How long does immunity to covid last?

One of the fears of many people in relation to covid has been that the immunity that develops after infection is so short lived that the infection will just keep going around and around and re-infecting everyone (until everyone is dead, I assume).

Two pieces of evidence have been presented to support this belief. The first concerns a few cases of “re-infection” that have been broadcast widely in media, even though virtually all of these cases have been either completely asymptomatic or only very mildly symptomatic the second time around – a sure sign the the immune system still remembers covid and is doing its thing to stop it.

The second concerns the fact that antibodies fade after infection. This builds on a fundamental lack of understanding of how the immune system works. Although the actively antibody producing cells diminish after an infection, these cells (so called “plasma cells”) are not responsible for immune memory. That role is filled by special “memory B-cells”, that lie dormant in the body, waiting for the infection to reappear. When it does, they quickly spring in to action and produce massive numbers of new antibody producing clones.

Now, however, covid has been around for a while, and we’re starting to get some pretty good data on how long immunity lasts after infection. There is a pre-print up on MedRxiv about a study that sought to gain a deeper understanding of what sort of immune memory is produced after a covid infection.

Before we get in to the details of the article, let’s talk a little bit about immune memory, so everyone is on the same page. Immune memory is the ability of the immune system to remember a pathogen after a first infection (or vaccination), and thereby respond much more quickly and effectively upon re-infection. It is mediated by three main types of cell. The first is the already mentioned memory B-cell, which is basically a dormant version of the antibody producing plasma cells. The second is the “memory killer T-cell”, which is a dormant version of the regular killer T cell (a.k.a CD8+ T-cell). Killer T-cells specialize in finding virus infected cells and getting them to commit suicide in a way that prevents the virus from spreading further.

The third is the “memory helper T-cell”, which among many other functions regulates the function of the other types of immune cell. Both killer T-cells and B-cells cannot become fully activated until helper T-cells have become activated. The central function of T-helper cells is shown by AIDS (Aquired Immune Deficiency Syndrome), a disease caused by the destruction of the T-helper cells by the Human Immunodeficiency Virus (HIV) – without the T-helper cells, other parts of the immune system cannot become fully activated, and the immune system is not able to function effectively.

In case you’re curious, the reason B-cells are called B-cells is because they mature in the bone marrow, so the B is for Bone marrow. T-cells mature in the thymus, so the T is for Thymus.

Ok, now you know enough to understand the results of the study. 185 people with confirmed covid-19 were recruited and had blood samples drawn. 92% had not required hospitalization, so only a minority had had severe disease. The ages of the participants varied from 19 to 81. The blood samples were collected from several different sites across the United States.

The results of the study were based on analysis of the participants blood. 79% of participants only provided blood at a single time point, which varied from six days post-infection to more than six months post-infection, while the remainder (21%) provided blood at multiple time points. In other words, this was not really a longitudinal study, since most participants only had their blood analyzed at a single point in time, although there was some longitudinal data. 41 participants provided blood samples at six months or longer after infection, and this is really the group we’re most interested in, since this is the group that can tell us if there is still a good level of immune memory six months after infection.

Let’s look at the results.

Among the 54 individuals measured at one month post infection, 98% had antibodies. Among the 41 individuals measured at six to eight months post infection, 90% had antibodies. As mentioned before, antibodies are produced by plasma cells, and although antibodies in the blood stream decline with time as the plasma cells start to disappear, there should still be memory B-cells present for much longer, which can quickly be activated upon re-infection. That’s why it’s actually more important to look at what’s happening with memory B-cells than with antibodies, if you want to know how long your body maintains the ability to mount an antibody response to an infection. So, what did happen with the memory B-cells?

The prevalence of memory B-cells increased at each time point measured up to five months post infection, at which point they reached a stable level. There was no sign of a decline in memory B-cells after the five month mark.

Next we have the killer T-cells. At one month post infection, 61% had detectable memory killer T-cells. At six to eight months, 50% had measurable killer T-cells. it was however only possible to test for these cells in 18 individuals at the six month mark, so the confidence interval is wide, and thus it’s really impossible to say exactly what the trajectory was between the one month and six month marks. What can be said though is that a large proportion of participants still had measurable killer T-cells at six months.

Finally we have the memory helper T-cells. 94% of those measured at one month had measurable helper T-cells. Among those measured at six to eight months, that number was 89% (again, this data is based on only 18 individuals).

So, what can we conclude?

First, it’s important to note that this study had some weaknesses. The first is that, with the exception of a minority of participants, the study was cross-sectional, not longitudinal. This means that we’re not comparing people with themselves over time, we’re comparing them with other people who happen to be at a different point in the time line. It would have been better to have longitudinal data for all participants. The second is that some of the groups studied were pretty small, which creates wide margins of error. Some of the data was based on less than twenty individuals, which is really a tiny number.

A third weakness is that this study isn’t looking at how many people get reinfected with covid after a certain amount of time, it is looking at biomarkers – in other words, it is using proxy data, which is clearly a less reliable type of information than seeing what is actually happening to people in the real world. It’s kind of like doing a statin study and looking at what happens to cholesterol levels instead of looking at how many people have died after certain time point.

Having said all that, it is clear from this study that there is significant immune memory at the six to eight month time point after infection. At six to eight months after infection, 90% of measured samples still had antibodies and T-helper cells specific for covid-19, and 50% still had measurable T-killer cells. If the decline continues linearly over time from what was seen in this study, then it is reasonable to assume that most people continue to be immune to covid after infection for at least a couple of years.

You might also be interested in my article about the number of years of life lost, on average, when someone dies of covid, or my article about whether face masks are effective against covid.

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