Immunity to coronavirus Covid 19: how long does it last?

Millions of people around the world have been infected with SARS-CoV-2, the virus that causes Covid-19. Countries are also now embarking on massive vaccination campaigns to control the virus and protect their most vulnerable citizens. One of the most important questions that remain to be resolved is whether vaccination and / or previous infection with SARS-CoV-2 offers lasting protection against this deadly virus. The good news is that immunology is finally shedding some clues.

To understand whether immunity is possible, and why it has even been called into question, it is important to consider the nature of SARS-CoV-2. It is a betacoronavirus, and several betacoronaviruses already circulate widely in humans; they are more familiar to us as a cause of the common cold. However, immunity to cold-causing viruses is not long-lasting, leading many researchers to question whether longer-term immunity to SARS-CoV-2 is possible.

However, studies looking at the closely related betacoronaviruses that cause Mers and Sars diseases offer a glimmer of hope. With these viruses, immunity has proven to be more durable. Could this also be true for immunity to SARS-CoV-2?

Well-trained protection

The first immune cells in the body to respond to an infection are designed to attack invading substances to try to control the spread of the infection and limit the damage caused. The immune cells that respond later and are responsible for immunity are known as lymphocytes, which include B cells and T cells. Lymphocytes take time to learn to identify the threat they face, but once trained they can be deployed quickly to find and destroy the virus.

Our T cells and B cells work together to fight infection, but they have quite different roles that allow them to deal with a wide variety of threats. B cells produce antibodies that neutralize infections. T cells are broadly divided into two types: helper T cells and cytotoxic T cells. Cytotoxic T cells directly kill viruses and cells that viruses have infected. Helper T cells support the functioning of B cells and cytotoxic T cells. Together, these are known as “effector” cells.

Studies have now shown the critical role these effector cells play in fighting Covid-19. Once the infection clears, these cells must die to avoid causing excessive damage to the body.

But some effector cells persist. In preliminary research that has not yet been reviewed by other scientists, functional T cells were detected six months after infection. Similarly, even patients who have had mild Covid-19 have detectable antibodies six to nine months after infection. However, antibodies decrease over time, so these SARS-CoV-2 antibodies could eventually disappear.

Remembering the danger

These findings raise real optimism about protection against reinfection. But what happens if the levels of effector lymphocytes eventually drop? Well, our immune systems have another trick up its sleeve to protect us in the long run, even after effector cells and people’s antibody levels have dropped. Once lymphocytes have been trained to deal with a virus, a group of cells remembers it and is saved for the future. These “memory” cells can be deployed quickly if the threat is found again.

Memory cells are incredibly powerful tools for our immune systems and can live very long lives, with studies showing that memory B cells for smallpox persist for at least 60 years after vaccination and for Spanish flu at least 90 years after the 1918 pandemic. To understand whether long-term immunity to SARS-CoV-2 is possible, it is critical to consider not just effector cells, but all types of memory cells: memory B cells, T helper cells and T cytotoxic.

Fortunately, memory cells can be identified by specific structures and proteins that they express on their surfaces, allowing researchers to distinguish them from effector cells. Now that Covid-19 has been with us for a year, researchers can make great strides in understanding memory responses to Covid-19. Evidence is emerging for memory T-cell responses that last six to nine months after infection, and a recent pre-print study (not yet reviewed by other scientists) has also identified what appear to be responses memory B cells.

Studies have also been investigating whether previous exposure to the virus confers protection, with research showing that in the UK’s second wave, previously infected healthcare workers were fully protected from reinfection or were asymptomatic if they contracted the virus again. These observational studies give real hope for the durability and potential of protective immunity.

We still have a lot to learn about Covid-19 immunology, but the pace of research is staggering, and the more we learn, the more power we have to defeat this virus. Our immune system is incredibly powerful, and these studies showing persistent immune responses nine months after infection are a real cause for celebration. They give us confidence that, with vaccination, we have a real chance to win the war against Covid-19.

• Sheena Cruickshank, Professor of Biomedical Sciences, Manchester University

– This article is republished from The Conversation under a Creative Commons license. Read the original article.