Read on Twitter
Scientific studies have shown that it’s possible to get reinfected with COVID-19, but also that immunity could last for years. How can that be and what does it mean for the hope we’re putting in vaccines? The answer is that it’s complicated…but potentially promising. A thread.
Let’s start with reinfection. It may sound simple to diagnose, but it’s actually kind of tricky because the virus from an infection may be able to stick around in your body in a sort of hibernation (called “latent infection”), and then become active again months later.
Thus, there is a high bar for proving that someone has actually been infected twice. How is that done? To answer that, we need to take a quick look at the structure of the virus. It has an outer shell made up of different proteins and that surrounds a molecule called RNA.
The RNA molecule carries all of the instructions needed to make the proteins, and just like a message can get changed as it is passed from person to person in a game of “telephone” the RNA keeps changing, or “mutating” over time as the virus is copied and passed along.
The rate of this change is still not fully understood, but to prove that someone has been infected two separate times, researchers need to sequence the RNA from both infections and show that there are significant differences.
There are relatively few reported cases where this has happened, but that may be because the samples aren’t always saved and sequenced. And, we don’t know how many people are reinfected but asymptomatic, and thus are never diagnosed the second time.
So, does the chance of reinfection mean that our hope in vaccines is misplaced? Thankfully not. This is where the most recent study offers exciting insights and some hope that vaccines could provide relatively long-term immunity.
This is also where it gets complicated. Our immune systems don’t fight every infection in the same way. Rather, they have a whole arsenal of tools, including different types of T cells and B cells, and multiple types of antibodies.
Researchers are still figuring out which of these weapons our bodies use to fight off COVID-19 infections, but initial data suggest that T cells are most important for dealing with initial infection whereas the B cells and antibodies offer protection from reinfection.
The big question is how long does this protection – called “immune memory” – last? An initial report in August showed that key “memory” B cells and T cells seemed to stick around or even increase over 3 months. https://www.medrxiv.org/content/10.1101/2020.08.11.20171843v2
But, 3 months is a short time, and that study had relatively few patients. In comparison, the data released last week include more patients, more immune system components, and timelines up to 6 months post-infection. https://www.biorxiv.org/content/10.1101/2020.11.15.383323v1
Potentially the most promising data from the study show that while antibody levels do seem to decrease over time, at 6 months post-infection, ~90% of people still have a type of antibody called IgG, which is thought to be key for immunity.
Also very encouraging are the data on memory B cells – these are the cells that essentially hold onto a piece of the virus and remember how to make antibodies in case they’re needed in the future.
The most recent data show that memory B cells increase post-infection and seem to reach a steady level after a few months. It’s not clear whether this trend continues or at what point they will decrease again, but their persistence over months is encouraging.
It’s important to note that the data I’ve shown you are for the spike protein on the virus. The researchers noticed similar trends for antibodies and B cells that specifically recognize the receptor binding domain, a part of the spike protein that allows the virus to enter cells.
So, how is it that this is really promising, but people are still getting reinfected? The answer is that we’re all different. If you look at the vertical axis on the two graphs I’ve shown you, it’s important to notice that it is exponential, not linear.
This means that the difference between each number label represents a 10-fold difference in the amount of antibody or B cells. So, while the trends are positive, the difference in immune memory between different people can vary by more than 100-fold.
That is why ~90% of people still have antibodies after 6 months, but 10% of people do not. And, among those 90% who do have antibodies, the people who have the most antibodies may be the most likely to be protected against future infection.
What does all of this mean for the promise of vaccines to offer long-term immunity? Initial data from vaccine clinical trials are showing ~95% protection from infection, which is great news. https://www.nih.gov/news-events/news-releases/promising-interim-results-clinical-trial-nih-moderna-covid-19-vaccine
What will be interesting for researchers to investigate more closely is the levels of antibodies, B cells, and T cells people have after receiving the vaccine and how these persist over time, as these data will give us an idea of how long the effect of a vaccine will last.
The take home message is that the data that are emerging are promising when it comes to controlling the pandemic. However, we’re not there yet and it is still tremendously important to stay distanced, wash your hands, and wear a mask!
