Adam Goriparthi
Half a year after infection, people who have recovered from COVID-19 still have antibodies against the virus which may drive long-lasting immunity. Recent findings suggest that cases of reinfection may be infrequent due to certain types of immune cells continuing to protect people against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) for at least six months. So, how does immunity work? And how long does it last against COVID-19? Adam Goriparthi investigates.
How does immunity work?
The immune system is the body’s natural defence to illnesses. When the body detects foreign substances (called antigens), the immune system works to recognise specific antigens and remove them.
SARS-CoV-2 antibodies are still detectable after six months of initial infection
We have two types of immunity:
- Innate (natural) immunity:‘general’ protection that we are all born with
- Adaptive (active) immunity:‘specific’ immunity that develops throughout life (developed through exposure to illnesses or vaccines).
A key component of the adaptive immune system are lymphocytes; B lymphocytes are triggered to make antibodies – proteins that lock onto specific antigens. Once generated, B memory cells and antibodies usually remain in the body in case the same pathogen enters again. Significantly, some COVID-19 patients have been unable to generate antibodies. A lack of germinal centres, where memory B cell production occurs, has been linked to severe COVID-19 and death.
Although antibodies can recognize antigens and neutralize them, they cannot destroy them without further aid of T lymphocytes; these cells destroy antigens tagged by antibodies. T cells also help signal other immune cells to do their jobs.
How long does immunity last?
Genetic variances may also occur in the virus and could explain the rare reoccurrences of COVID-19 infection
Seemingly, immunity lasts 5-7 months. SARS-CoV-2 antibodies are still detectable after six months of initial infection (albeit in diminishing levels). A study reported that those antibodies that remained half a year after infection typically bound to and neutralised the virus more effectively than those generated after only a month.
Furthermore, levels of the memory B cell that manufacture these antibodies do not dwindle with time. The findings suggest that individuals were generating long-lasting memory B cells and re-exposure to the virus would likely trigger a quick and effective immune response against it. This offers some hope for long-term immunity and also suggests benefits for vaccine administration.
Interestingly, the memory B cells’ genetic sequences seemingly alter over time, suggesting the B cells and antibodies evolve in response to infection. This occurs through viral persistence, which is when a pathogen (like COVID-19) hides in the body or in cells’ DNA, even after cessation of symptoms. There have been reports of patients’ gastrointestinal tracts containing traces of SARS-CoV-2 half a year after initial infection and could be how immunity is maintained.
Genetic variances may also occur in the virus and could explain the rare reoccurrences of COVID-19 infection. Analysis of SARS-CoV-2 showed significant differences between each genetic variant associated with individual cases of infection. This observation implies that the patient was infected by SARS-CoV-2 twice, each time by a genetically distinct virus.
Compared with asymptomatic patients, the T cell responses of those with symptoms were 50% stronger
Thus, exposure to SARS-CoV-2 may not promise total immunity in future cases. Therefore, even those previously diagnosed with the virus should still take precautions to avoid infection. Despite being rare, the implications of reinfections are very relevant for the development and application of future vaccines.
Transmission electron microscope image of SARS-CoV-2. The spikes seen on the edge of the cells are part of the virus’ antigens. (Colourised)
Another study in early 2020 involved 100 individuals who had recently recovered from COVID-19, having had only mild or no symptoms. Researchers used blood samples to track changes in antibody levels targeted at different viral proteins. Half a year later, all the participants had (helper) T cells remaining in their blood. A particularly significant response of these cells is the production of IL-2, an immune-signaling molecule that helps combat viral infections.
Compared with asymptomatic patients, the T cell responses of those with symptoms were 50% stronger. It is suggested that greater cellular immunity could reduce the likelihood of reacquiring the virus in those with symptoms.
The exact duration of COVID-19 immunity beyond 6 months is not fully understood
The responses of T cell correlated with levels of respective antibodies to the viral proteins during the six months. This suggests that both antibody-producing B cells and T cells were collaborating in the immune response.
Arguably the study was limited – a small sample that only focused on low-risk people. Nevertheless, there is strong reasoning that getting the virus can induce immunity – especially in young people and asymptomatic cases.
But how long will immunity last? Immunity to other coronaviruses wanes after a year in most cases. The exact duration of COVID-19 immunity beyond 6 months is not fully understood given the understandable lack of long-term studies.
Patients infected with the SARS virus in 2003 still present memory B cells for that strain; therefore, a similar result could be seen in SARS-CoV-2 as immunological memory tends to be long lasting.
Vaccine optimism
Knowledge of the processes underlying the immune response can allow us to better research vaccines
Given the findings, it makes sense that vaccines against COVID could provide long-lasting immunity. Vaccines echo the immune response: the introduction of a weakened pathogen triggers the immune response so that antibodies can be synthesised and utilised in future re-encounters – causing a less severe infection.
Now that the Pfizer/BioNTech vaccine is slowly being rolled out, measuring the memory B cell count and patient immunity could be useful in the assessment of the vaccine’s effectiveness.
There is still much to learn before we can fully understand how immunity to COVID-19 works. While immunity is reported for at least half a year in most cases (and vaccines are currently being rolled out globally), the virus is still being investigated. Despite rare cases of reinfection, initial infection to the virus facilitates future protection.
Our knowledge of the processes underlying the immune response can allow us to better research vaccines and understand the precautions that should be taken by those previously infected.
Adam Goriparthi
Featured image by NIAID from Flickr. Image licence found here. No changes made to this image.
In article image by Prachatai from Flickr. Image license found here. No changes made to this image.
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