"There was a lot of concern originally that this virus might not induce much memory," Shane Crotty, an infectious disease professor at the University of California, San Diego and one of the study researchers, told MIT Review. "Instead, the immune memory looks quite good."
According to the National Institutes of Health, the immune system remembers a virus after a person gets infected with it. Immune cells and proteins in the body can then recognize and kill the virus if that person catches it again, preventing a second infection. This "immunological memory" is the basis for adaptive immunity.
There are several components of adaptive immunity, including antibodies, helper T cells, killer T cells and memory B cells. Antibodies recognize foreign substances such as viruses and neutralize them. Helper T cells, on the other hand, activate other immune cells while killer T cells eliminate pathogens. Finally, memory B cells reactivate and produce new antibodies when the body needs them.
But it was unclear how long immunity against COVID-19 would last. To that end, Crotty and his team measured the levels of each of these immune components in 188 COVID-19 survivors and patients. More than 40 of the participants provided samples of their blood six to eight months after catching the virus.
The researchers found antibodies specific to the spike protein of SARS-CoV-2, which the virus uses to infect cells, in 98 percent of the participants a month after symptom onset. Levels of these antibodies declined only modestly six to eight months after infection, consistent with previous studies.
Meanwhile, virus-specific memory B cells increased over time. The participants had more memory B cells six months after symptom onset compared to a month afterward. Though memory B cells reached a plateau after a few months, they did not decline over the period studied.
Levels of virus-specific T cells also remained high. Six to eight months after infection, 92 percent of the participants still had detectable helper T cells while around half still had detectable killer T cells.
"This implies that there’s a good chance people would have protective immunity, at least against serious disease, for that period of time, and probably well beyond that," Crotty said.
The researchers noted that the immune system's response to COVID-19 varies from one person to another. People with a weak immune system may be more susceptible to reinfection or are more likely to infect others than those with a robust immune system. But overall, the findings bode well for COVID-19 survivors.
"Several months ago, our studies showed that natural infection induced a strong response, and this study now shows that the responses last," said Daniela Weiskopf, a researcher at the La Jolla Institute for Immunology (LJI) in California and a co-author of the study.
Past studies suggested that survivors might be vulnerable to reinfection since COVID-19 antibodies steadily plummet after recovery. For instance, a study released last November indicated that antibodies peak 24 days after infection and drop by half just three months after symptom onset. But Alessandro Sette, one of the study's researchers who heads his own lab at LJI, allayed fears of a second infection.
"Of course, the immune response decreases over time to a certain extent, but that's normal. That's what immune responses do. They have a first phase of ramping up, and after that fantastic expansion, eventually the immune response contracts somewhat and gets to a steady state," Sette explained. (Related: Infected patients develop long-term immunity to coronavirus.)
The researchers continue to analyze blood samples from COVID-19 patients to track their responses 12 to 18 months after symptom onset. They are also working to understand how adaptive immunity differs across people of different ages and how that may influence disease severity.
Learn more about how the body's built-in defenses ward off COVID-19 at ImmuneSystem.news.