It has been over 6 months since the first threat covid-19 hit international news, with many now believing it arrived in late 2019, and after 4 months of lockdown it is time to address the elephant in the room: immunity. What is it, and what is current research telling us about coronavirus immunity?
At the end of May (2020), I wrote an article about how our immune systems work with the promise of explaining immunity once more research had been conducted for covid-19. First, what is immunity? Generally speaking, our immune systems get stronger as we age because we have been exposed to more pathogens and developed more immunity. There are three types of immunity: innate, adaptive (acquired) and passive. From day one on Earth, our bodies are fighting infection. Our innate immune response is more general and non-specific, a first line of defence encompassing our skin and mucus membranes of the throat and gut for example. If disease causing pathogens get passed our innate defence, adaptive/acquired immunity takes over. As we are exposed to disease or get vaccinations, we build up a library of the antibodies to the different pathogens (to learn more about antibodies, see the Understanding Our Immune System article). This type of immunity is acquired over time, whilst the final type is passive immunity where antibodies are “borrowed” from somewhere else (i.e. maternal or artificial), but do not protect us indefinitely. Here, we do not have to have been exposed to the pathogen to have antibodies which can combat the disease immediately, but they may only last in our systems for a few months at a time. For example, in plasma therapy, or antibodies which are acquired through the placenta or a mother’s breast milk. Most importantly, we do not keep a memory of the antibodies we acquire through passive immunity.
Generally, vaccinations are created using live attenuated (weakened) viruses, or with inactive viruses. However, in a handful of instances, scientists have used antibodies to create a vaccination utilising passive immunity. Examples include hepatitis and respiratory syncytial virus. In an ideal world we would have a vaccination for covid-19 which uses adaptive immunity to ensure we can always fight the virus. Numerous vaccinations need “boosters” through our lives (such as the tetanus vaccine), whilst others need to be administered yearly (i.e. the flu jab) as the pathogen is constantly changing and the vaccine needs to cater for that change year on year. Covid-19 is a coronavirus which is likely to mutate over time and thus require vaccine alterations to match. With over 12 million confirmed covid-19 cases worldwide, approaching 550,000 deaths, scientists are racing for immunity answers.
Even if researchers are able to create a successful vaccine which causes an immune response and gives some immunity, we still need to know how long that immunity lasts, how strong the immune response is and whether it can protect us from future contact with the virus. These are the million-dollar questions. We’ve learnt plenty about how the virus enters the body and causes symptoms (with new ones appearing almost weekly), and there are almost 200 vaccines in development, but we still need answers about immunity. This is the job of immunologists around the globe. Other coronaviruses in human circulation are linked to common colds, causing an immune response to clear the infection but immune memory does not last long (a few months). However, the coronaviruses which caused MERS and SARS outbreaks, lead to a much more significant immune responses which were remembered by sufferers’ immune systems from just a few years to 12! How will covid-19 fair?
Researchers first port of call is to measure covid-19 antibody levels in those who have had a positive test. Research has shown that 90% of people who suffered severe symptoms had high levels of antibodies in their blood after 2 months, whereas just 40% of asymptomatic cases have “normal” covid-19 antibody levels. Antibodies are not the only factor to monitor though, T cells also play an important role in immune response and those involved in past infections may be helping individuals suffering from covid-19. However, we don’t yet know the best level of “neutralising antibodies” that are needed, and have to instead look at past immune responses to other, similar infections. Such studies could consider two potential immunity methods: sterilising immunity (prevents infection but only lasts a few months), or protective immunity (prevents and eases symptoms, lasts much longer, but does include infecting the individual to produce an immune response (i.e. most vaccines)).
The only answer we do have is that every step forward leads to more questions, but that is the nature of science. We will need survivors of covid-19 to be tested at least a year after initial infection, to start to form a comprehensive picture of what immunity to this disease may look like. In the case of immunity, time will tell.