Evolution of SARS-CoV-2
Q: I heard that there were two strains of the virus going around; one more aggressive and one less aggressive. Is there any truth to that?
A number of articles have recently been shared online suggesting that there are two strains of the novel coronavirus in circulation. Many articles state that there is a ‘more aggressive’ and ‘less aggressive’ strain. Much of this information is based on varying interpretations of one study carried out by a group of scientists in Beijing. We will walk you through the study, so that you can understand for yourself what we can take away from the data presented.
Firstly, the purpose of the study is to investigate the genetic origin of the SARS-CoV-2 virus - the virus that causes the disease COVID-19. Which species did this virus come from? How was this virus able to evolve in such a way that animal to human transmission became possible? These are the primary questions that this study tried to answer, and you could imagine that gaining a better understanding of this could help scientists and governments around the world to better prepare for and monitor future outbreaks.
To go about answering these two questions, the scientists compared the sequence similarity of the viral RNA from the SARS-CoV-2 virus with the sequences of different but closely related coronaviruses; for example one common to bats and one found in pangolins. It is believed that the virus was transmitted to humans from pangolins. The scientists were able to build a good understanding of the ‘genetic history’ of this virus and show which changes - mutations - in its genetic sequence may have enabled it to make the bold leap into humans.
So, why have so many journalists been writing about more or less aggressive strains of the SARS-CoV-2 virus? Well, the authors of the study were also interested in the continued evolution of the virus, but in particular changes that may have occurred around the time it was transmitted to humans. All viruses gather mutations, small changes in their genetic make-up, every time they reproduce. As these mutations are passed down to the next generation, it’s possible to trace how quickly viruses are evolving by seeing how many mutations they are accumulating. In this study, 103 publicly available SARS-CoV-2 genomes were compared to try to investigate this. Overall they found all of the virus genomes to be very similar to each other, suggesting that they have not evolved significantly since the outbreak. By identifying particular patterns of small genetic changes, they noticed that there were two different ‘strains’ of the virus which they called ‘L’ and ‘S’ types. They were interested in finding out which of these types is most closely related to similar coronaviruses found in animals such as pangolins in order to understand the evolution of the virus. They conclude that the L type is more ‘aggressive’ than the S type, but in this context they are referring to its prevalence - the data suggests that it has a higher transmission rate - not the severity of disease it causes.
So this group’s conclusion was that there are two strains of coronavirus. But what does this actually mean for us? It's important to keep in mind that mutations gather naturally over time in all living things, and because genes only make up a small percentage of all of your DNA, the vast majority of mutations have no effect. Ian Jones, Professor of Virology at the University of Reading, points out that “the differences between the two identified strains are tiny. Many of the genetic differences won’t affect the production of proteins, and so won’t change the way the virus works, or the symptoms it causes.” The study was completely based on comparing the sequences of the viral genomes - they didn’t have any clinical information such as the severity of symptoms the viruses caused. There were more than 650,000 confirmed cases worldwide at the time of writing, and the authors themselves conclude that more information is required to verify their findings. It is certainly interesting and important to understand the mechanisms of how SARS-CoV-2 has evolved to enable transmission from its original animal host to humans, but so far evidence does not support the suggestions that we are fighting a rapidly changing enemy.
Over time, we will continue to see interesting studies characterising many aspects of the disease as scientists across the world work hard to fully understand it. The combination of information resulting from the tireless work of thousands of scientists will eventually lead to our victory over COVID-19.