Three separate studies just published in the journals Immunity and Science Immunology suggest COVID-19 antibodies persist in survivors for at least three months—if not longer. Additionally, two of the three studies point to the IgG class of antibodies as the longest-lasting in patients who have contracted COVID-19, with or without symptoms.
Given its novelty, there has been and continue to be many, many questions surrounding COVID-19. One of the most persistent has been the idea of immunity. Once someone survives a COVID-19 infection, are they then immune to reinfection? If so, for how long? And does this apply to those who are asymptomatic, as well?
Understanding reinfection rates can have a huge impact on public health policy. Thus far, studies have indicated herd immunity is not an effective solution to end the COVID-10 pandemic. And while that still may be true, the studies published this week provide more evidence of long-lasting protective antibodies.
Massachusetts General Hospital
Researchers at Massachusetts General Hospital obtained blood samples from 343 patients with COVID-19—most of whom had severe symptoms—and compared them to blood samples taken from 1,500 individuals prior to the pandemic. The COVID-19-infected blood samples were taken up to four months after a patient's symptoms emerged.
According to their paper, first author Richelle Charles, MD, and her team isolated plasma from the blood and applied it to laboratory plates coated with the receptor-binding domain (RBD) of the coronavirus’ spike protein to examine how different types of antibodies in the plasma bound to RBD.
The team found immunoglobulin G (IgG) levels remained elevated in COVID-19 survivors for four months, and were associated with the presence of protective neutralizing antibodies, which also demonstrated little decrease in activity over time.
Additionally, the researchers showed that those 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," says Charles.
That information is critical as 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.
University of Toronto
Researchers at the University of Toronto also focused on the IgG class of antibodies, finding that they are detectable for up to 115 days (just shy of 4 months) in COVID-19 survivors.
Graduate students Baweleta Isho, Kento Abe, Michelle Zuo and Alainna Jamal used both saliva and blood samples from 402 COVID-19 patients to measure and compare antibody levels for almost four months post-symptom onset. Compared with 339 pre-pandemic controls, patients with COVID-19 showed peak IgG levels at 16 to 30 days after the appearance of symptoms. Even so, researchers found that antibodies of the IgG class that bind to the SARS-CoV-2 spike protein were detectable for at least 115 days. The study is the first to show these antibodies can also be detected in the saliva.
While the team admits there is a lot still unknown about antibody responses to SARS-CoV-2 infection, including how long the antibodies last beyond this period or what protection they afford against re-infection, they say this research could have broader implications in the development of an effective vaccine.
University of Arizona
Meanwhile, researchers with the University of Arizona took a slightly different approach, focusing on antibodies that bind to two different parts of the SARS-CoV-2 virus—one common and one less common.
As discussed in the Massachusetts General Hospital study, most antibody tests look for antibodies at S1, which includes the receptor-binding domain wherein the spike protein binds to a protein receptor to infect cells. But the UArizona Health Sciences team developed a test that also analyzes the S2 region of the spike protein.
The researchers, including Deepta Bhattacharya and Janko Nikolich-Zugich, believe the analysis of only certain antibodies may have led earlier studies to report short-term immunity. For example, when a virus first infects cells, the immune system deploys short-lived plasma cells that produce antibodies to immediately fight the virus. Those antibodies appear in blood tests within 14 days of infection. The second stage of the immune response is the creation of long-lived plasma cells, which produce high-quality antibodies that provide lasting immunity.
Focusing on the long-lived plasma cells, Bhattacharya and Nikolich-Zugich studied and tracked the production of COVID-19 antibodies from a sample of nearly 6,000 survivors. They found SARS-CoV-2 antibodies present in blood tests at viable levels for at least 5 to 7 months, although they believe immunity actually lasts longer.
"The latest time-points we tracked in infected individuals were past seven months, so that is the longest period of time we can confirm immunity lasts," Bhattacharya said. "That said, we know that people who were infected with the first SARS coronavirus, which is the most similar virus to SARS-CoV-2, are still seeing immunity 17 years after infection. If SARS-CoV-2 is anything like the first one, we expect antibodies to last at least two years, and it would be unlikely for anything much shorter."
Photo: Deepta Bhattacharya, an associate professor of immunobiology at the UArizona College of Medicine - Tucson, gets blood drawn to take the antibody test he helped develop. Credit: University of Arizona Health Sciences, Kris Hanning