How long does immunity last after having COVID-19?

This blog was co-authored by Allison Jackson & Fiona Henriquez

The coronavirus pandemic has affected the lives of everyone in 2020. People are, quite rightly, wanting to know if they catch the virus, and are they then protected from further infections? Recently, a team of researchers at Imperial College London and their collaborators published a pre-print showing a survey they conducted looking at the prevalence of antibodies against the virus that causes COVID-19, the novel coronavirus SARS-CoV2, decreases over time. In many media outlets, including the Guardian, the Herald Scotland and the BBC Science Focus Magazine the headlines screamed that immunity to coronavirus could be lost in months. But is this really true?

What did the study do?

The Real-time Assessment of Community Transmission (REACT) Study surveyed 365,000 adults in England using a home testing kit to see whether they had antibodies to SARS-CoV2. They sampled three random groups of adults in England using a self-administered home antibody testing kit. Each sample was a different group of adults that had volunteered for the study.

What is an antibody?

An antibody (also known as immunoglobulin –Ig) is a molecule produced by our immune system in response to infection. Antibodies are created to be highly specific to a part of an invading microbe, such as SARS-CoV2. There are five different classes of antibodies in the human body and they are produced for distinct purposes and in specific locations. When you get infected by a microbe for the first time, for example with the novel coronavirus, the first antibody that is produced is IgM. It is highly efficient during new and early infections. Upon secondary exposure to the same microbe, the body’s immune system has acquired a memory from the first exposure and now the most abundant antibody that is produced is IgG. IgG will be present at low levels after an initial infection. SARS-CoV2 predominantly affects the respiratory system and as a result IgA is also produced. Like any other molecule secreted from cells, antibodies degrade after a period of time. IgG has the longest half-life of 20-24 days.

What were the results?

The first survey of 105,000 individuals in June 2020 found that 6% of the adult population had detectable IgG antibodies. The third and final survey, conducted in September 2020, showed the levels had decreased to 4.4%.

What did the researchers conclude?

The researchers conclude the prevalence of antibodies decreased between rounds 1 and 3 – but not in healthcare workers, presumably due to repeated exposure. They suggest this is consistent with evidence that immunity to seasonal coronavirus declines after infection, but that the relevance of this decrease has not been determined for SARS-CoV2. They also suggest their first sample may have underestimated the total of those infected in the first UK wave. The limitations of their study are described – namely the possibility that people exposed to the virus were less likely to take part over time, which may explain the decrease in antibody prevalence.

What does this mean for people’s immunity to the virus that causes COVID-19?

Viruses must replicate inside a host cell. They hijack host cell machinery so that the virus can replicate, produce its own components and be packaged up into about a thousand (in the case of SARS-CoV2) new virus particles in order to be released from the host cell before going on to infect other cells. Damaged host cells release signals that flag to the immune system that a viral infection is present and needs to be eliminated. Antibodies are only part of this process.

The main cells that are effective in eliminating viral infections are actually T cells, which develop in the thymus, hence the “T”. A subset of T cells, cytotoxic T cells, target infected host cells to kill them. Cytotoxic T cells are activated through a cascade of events that is initiated by ingestion of the virus by a dendritic cell. The dendritic cell proceeds to present the virus through specialised cellular pathways to a T helper cell, that in turn activates the cytotoxic T cell. T helper cells also provide a signal to make more of the antibody-producing cells. All these check points are necessary and highly regulated because, if not, there is a surge of immune over-activity that can lead to the so-called ‘cytokine storm’. There is evidence that suggests that a subgroup of COVID-19 patients do experience the ‘cytokine storm’ and, it is in these cases that the anti-inflammatory drug dexamethasone works well to reduce the over-active immune response.

Once the infection is resolved, the immune system returns to a resting state with only specific antibodies (for a brief period) and memory cells that recognise the virus, in circulation. Memory cells are those cells that vaccination seeks to create, so that the next time a person becomes infected, the immune system responds efficiently and rapidly to the virus infection. At this point, antibodies are generated swiftly and act to block entry of the virus to the host cell (Figure 1) or surround the virus so it is easily identified by the other cells of the immune system.

What did the media say?

Given that we have only known about SARS-CoV for less than a year, and the huge disruption it is having on our lives, one of the main questions researchers are searching for answer to is ‘will someone who has been infected, be protected from further infections?’. This study is in important step on the way to answering this question, however it is only a small part of the answer. The media headlines however have misrepresented the story and said that immunity wanes in months, without explaining that they are really talking about antibodies, which is not the same thing. This is frustrating because if you read the University’s press release, it is clear the researchers are talking about antibodies decreasing and not immunity. The sensationalist headlines will mean that the many people who have already been infected will undoubtably be concerned about what this really means for them.

Almost all the articles we read made claims that the researchers themselves did not make – particularly in their headlines or the first paragraphs. The Guardian suggested ‘the virus could reinfect people year after year, like common colds’. The Herald Scotland said ‘immunity to coronavirus in recovered patients may only last a few months’. The BBC was more accurate, saying ‘antibodies fall rapidly after infection’ but the Metro said ‘immunity to coronavirus may only last a few months’.

Points to remember

Immunity is a complicated process. The researchers do not discuss other mechanisms of immunity, namely cytotoxic T cells in their pre-print. They also only detected IgG antibodies, and as you can see, there are multiple types, although IgG is the dominant type. You would expect that antibody levels decrease over time after infection, but they would increase again if the person encounters the virus for a second or third time.

One thing to note is that these results of this study are, for now, a pre-print, which is a full draft research paper that is shared publicly before it has been peer-reviewed. Pre-prints allow rapid data and information sharing, which is particularly important in helping researchers to understand how SARS-CoV spreads, causes disease, and how the body responds to infection. However, this means that the results have not been submitted for peer-review and have not been scrutinised to ensure the research has been conducted to the highest quality standards.

The bottom line

Currently we don’t know how antibody levels relate to protection from re-infection of this novel coronavirus. Until researchers unpick all the complex components of the immune system and their roles, everyone should play it safe and follow the government guidelines on social distancing, hand washing and wearing face coverings, and getting a test if you have symptoms.