Covid-19 Long-term Immunity: 8 months
A new study proposes that long-term immunity lasts at least 8 months post-infection.
Hello All! After today, I will be posting a biotechnology article bi-weekly until the end of the spring semester. I will be attending classes full-time now, so unfortunately, I will have less time to research and write an article every week.
After today, my next article will be published on 1/21/21, so keep your eyes open!
Since the start of the Covid-19 pandemic in February 2020, scientists and the general public have been asking the question, “How long does immunity last after infection with Covid-19?”
A research group in California have proposed an answer: at least 8 months.
This research group measured the concentration of specific immune cells (B Cells, CD4+ T cells, and CD8+ T cells) and neutralizing antibodies present in 188 subjects who exhibited a range of Covid-19 infection severity; asymptomatic to severe. The group performed measurements up to 240 days (about 8 months) post-symptom onset (PSO).
The group measured the concentration of neutralizing antibodies for 240 days PSO. Their data indicates that the effectivity of antibodies to recognize and neutralize the SARS-CoV-2 spike protein remain reasonably stable over 6+ months. However, antibody effectiveness varied between individual subjects - almost a 200-fold difference.
A fold change is a ratio between two measurements. For example, we’re measuring apples harvested; the measurement from Year 1 is 60 apples and the next year, Year 2, the measurement is 30 apples. The ratio is 60/30 (60 over thirty or 60 divided by 30) which is equivalent to 2-fold.
This ~200-fold difference suggests that individual people may have very different immune responses to Covid-19 post-infection. In other words, some people may not have long-term immunity to Covid-19; they could be reinfected.
The research group also examined B cell and T cell (CD4+ and CD8+) responses. Memory B cells are responsible for secreting neutralizing antibodies. CD8+ “killer” T cells neutralize or “kill” infected cells while CD4+ “helper” T cells assist in initiating and regulating the adaptive immune response; the immune response responsible for memory.
The group’s data indicates that spike memory B cells are abundant at 6+ months post-infection and the CD4+ and CD8+ SARS-CoV-2 specific T cells gradually decline over the 8 months with a half-life of 3-5 months.
Biological half-life is the time it takes for a maximum concentration of a biological substance to be reduced to half of its maximum concentration, often through natural processes in the human body.
Based on the results of this study, two things are clear:
There is notable variance between individuals’ long-term immune responses against SARS-CoV-2. Some people had strong neutralizing antibodies at the 8 month mark, while others did not.
The body’s ability to initiate adaptive immunity against SARS-CoV-2 and neutralize infected cells decreases over 8 months.
The important take-away message here is that some SARS-CoV-2-infected people who did not have durable long-term immunity may be reinfected with the virus in the near future. Also, this group found that ~95% of their subjects had measurable immune responses after 5-8 months PSO, indicating the possibility of long-term immunity against reinfection with SARS-CoV-2 in the majority of individuals.
Keywords:
B cells: Immune cells responsible for producing antibodies.
CD4+ T cells: Immune cells responsible for initiating and regulating the adaptive immune response.
CD8+ T cells: Immune cells responsible for “killing” infected or irregular cells.
Additional Readings:
COVID-19 immunity could be long term
Immunological memory to SARS-CoV-2 assessed for up to 8 months after infection