If COVID-19 antibodies decline after 2-3 months, are COVID-19 antibody tests really USELESS?
Keywords: SARS-CoV-2, herd immunity, immunity passport, neutralizing antibody
As the pandemics of COVID-19 is still ongoing, we are all anxious to know whether herd immunity might stop or slow down the spread of the disease, and if the "immunity passport" works while we gradually re-open the community, which relies on the duration and strength of the immunity.
A new study published on Jun 18, 2020 in Nature medicine reported that antibodies decrease within 2-3 months after initial infection (Quan-Xin Long, 2020), suggesting whether achieving herd immunity is possible and if the antibody test would only be effective in the immediate few month postinfection.
Dr. Huang and his colleagues compared the antibody response in the Wanzhou District. 37 asymptomatic people and 37 symptomatic people were followed up for 8 weeks. Asymptomatic people showed weaker immune responses, and antibodies from both asymptomatic and symptomatic people dropped to undetectable levels in 40% vs. 12.9% of the people tested. (Quan-Xin Long, 2020)
Since this study was released, it has instigated the notion that immunity against SARS-CoV-2 is short term. Furthermore, this controversial notion was covered extensively by mainstream media (i.e.: CNBC) and widely interpreted that antibody tests which detect virus specific antibodies are useless and the idea of "immunity passport" is risky.
Before we make such conclusions, we may need to consider how adaptive immunity works and how it protects us from reinfection? Especially whether it is right for us to infer that the decline in the level of antibodies really translates to loss of immunity towards SARS-CoV-2.
Decrease of antibodies may not correlate directly to the loss of long-term immunity
1. Decline of antibody level during convalescence stage is quite normal
A previous study of 2009 Influenza Pandemic in Norway shown that IgG antibody levels dropped significantly between 3 and 32 weeks post-infection in the convalescent patients, while severely ill patients have higher antibody responses than patients with mild disease, (Mohn, 2015) which also correlate with Dr. Huang's study in Nature Medicine. (Quan-Xin Long, 2020) Maintaining a high level of antibodies may relate more to the disease status than the immunity against reinfection. The cytokine level in asymptomatic vs. symptomatic people in the same paper also suggests the infection status vs. antibody level. (Quan-Xin Long, 2020) Decline of antibody levels during the convalescence stage is quite normal.
2. Long-term immunity memory can quickly kick in during reinfection
After initial infection, the body generates antibodies of various isotypes that can be reactivated upon antigen re-exposure. Two major cell types are responsible for the long-term immunity memory: long-lived plasma cells (LLPC) and memory b cells (MBC). LLPCs reside in bone marrow and is also found in gut-associated lymphoid tissue (GALT) in recent studies. (Differences between Primary and Secondary Immune Response, 2018)
Long-lived, pathogen-specific antibodies keep titers in serum that lasts decades following infection or vaccination, continue to secrete antibodies without the presence of the antigen. (Shivana M. Lightman*, 2019) (Brandtzaeg P, 2005) Neutralizing antibodies, including IgG and IgA, resides in LLPC and are expressing continuously in a low basal level independent from antigen re-exposure. The vicinity of LLPCs niche also enables the close contact of neutralizing antibodies to pathogens, prohibiting the virus from amplification before MBC takes several days to elevate the total antibody level. (Charles A Janeway, 2001) (Barbara J. Hebeis, 2004) (Tomohiro Kurosaki, 2015)
Figure 1. Differences in the Primary and Secondary Immune Response (Differences between Primary and Secondary Immune Response, 2018)
Figure 2. Each human immuno-globulin isotype has specialized functions and a unique distribution (Charles A Janeway, 2001)
MBCs, on the other hand, can quickly be reactivated after re-exposure (Barbara J. Hebeis, 2004) (Tomohiro Kurosaki, 2015). It is not necessary to keep the antibody expression plasma cell at a high level for a long time after the patient is recovered and viruses are cleared. The decline of the antibodies does not necessarily means loss of immunity, while the immune system has memory to protect us from reinfection.
3. Low level of high-affinity neutralizing antibodies may be enough to ensure the effectiveness of protective immunity
Although some patients show undetectable antibody levels by serology kits (Quan-Xin Long, 2020), whether the detection threshold is the same as the protective immunity threshold has not been studied thoroughly, yet. Among all types of antibodies, neutralizing antibodies that only consist a very small portion of total virus-specific antibodies play an important role in viral protection as the first response (Chapter 6 - Immunity and Resistance to Viruses, 2017) (Laura A. VanBlargan, 2016). Such antibodies may be enough for protective threshold even if its level is far below the detection threshold.
Rogers T. et al isolated SARS-CoV-2 antibodies from a small animal model and found only a small portion of the virus specific antibodies are neutralizing antibodies while the majority are binding antibodies (Thomas F. Rogers, 2020). Binding antibodies are likely to trigger opsonization by activating the complement system, and neutralizing antibodies block the entrance of viruses and the following amplification in hosts.
Among all neutralizing antibodies, the ones that neutralize the RBD domain have the best potency, make up the least amount of antibodies present (0.2-0.4%) (Thomas F. Rogers, 2020). It is very likely that the most effective antibodies may not be detectable long after infection but still function well, especially in the early secondary immune response that blocks virus entrance to the human body, leaving enough time for MBCs to proliferate and produce more antibodies for opsonization.