News
Immunity, natural infection & vaccines
As Sri Lanka continues to see rising COVID-19 numbers, Consultant Clinical Microbiologist Dr. Mahen Kothalawala attached to the National Hospital of SriLanka (NHSL) focuses on two crucial aspects.
Immunity and what lies ahead in the face of the inexorable march of COVID-19 are what interests him.
The ‘immune response’ against SARS-CoV-2
(the new coronavirus)
A human being’s immune system is organised to protect his/her body’s systems from invading pathogens (a virus like the new coronavirus, a bacterium or any other micro-organism which causes disease). For this all-important function, the immune system demonstrates a multi-layered organisation. While each layer has distinct functions, all layers work together to ‘eliminate’ or ‘contain’ the pathogen.
It is when a pathogen escapes all layers of the immune system that it is able to cause illness in the person.
The multi-layers of the immune system are:
Protective surfaces and the intrinsic system – this prevents the entry of organisms through ‘the portal of entry’.
The adaptive system – this is involved in mounting a more ‘refined’ and ‘targeted’ response, once the organism has breached (broken through) the protective surfaces.
All these layers of the immune system work in tandem to keep the host’s (person’s) systems safe.
Immune response to ‘Natural COVID-19 infection’
Immunologically ‘naïve’ individuals and ‘primed’ individuals respond differently to natural infection.
An immunologically-naïve (not exposed earlier) person will have a high likelihood of ending up with severe COVID-19. Most hospitalisations and deaths have been reported in this group.
An immunologically-primed (exposed earlier) person will have a low probability of developing complications and death following infection. Though the disease severity can be reduced, primed immunity may not be sufficient to prevent the person from getting the ‘infection’.
The primed immune response to SARS-CoV-2 takes two forms:
Antibody response – this is characterised by the production of antibodies against many viral antigenic epitopes. ‘Neutralising’ antibodies directed against spike protein form the ‘backbone’ of the antibody response. These neutralising antibodies formed following natural infection (or vaccination) will ‘contract’ over time and in a few months after the infection, may not be detectable at all. The contraction of the antibody response overtime is a ‘natural’ phenomenon.
Though neutralisation antibodies contract overtime, the ‘immune memory’ generated by the primed response will remain for a long time. With subsequent exposures to the virus, the memory cells kick in to produce a rapid immune response and protect the host (person) from severe manifestations of COVID-19.
Cell-mediated immunity (CMI) – this is characterised by the production of specific cellular elements (CD4 cells and CD8 cells). CMI too will have a memory and responds the same way to subsequent exposures. T-cells, B-cells and T & B memory response are sufficient to prevent ‘complications’ of COVID-19. The antibody response mediated by memory B & T cells occurs with a slight delay but once the immunity is established, it is sufficient to stall progression to severe COVID-19.
The response mediated by T cells, B cells and immune memory fails to prevent ‘minor symptoms and infection’. Once primed, the cell-mediated immunity and immune memory last for a long time.
CMI generated against SARS-CoV-1 is detectable in those who have recovered from SARS (Severe Acute Respiratory Syndrome), even after 17 years. However, CMI and memory response are difficult to measure, but remain intact for months to years.
Immunity induced by
vaccines
Vaccines, mimic an‘infection-like’ situation to prime the immune system. The immunity induced by the 1st dose is known as the ‘primary response’ and the immunity achieved by the 2nd dose as the ‘secondary response’.
A vaccine primed system can mount a robust immune response to a subsequent ‘exposure’ to the virus. But, it may fail to protect the host (person) from acquiring the infection.
Vaccines – ‘Primary Series’, ‘Extended Primary Series’
and ‘Boosters’
Primary Series – this is the number of vaccine doses required for the vaccine to reach its full effectiveness. At the end of the primary vaccine series, a recipient usually becomes adequately protected.
Extended Primary Series – if any doubts exist on immunity following the primary series, one or more additional dose/s can be given. These additional vaccine doses are known as extended primary series. Additional doses of vaccines are needed for patient groups with compromised immunity.
Boosters – these are required months or years after the primary series to refresh the immune system’s memory and maintain protection.
COVID-19 vaccines included in the primary series are sufficient to generate immunity against ‘severe disease and complications’.
The Primary Series of different vaccines:
AstraZeneca (COVISHIELD) – primary series has two doses given 4 weeks apart
Sinopharm (BBIBP-CoV) – the primary series has two doses given 4 weeks apart.
Comirnaty (Pfizer) – the primary series consists of two doses given 3 to 4 weeks apart.
| Future of the pandemic There are four ‘possible’ scenarios, according to Dr. Mahen Kothalawala. They are: Variants will continue to fuel the outbreak – this is an unlikely scenario. No Variants of High Consequence have been encountered yet. Whatever the variant that emerges, the immune system of the people has managed to protect them so far, by mounting an appropriate immune response as protection. Virus will become seasonal and will show a ‘seasonal pattern of transmission’ – the chances of it becoming a seasonal virus like the flu are possible. In such a scenario, we will see surges and a few deaths in each season. Virus will become attenuated and be similar to the common cold coronaviruses (acquiring an endemic pattern of transmission). The virus will disappear suddenly from the world – SARS (2004) and MERS (2012) outbreaks suddenly disappeared from circulation. Both entities were caused by coronaviruses. Will SARS-CoV-2 too also disappear? The likelihood of a sudden disappearance of this virus is low. Dr. Kothalawala’sexpectation is: “The SARS-CoV-2 will become attenuated and behave like other circulating human coronaviruses. The time period for this to happen would depend on community immunity. Therefore, till the outbreak ends, we and the virus may have to co-habit. As long as vaccine distribution inequities exist, the chances of new variants emerging would be high.”
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