Earlier this year we wrote a post focused on COVID-19-related immunity; and based on the data available at the time it wasn’t really clear how long immunity lasts or if people could catch COVID-19 more than once. However, because of the urgency involved in researching SARS-CoV-2 there’s a considerable amount of new data available surrounding COVID-19 immunity and reinfections. In the following post, we summarize two recent scientific articles on these subjects.
Immunity to SARS-CoV-2 - update
If you haven’t read our previous post about immunity, you can find it here. We describe how the human immune system works and clarify that immunity isn’t binary (either you have it not) - it’s more of a spectrum that changes over time. A wide variety of factors both on the human side and the virus side of things may impact immunity. From the human side - time is really important. Consequently, time is also important on the virus side of immunity because of mutations.
Mutation is when there is a change in the DNA/RNA sequence that makes up a gene. Mutations can occur for a number of reasons and they don’t always result in a physical change in the organism. In RNA viruses - like SARS-CoV-2 - mutations can occur relatively quickly resulting in different strains of the same virus. These changes can impact how our immune system sees the virus in the body. You can think of it like when a coworker gets a haircut - they changed their appearance from what you’re used to seeing - you will recognize them but it might take you longer than it would have before they changed their look.
Currently, one of the main concerns about SARS-CoV-2 mutations is focused on whether or not the mutations will change the spike protein - a special surface part that the virus uses to enter and infect human cells. This is because 1) many vaccines that are being developed are focusing on generating a robust immune response by recognizing the spike protein; 2) changes in this protein could make the virus better at infecting people and 3) these alterations may make the immune system less effective at recognizing and attacking this protein.
The Impact of Mutations in SARS-CoV-2
The first study we’re discussing today is focused on characterizing how the SARS-CoV-2 spike protein is naturally mutating (Li, et al 2020). To do this, Li and colleagues analyzed the genetic code from a plethora of SARS-CoV-2 (more than 13,000 sequences) collected from all over the world. Then they made 106 mutant viruses in the lab to mimic these mutations and looked at how animal cells - including human cells - responded to these altered viruses.
Some of these mutations appear to make the virus better at infecting human cells, while other mutations reduce the potential of the virus to cause an infection/to infect. The same is true for how mutations change immune system (here, antibody) recognition - sometimes the mutation made the virus more sensitive to antibodies and sometimes mutations made the virus less sensitive to antibodies.
Some of these mutations appear to make the virus better at infecting human cells, while other mutations reduce the potential of the virus to cause an infection/to infect.
The good news so far is that these researchers only detected one combination of mutations that made the virus both more infectious AND less sensitive to antibodies. Further, the genetic code from this mutated virus has only been found one time in the currently available SARS-CoV-2 sequence databases.
Plot showing how the mutated SARS-CoV-2’s behavior changed with alterations in their genetic code. Only 1 mutated virus was both better at infecting human cells and less sensitive to antibodies - this type of mutated virus has only been identified once in more than 13,000 different types of SARS-CoV-2.
Virus image credit: https://www.cdc.gov/media/subtopic/images.htm
A positive spin. Is this good news really all that good? (Yes, it is)
These findings are important because by gaining a better understanding of how the virus is changing over time and how our immune system reacts to these changes, researchers can generate a better-longer-lasting vaccine. What about long term immunity?
Okay, so now we know that the virus is doing what it can do to avoid detection from our immune system -mutating- but that it’s not always doing a good job. But what about our immune system? How good is its long term memory?
Initial research on how long antibodies stuck around post-COVID-19 infection found that after about 3 months, the human body had lost most of these virus-attacking antibodies. A more recent study by Ripperger and colleagues, however, has increased that number to about 5-7 months - depending on how strongly your immune system responded to a SARS-CoV-2 infection AND depending on which antibody is measured.
For example, the antibody that our immune system produces to recognize the spike protein, is one of the longer lasting ones. Their results also showed that the severity of the infection dictates how many antibodies are produced and then how long your antibodies will stay around after clearing the infection. So if you had a mild form of COVID-19, then your immunity may be shorter lived than someone who had a severe infection.
...if you had a mild form of COVID-19, then your immunity may be shorter lived than someone who had a severe infection.
Again, this is good news because if antibodies are staying active longer, then our chances for re-infection go down and hopes for a long lasting vaccine can go up.
What about reinfections?
This brings us to the final point of this post - re-infections. The pandemic has now been going on long enough that researchers have been able to definitively identify a few cases of re-infection (Ripperger et al 2020). A recent blog post on the Nature website discussed 2 of these cases. In the first case an 89 year old patient with cancer died after catching COVID-19 a second time. It is well known that cancer and cancer treatment have a severe negative impact on our immune system. The other reinfected person was hospitalized after recovering from a first mild SARS-CoV-2 infection. Based on the data we discussed above, both of these patients may not have raised a robust immune response during their first infection. Also, the data also demonstrates that these two patients got infected with different strains of SARS-CoV-2 during both infection events - totalling 4 slightly different strains of SARS-CoV-2.
This means that there doesn’t seem to be an extra-deadly MUTATED strain of SARS-CoV-2 circulating. The current data doesn’t suggest that SARS-CoV-2 is rapidly mutating to become more and more infectious. Instead we’re seeing that 1) mutations are occurring and rarely in the spike protein; 2) when our body effectively makes antibodies against the spike protein they last longer than we previously thought, and 3) a vaccine aimed at using antibodies against the spike protein is still a good idea.
Take home messages: Scientists are working around the clock to produce a safe and effective vaccine and research has already started to show that they're on the right track. But until then, if you’ve already had COVID-19, your immunity period depends on a number of variables - maybe you have those ‘long-lasting’ antibodies but maybe you don’t. The safest option is to keep following social distancing guidelines, wearing your mask, and washing your hands.
Glossary:
Mutation is a change in the nucleotide sequence of the genome of an organism.
RNA viruses is a term used to describe viruses who's genetic code consists of a single strand of molecules.
References:
Lowe, Derek. Immunity and Re-Infection. Science Magazine blog. October 2020. https://bit.ly/31y5ybL
Li, Qianqian et al.The Impact of Mutations in SARS-CoV-2 Spike on Viral Infectivity and Antigenicity. Cell. September 2020. https://doi.org/10.1016/j.cell.2020.07.012
Ripperger, T.J., et al., Orthogonal SARS-CoV-2 Serological Assays Enable Surveillance of Low Prevalence Communities and Reveal Durable Humoral Immunity. Immunity. October 2020. doi: https:// doi.org/10.1016/j.immuni.2020.10.004.
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