THURSDAY, Dec. 31, 2020 -- Folks who contract COVID-19 can expect to gain some durable immunity against future infection, according to a new study of memory cells within the immune systems of coronavirus patients.
Previous studies have raised concern that COVID-19 patients might lose their immunity quickly once they recover, because the first wave of coronavirus antibodies tends to wane after the first few months.
But painstaking work by Australian researchers has revealed that people infected with the SARS-CoV-2 virus have immune memory to protect against reinfection for at least eight months.
Samples from 25 COVID-19 patients found that they continued to carry stable levels of virus-specific memory B cells as long as eight months after their infection, the researchers reported online recently in the journal Science Immunology.
"Because of the biology of the persistence of these memory cells, it anticipates that we will have rather durable immunity," said Dr. William Schaffner, a professor of infectious diseases at Vanderbilt Medical Center in Nashville, Tenn.
"It can't tell us for exactly how long, but it does conform with the observation that documented second infections have been, to this point, really quite rare," said Schaffner, who was not involved with the new study.
This discovery indicates that COVID-19 vaccines will create lasting immunity, said senior researcher Menno van Zelm. He's an associate professor at the Monash University department of immunology and pathology, in Melbourne.
"This has been a black cloud hanging over the potential protection that could be provided by any COVID-19 vaccine and gives real hope that, once a vaccine or vaccines are developed, they will provide long-term protection," van Zelm said in a journal news release.
After the body successfully fends off an infection, some of the immune cells that were part of the defense are converted into "memory" cells that remember the virus or bacteria they just fought.
These cells live in the body for a long time, and provide a blueprint for the immune system to fight off any future infections from the same pathogens.
For the new study, the researchers analyzed 36 blood samples taken from 25 COVID-19 patients, and found that antibody levels did indeed show a marked decline around 20 days after the onset of symptoms.
But memory B cell levels continued to rise as far out as 150 days, the investigators found, suggesting that the body remained ready and able to quickly churn out virus-specific antibodies when faced with a future infection.
Memory B cells specific to COVID-19 were found in samples taken 242 days after symptom onset, the results showed.
According to Schaffner, "It does really suggest that once we experience the infection, our immune response recognizes the foreignness of COVID and sets up a mechanism so it will be able to respond very quickly down the road should we be exposed to the virus a second time."
Because COVID-19 has only been around for about a year, experts still can't say exactly how long that immune protection will last, noted Dr. Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, in Baltimore.
The possibility remains that people might need regular COVID-19 vaccine shots, just as there are annual flu shots.
"It appears, like with many infectious diseases, recovery from COVID-19 is associated with a period of immunity that extends at least several months," Adalja said. "It will be important to determine how long immunity -- natural and vaccine-induced -- lasts in order to optimize vaccine schedules; it will likely be a year or more."
In the meantime, these results indicate that COVID-19 survivors who fall ill a second time probably have picked up some other bug, he noted.
"For those who have recently recovered, it's important to think of other causes for symptoms, as reinfection is very rare in the months post-infection," Adalja said.
While these memory B cells provide welcome news of lasting protection, they circulate in such low levels in the body that it's unlikely they could form the basis of another new type of COVID-19 test, Schaffner said.
"The cells that are circulating are there in very small numbers. So you have to pick them out, and you have to pick them out against a lot of background noise," Schaffner said. "Part of the reason this got into this prestigious journal is that these investigators really went to great lengths to quite definitively find these small circulating memory B cells."
Arizona State University has more about immune memory cells.
SOURCES: William Schaffner, MD, professor of infectious disease, Vanderbilt Medical Center, Nashville, Tenn.; Amesh Adalja, MD, senior scholar, Johns Hopkins Center for Health Security, Baltimore; Science Immunology, Dec. 22, 2020, online