treatments to take it from potentially fatal disease to one that is nothing more than a cold” or until a vaccine is developed.

He noted there are four other coronaviruses in widespread circulation every year that “amount to about 25% of the common cold.”

This study may help narrow the window as to when convalescent plasma – plasma that is taken from people who have recovered from COVID-19 and that is used to help people who are acutely ill with the disease – will be most effective, Dr. Creech explained. He said the results suggest that it is important that plasma be collected within the first couple of months after recovery so as to capture the most antibodies.

This study is important as another snapshot “so we understand the differences between severe and mild disease, so we can study it over time, so we have all the tools we need as we start these pivotal vaccine studies to make sure we’re making the right immune response for the right duration of time so we can put an end to this pandemic,” Dr. Creech concluded.

The study was supported by grants from the AIDS Healthcare Foundation, the Doris Duke Charitable Foundation, the National Institutes of Health, the James B. Pendleton Charitable Trust, and the McCarthy Family Foundation. A coauthor reports receiving grants from Gilead outside the submitted work. Dr. Creech has disclosed no relevant financial relationships.

This article first appeared on Medscape.com.

Rapid drop of antibodies seen in those with mild COVID-19