Have you already been infected with the novel coronavirus? If you have, does this give you an ‘immunity passport’, meaning that you can start performing ‘normal’ daily tasks such as going to the grocery store or dining in restaurants without fearing for your health? In this week’s post we are going to discuss what is currently known concerning immunity to COVID-19.
How does immunity work?
First of all, it’s important to remember that immunity isn’t a binary thing (i.e. full immunity or no immunity at all), a person’s level of immunity can be thought of as a spectrum that changes with time. Usually, when people become infected with a virus, the body has two response systems. The first is rapid non-specific, innate immune response, during which the body employs cellular tools such as macrophages to delay the spread of the virus within the body.
Then there's the adaptive immune response which involves the generation of special proteins called antibodies that bind to the virus and it becomes incapable of infecting more cells (www.who.int). You may have heard about the antibody COVID-19 test, that tells you if you’ve already had the virus or not. The whole premise of this test is based on detecting these proteins generated by the body during an adaptive immune response.
Because antibodies usually don’t always stick around very long after our body clears itself of a pathogen, we have special cells called B cells whose job is to regenerate antibodies if the body comes in contact with the same virus again. Another result of the adaptive immune response is that the body produces special cells called T cells, which look for already infected cells and eliminate them.
These two processes (non-specific innate response and adaptive immune response) allow the body to completely rid itself of a virus, and sometimes these responses also prevent the body from getting re-infected with the same virus. Your level of immunity depends largely on your body’s response to re-exposure to the same virus (the two steps we discuss above) - in part on the B cells you generated the last time you were infected.
What do we know about the immune system and SARS-CoV-2?
Our knowledge concerning the innate immune response to the new SARS virus - like many other things about this virus - is limited. However, researchers think that because of the similarities between SARS-CoV-2 and other SARS viruses, our body's initial recognition of this virus may also be similar (reviewed in Vabret et al. 2020). Some studies on closely related SARS viruses have shown that early steps in the innate immune response can prevent a full blown infection, so knowing how the innate immune system responds to SARS-CoV-2 is really important.
After the innate immune response, the adaptive immune response begins. Today we want to focus on a few specific elements of the adaptive immune response, one of which you’ve likely heard about in the news - antibodies.
A recent non-peer reviewed study claims that immunity to COVID-19 may weaken overtime. The authors found that most people who were infected with this virus did generate antibodies and that people with more severe infections made more antibodies. However, when researchers collected samples from the same patients over the course of three months the number of antibodies in their blood had dropped significantly (Seow et al. 2020). This means that if antibodies are our main line of defense against this virus, we could see people get reinfected seasonally. These results are also important for the development of a vaccine because if the vaccine relies on antibody generation to confer immunity then people may need to be re-vaccinated often. Also, this type of immune response could mean that the herd immunity isn't a possible option to end the pandemic. Herd immunity is the idea that if enough people become immune to infection they will provide others (who haven’t been infected) with indirect protection (www.jhshp.com). But if the herd doesn’t/can’t maintain immunity post-infection then this type of protection isn’t possible. It is important to remember that these results have not been reviewed by the scientific community and need to be thoroughly discussed before being officially published.
It’s also important to remember that that group of researchers were only measuring one of the many complicated immune responses. However, as we said previously other responses (T cells and B cells) from the adaptive immune system play a role in conferring long-term immunity to a pathogen. Researchers have found that most people who are infected with SARS-CoV-2 do generate B cells (reviewed in Vabret et al. 2020). However, how many and what kind of B and T cells people make varies a lot between individuals. A recent study that analyzed samples from 125 COVID-19-positive individuals found that 80% of patients made a significant amount of B and T cells (Mathew et al. 2020). However the sub-types of these cells that people generated were extremely different depending on the person. Which means that there are likely other biological factors we need to investigate before fully understanding immunity to COVID-19.
It’s also not clear how good B cells are at remembering the SARS-CoV-2 virus. Several studies have found evidence of long term immunity to this virus, but these results are based on small sample sizes or using primates. The long term memory of B cells means long term immunity, while short term memory of these cells means...we have a lot of work to do before we’ll be jamming out at our favorite summer music festivals.
Take-home message
Ultimately, all of these studies are only looking at small pieces of a very large and complicated immunological puzzle. It is going to take time for scientists to figure out how/if re-infection is possible. Until more data has been collected it is probably a good idea to act like immunity passports don’t exist - even if you think/know you’ve already been infected with COVID-19.
Glossary
innate immune response - the first non-specific response of the immune system to a pathogen
pathogen - a bacterium, virus, or other microorganism that can cause disease
macrophage - a special type of cell made by the immune system that identifies and engulfs damaged or infected cells
adaptive immune response - the second specific response of the immune system which often results in the generation of antibodies, B cells, and T cells
antibodies - a special protein in the blood that is generated to identify and stop pathogens that the body has previously encountered
B cell - a special cell that remembers previous pathogens and when the body comes into contact with a pathogen again these cells generate antibodies to prevent it from doing further harm
T cell - a special cell that attacks other cells that are infected with a pathogen
Works Cited
https://www.who.int/news-room/commentaries/detail/immunity-passports-in-the-context-of-covid-19
Vabret, N et al. Immunology of COVID-19: Current State of the Science. Immunity 52 (2020).
Seow, J et al. medRxiv prepint. July 2020.
https://www.jhsph.edu/covid-19/articles/achieving-herd-immunity-with-covid19.html
Mathew, D et al. Deep immune profiling of COVID-19 patients reveals distinct immunotypes with therapeutic implications. Science (2020).
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