“Selective Pressure“

By Katelyn Jetelina, Your Local Epidemiologist

[This is an email distributed by Dr Katelyn Jetelina on her own time. At the bottom of this re-post there are the URL links she supplies to subscribe. From time to time we will re-post these emails here, when they represent advances in the understanding of the fundamental science so significant, they must broadly be understood and distributed. “Selective Pressure” in our opinion, represents such an advance.]

“Last week, I described a variant -B.1.526- that was first discovered in New York City. In my newsletter, I quoted a geneticist: “The overall pattern of mutations suggests that B.1.526 arose in part in response to selective pressure from antibodies”. Thanks to your comments and emails I realized that I didn’t translate “selective pressure”. This post is meant to clear that up that confusion…

What is selective pressure?

“The virus’s primary goal is NOT to kill. It’s goal is to survive. So, if the virus is having a hard time surviving (through expanding herd immunity, vaccine-induced immunity, antiviral therapies) it’s pressured to evolve. In other words, selective pressure happens when populations (or individuals) force the virus to change because we are becoming immune. Keep in mind, this is not unexpected. Scientists are catching these variants because they are looking for them.

How did COVID19 selective pressure start?

“We have a few guesses, but at this point they’re merely hypotheses. We think that a combination of individual events and population events are causing the pressure. For example…

  1. “Individual. The UK variant is rumored to have started in one chronically infected person with a weakened immune system who was being treated with plasma (antibodies from a recovered patient). This would have given a strong advantage to any variant that could resist the therapeutic antibodies.
  2. “Population. In October 2020, it was thought that Manaus (a small, secluded town in Brazil) reached herd immunity due to an intense wave of COVID19 disease. At the time, scientists estimated that 76% of the population was “naturally” infected. However, the town is now reinfected by COVID19 at alarming rates. After sequencing the virus, it’s obvious that transmission is due to a new variant (called P.1) and the population is not protected by previous infections’ antibodies. The resulting high level of immunity in the population may have given an advantage to mutations in order for the virus to survive. (As I’ve said before, it’s under great debate whether Manaus ever reached herd immunity in the first place, but important to follow nonetheless.)

How do we know selective pressure is happening? 

“We don’t really know. There is not lab test or a quantitative study that definitively tells us. We rely on more of a qualitative assessment. And, so far, there are two overarching themes that are telling us selective pressure.

  1. “Variants are starting to emerge in different places around the globe. Despite arising independently on different continents, the variants share striking similarities. Importantly, these mutations are the same (or similar) on the spike protein, which gives the virus a big advantage to surviving. For example, the variant first discovered in South Africa has the same exact mutation on the spike protein as the one just discovered in New York and the one discovered in Brazil.
  2. “Selective pressure could explain why the variants are acquiring several mutations so quickly. For example, B.1.1.7 has 17 mutations, which is quite a lot this early. It’s important to note that the rate of mutations is still slower than the flu but much faster than, for example, measles.

So…selective pressure is a good sign?

“Yes and no. Selective pressure means that that the global population is starting to effectively fight the virus. However, it’s now a game of cat and mouse. Can we vaccinate faster than the virus mutates? Only time will tell.

What can we do to give us an advantage in the race?

“We can do three things right now:

  1. “Reduce transmission. The less this virus jumps from one person to the other the less chance is has to change. This is why isn’t important for vaccinated people to stay masked because transmission isn’t 100% stopped by the vaccine.
  2. “Keep up public health measures. We can exert selection pressure through behavior. By masking, for example, we could exert pressure that favors other routes of transmission. And, by doing this, it would probably make this virus less harmful.
  3. “Vaccine manufacturers are making modifications to vaccine design (out of precaution) to ensure that we stay one step ahead.”

“Love, YLE”

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