Two studies published yesterday show that COVID-19’s immune responses last up to 8 months, although the authors focus on different reasons.
The first study, published in Scientific Immunology, followed a small cohort of Australians from day 4 to day 242 after infection. All patients demonstrated the presence of memory B cells – immune cells that “resemble” viral proteins and can trigger the rapid production of antibodies when reexposed to the virus – up to 8 months after the initial infection.
The second study investigated antibody responses in 58 patients confirmed with COVID-19 in South Korea, 8 months after asymptomatic or mild SARS-CoV-2 infection, finding high rates of serum antibodies. These results, published in Emerging infectious diseases, are contradictory both with the antibody data from the first study and with previous research that showed antibodies decreasing after 20 days, but the authors suggest that variations in the characteristics of the immunoassay and in the manufacture may be responsible for the difference.
Memory B cells are essential for detecting the immune response
For the Australian study, the researchers obtained blood samples from 25 patients confirmed with COVID-19 with a variety of disease severities and 36 healthy control patients from March to September, assessing the antibody status of each patient and the levels of virus-specific immune cells.
The study authors found that, on day 6 post-infection, all patients had immunoglobulin (Ig) G antibodies to the viral receptor binding domain – a protein on the viral surface that binds to cell receptors, allowing entry and infection – and the nucleocapsid protein, the protective protein shell of the virus. The patient’s IgG levels began to decrease 20 days after the onset of symptoms.
In contrast, the study authors found that memory B-cell levels continued to increase until 150 days after infection and remained detectable 240 days after the onset of symptoms, suggesting that the patient’s immune system was primed to respond to reinfection. Because of their prolonged presence, cells may be a better indicator of long-term immune response than serum antibodies, say the authors.
The study results provide hope that vaccines will provide a similar duration of protection, and the authors say that cellular immunity may explain why there are few documented cases of SARS-CoV-2 reinfection.
“These results are important because they definitely show that patients infected with the COVID-19 virus do indeed retain immunity against the virus and disease,” said senior author Menno van Zelm, PhD, in a press release from Monash University yesterday.
“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.”
Antibodies 8 months after infection in Korea
In the second study, the researchers measured specific antibodies to SARS-CoV-2 using four commercial immunoassay tests on isolated patients at a community treatment center at Seoul National University Hospital from March 5 to April 9. Three of the four trials showed high rates of seropositivity (69% to 91.4%; P <0.01), in contrast to another previous study showing that asymptomatic patients become seronegative 2 to 3 months after infection.
“Rates differed according to immunoassay methods or manufacturers, thus explaining the differences in rates between studies,” wrote the authors. For example, they said, in July BMJ study reported that chemiluminescent immunoassay tests had 97.8% sensitivity to IgG or IgM, whereas enzyme immunoassay tests had only 84.3%.
The neutralization activity of the virus – essential for protection against reinfection – was detected in only 53.4% of the study participants at 8 months after infection, considerably less than the rate of positivity for immunoassays.
“Despite concerns about declining immunity, appropriate immunoassays can detect antibodies to SARS-CoV-2 8 months after infection in most asymptomatic or mildly symptomatic people,” concluded the authors.