Two studies published yesterday show that immune responses to COVID-19 last as long as 8 months, although the authors focus on different reasons.
The first study published in Scientific immunology, followed a small group of Australians from 4 to 24 days after infection. All patients showed the presence of memory B cells – immune cells that “remember” viral proteins and can trigger rapid antibody production when re-exposed to the virus – as long as 8 months after the initial infection.
Another study examined antibody responses in 58 confirmed patients with COVID-19 in South Korea 8 months after asymptomatic or mild SARS-CoV-2 infection, finding a high serum antibody rate. These results published in Emerging infectious diseases, contradict both the antibody data from the first study and previous studies that showed that antibodies decline after 20 days, but the authors suggest that differences in immunoassay and production characteristics may be responsible for the difference.
Memory B cells are key to detecting an immune response
For an Australian study, researchers took blood samples from 25 confirmed COVID-19 patients with a range of disease severity and 36 healthy control patients from March to September, assessing antibody status and the level of virus-specific immune cells.
The study authors found that by day 6 after infection, all patients showed immunoglobulin (Ig) G antibodies for the viral receptor-binding domain – a viral surface protein that binds to cellular receptors, allowing entry and infection – and a nucleocapsid protein, protective protein shell of the virus. The patient’s IgG level began to decline 20 days after the onset of symptoms.
In contrast, the study authors found that memory B cell levels continued to rise up to 150 days after infection and remained visible 240 days after the onset of symptoms, suggesting that the patient’s immune system was prepared to respond to reinfection. Because of their prolonged presence, cells may be a better indicator of a long-term immune response than antibodies in serum, the authors say.
The results of the study give hope that vaccines will create a similar duration of protection, and the authors say that cellular immunity can explain why there are few documented cases of re-infection with SARS-CoV-2.
“These results are important because they definitely show that patients infected with the COVID-19 virus actually retain immunity against the virus and the disease,” said the senior author, Dr. Menno van Zelm in a statement from Monash University yesterday.
“This is a black cloud that hangs over the potential protection that any COVID-19 vaccine could provide and gives real hope that, once a vaccine or vaccine is developed, it will provide long-term protection.”
Antibodies 8 months after infection in Korea
In another study, researchers measured antibodies specific for SARS-CoV-2 using four commercial immunoassay tests of isolated patients at the National University Hospital Center in Seoul from March 5 to April 9. 69% to 91.4%; Str <0.01), in contrast to another earlier study showing that asymptomatic patients became seronegative 2 to 3 months after infection.
“Rates varied according to immunoassay methods or manufacturers, thus explaining differences in rates between studies,” the authors wrote. For example, they said, July BMJ the study reported that chemiluminescent immunoassays had 97.8% IgG or IgM sensitivity, while enzyme-linked immunosorbent assays had only 84.3%.
Virus neutralizing activity – essential for protection against re-infection – was detected in only 53.4% of study participants in the 8 months after infection, which is significantly lower than the positivity rate for immunological tests.
“Despite concerns about reduced immunity, appropriate immune tests can detect antibodies to SARS-CoV-2 at 8 months after infection in most asymptomatic or mildly symptomatic individuals,” the authors concluded.