- James Gallagher
- BBC, health and science reporter
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Image source, Getty Images
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It is clear that we are now dealing with a virus that spreads much more easily – probably more than twice as easily – than the version that emerged in Wuhan in late 2019.
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The Alpha variant, first identified in Kent, UK, made a big leap in transmission capacity. Now the Delta, first found in India, has taken an even bigger leap.
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This is evolution in action.
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So are we doomed to a never-ending parade of new and improved variants that become increasingly difficult to contain?
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Or is there a limit to how much the coronavirus can get worse?
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The journey this virus is on is worth remembering. Has given the jump ofsde infect a completely different species (their closest relatives are bats) to us.
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It’s like you when you start a new job: you are competent, but there is room for improvement.
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The first variant was good enough to start a devastating pandemic, but now is learning at work.
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When viruses jump into humans, it would be “very rare for them to be perfect,” explains Wendy Barclay, a virologist at Imperial College London. “They settle in and then have a great time.”
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Image source, Getty Images
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The clearest way to compare the pure biological spreading power of viruses is to look at their reproduction number.
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There are examples of viruses, he says, from flu pandemics to Ebola outbreaks, taking the leap and then accelerating.
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So how far could it go?
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The clearest way to compare the pure biological spreading power of viruses is to look at their R0 (the reproduction number).
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That is the average number of people to whom each infected person transmits a virus, if no one were immune and no one took extra precautions to avoid becoming infected.
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That number was around 2.5 when the pandemic started in Wuhan and could go up to 8.0 for Delta variant, according to disease modelers from Imperial College.
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“This virus has surprised us a lot. It goes beyond what we feared,” says Aris Katzourakis, a researcher studying viral evolution at the University of Oxford.
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“The fact that it happened twice in 18 months, two lineages (Alpha and then Delta) each 50% more transmittable is a phenomenal amount of change. “
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He thinks it is “silly” to try to put a number to how high he could go, but he can easily see more jumps on broadcast for the next two years.
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Other viruses have a much higher R0, and the record holder, measles, can cause explosive outbreaks.
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Image source, Getty Images
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The virus can still “improve”, that is, become more efficient in its mode of transmission.
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“There is still room for it to go higher,” says Barclay. “Measles is between 14 and 30, depending on who you ask, I don’t know how it will develop.”
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So how are the variants doing it?
There are many tricks that the virus could use to improve its spread, such as:
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- improve the way it enters the cells of our body
- survive the longest in the air
- increase viral load so that patients breathe or cough more viruses
- change in the process of infecting another person
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One of the ways in which the Alpha variant became more transmissible was by improving the way it bypasses the mechanism that alerts of its intrusion, called the interferon response, within the cells of our body.
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But this does not mean that by the time we get to Omega in the Greek alphabet of variants we will end up with an unstoppable beast.
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“As a last resort, there are limits and there is no superdefinitive virus that has all combinations of negative mutations, “says Katzourakis.
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There is also the concept of evolutionary tradeoffsTo get better at one thing, you often have to get worse at another.
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Image source, Getty Images
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The new variants are more transmissible than the original.
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The fastest vaccination program in history will give the virus a different hurdle to overcome and push it in another evolutionary direction.
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“It is quite possible that changes in the virus that make it better to avoid vaccines could end up compromising its transmission capacity in an absolute sense,” says Katzourakis.
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He thinks the Beta variant – which has a mutation called E484K that helps evade the immune system but has failed to get off the ground – is an example of this.
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However, Delta has mutations that help it spread and partially bypass immunity.
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It is still difficult to predict what the optimal strategy for the coronavirus will turn out to be. Different viruses use different techniques to keep infecting.
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Measles is explosive, but it leaves a lifelong immunity, so you always have to find someone new. Influenza has a much lower R0, just over 1, but it constantly mutates to evade immunity.
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“We are in a really interesting, middle and somewhat unpredictable phase, it’s hard to predict how it will play out a year from now, “says Barclay.
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One thing that is often claimed, but which attracts scorn from scientists, is that the virus must become milder to spread more easily.
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Image source, Getty Images
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The increasingly transmissible variants are a nightmare for the poorest countries.
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There is very little evolutionary pressure on the virus for that to happen. The virus has already been transmitted to the next person long before it kills the infected person.
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And the people who spread it the most (the youngest) are the ones who don’t get very sick.
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In rich countries with good vaccination campaigns, it is expected that the next variants may not represent a major problem due to widespread immunity.
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But these increasingly transmissible variants are one nightmare for the rest of the world, where they make it increasingly difficult to contain covid-19.
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