Since the outbreak of COVID-19, scientists have strived to understand the intricacies of the human immune response to the novel coronavirus, SARS-CoV-2. Whilst there is now little doubt that humans can, thankfully, generate an effective immune response – involving production of viral-specific antibodies – recent studies suggest that the quality and duration of such an immune response remains in question.
When a pathogen, or disease-causing organism, invades our cells, many immunologists describe a situation in which our immune system goes to war, fighting the intruder in two stages of defence: the innate response and the adaptive response. Our innate immune system is first to the scene and takes on the role of damage control before presenting fragments of the pathogen, known as antigens, to cells of the body’s adaptive immune system in order to allow expansion of a highly-dedicated attack force in the form of viral-specific T cells – which originate in the thymus – and B cells – which originate in the bone marrow.
Antibodies (also known as immunoglobulins) are Y-shaped protein structures that are produced by B cells and can bind to antigen, or ‘danger flags’ on outer surface of an invading pathogen, preventing viral entry to the host cell by a process known as neutralisation. However, it’s important to note that they are several classes of antibody – IgM, IgA, IgG, IgE and IgD, each with distinct structures and function. When scientists talk about viral immunity, they are most often referring to how well the body produces IgG, however in order to convert IgM to IgG, B cells require additional signals, from other cells of our immune system. Therefore, the presence of only IgM is most likely to signify early stages of infection, as a process known as class switching has not yet taken place.
IgG is known to have a higher affinity then IgM, however this does not rule out the importance of IgM in our immune response against COVID-19. One study have found that the levels of IgG correlated with disease severity  however this may not account for long-term immunity, as one might predict. A study led by researchers at Kings College London, has shown IgG level may decline within 3 months of initial detection . However, the authors explain that some individuals displayed immunity, in the absence of IgG, highlighting that IgM and IgA may “facilitate neutralization in the absence of measurable IgG”.
Although there is little doubt humans can develop immunity to SARS-CoV-2, this research raises the question of how scientists can best to evaluate vaccine candidates and highlights the importance of ongoing PCR testing, alongside collecting blood samples for testing in hospital facilities.
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Click here to learn more about how scientists use PCR to diagnose COVID-19
Today, it was announced that the UK government is planning to make thousands of rapid antibody tests available to the public, after the leader of the UK Rapid Test Consortium (UK-RTC), Chris Hand explained the test was “98.6 per cent accurate” (The Guardian).
Talking to the New Scientist, Danny Altmann, a professor of Immunology as Imperial College London explains that “natural immunity to coronaviruses can be short-lived” however, many other leading scientists have suggested that “T cell memory of SARS-CoV-2 may last longer than antibodies” (such as Zania Stamataki, a leading viral immunologist at University of Birmingham, writing in The Guardian). Dr Al Edwards, from the University of Reading, explained that vaccine approaches using weakened versions of the virus may be less likely to provide sufficient immunity (The Telegraph), however researchers at Oxford University remain hopeful that their viral-based candidate, ChAdOx1 nCoV-19, will bring encouraging results – some of which are due to be released next week, according to Fergus Walsh, a medical correspondent at the BBC [see article].
Immunity to COVID-19 could be lost in months, UK study finds Ian Sample, The Guardian
Can we become immune to the coronavirus? What the evidence says so far Graham Lawton, New Scientist
There’s no guarantee a coronavirus vaccine will last a lifetime – antibodies are only part of the story Sarah Newwy and Paul Nuki, The Telegraph
Coronavirus: UK plans millions of antibody tests after trial success – report The Guardian
I’m a viral immunologist. Here’s what antibody tests for COVID-19 tell us Zania Stamataki, The Guardian
Coronavirus: encouraging results in vaccine trials Fergus Walsh, BBC News
Potent antibodies found in people recovered from COVID-19 Sharon Reynolds, NIH
- Liu, X. et al (2020) Patterns of IgG and IgM antibody response in COVID-19 patients Emerging Microbes & Infections, 9:1, 1269-1274, DOI: 10.1080/22221751.2020.1773324
- Seow, J. et al (2020) Longitudinal evaluation and decline of antibody responses in SARS-Cov-2 infection medRxiv DOI:10.1101/2020.07.09.20148429
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