Lasting Defense?

Protective antibodies persist for months in patients who survive serious COVID-19

antibodies vs covid

3D illustration of antibodies and a coronavirus. Image: Dr_Microbe/Getty Images.

This article is part of Harvard Medical School’s continuing coverage of medicine, biomedical research, medical education and policy related to the SARS-CoV-2 pandemic and the disease COVID-19.

People who survive serious COVID-19 have long-lasting immune responses against SARS-COV-2, according to a new study led by Harvard Medical School researchers at Massachusetts General Hospital.

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The study, published in Science Immunology, offers hope that people infected with the virus will develop lasting protection against reinfection. The findings also demonstrate that measuring antibodies can be an accurate tool for tracking the spread of the virus in the community.

The immune system produces proteins called antibodies in response to infection with SARS-CoV-2, the virus that causes COVID-19.

“But there is a big knowledge gap in terms of how long these antibody responses last,” said senior study author Richelle Charles, HMS assistant professor of medicine and an investigator in the Division of Infectious Diseases at Mass General.

To help close the gap, Charles and colleagues obtained blood samples from 343 patients with COVID-19, most of whom had severe cases. The blood samples were taken up to four months after a patient’s symptoms emerged.

Blood plasma was isolated and applied to laboratory plates coated with the receptor-binding domain (RBD) of the virus’ spike protein, which SARS-CoV-2 uses to enter and infect human cells. The team studied how different types of antibodies in the plasma bind to RBD. The results were compared to blood samples obtained from more than 1,500 individuals prior to the pandemic.

The researchers found that measuring an antibody called immunoglobulin G (IgG) was highly accurate in identifying infected patients who had symptoms for at least 14 days. The standard PCR nasal swab test for SARS-CoV-2 loses sensitivity over time. Augmenting it with a test for antibodies in patients who have had symptoms for at least eight days, at which time 50 percent are producing antibodies, would help identify some positive cases that might otherwise be missed, according to Charles.

The researchers found that IgG levels remained elevated in these patients for four months and were associated with the presence of protective neutralizing antibodies, which also demonstrated little decrease in activity over time.

“That means that people are very likely protected for that period of time,” Charles said. “We showed that key antibody responses to COVID-19 do persist.”

In another finding, Charles and her colleagues showed that people infected with SARS-CoV-2 had immunoglobulin A (IgA) and immunoglobulin M (IgM) responses that were relatively short-lived, declining to low levels within about two and a half months or less, on average.

“We can say now that if a patient has IgA and IgM responses, they were likely infected with the virus within the last two months,” Charles said.

Knowing the duration of the immune response by IgA and IgM will help scientists obtain more accurate data about the spread of SARS-CoV-2, according to study co-senior author Jason Harris, HMS associate professor of pediatrics and an infectious disease specialist at Mass General.

“There are a lot of infections in the community that we do not pick up through PCR testing during acute infection, and this is especially true in areas where access to testing is limited,” Harris said. “Knowing how long antibody responses last is essential before we can use antibody testing to track the spread of COVID-19 and identify ‘hot spots’ of the disease.”

Adapted from a Mass General news release.