Apr 23, 2020 - Science

How (corona)viruses change

Illustration: Sarah Grillo/Axios

Viruses change as they spread — the novel coronavirus included.

Why it matters: A key question for the development of diagnostic tests, vaccines and treatments is how much a virus mutates — and how efforts to fight it may have to adjust to keep up.

How it works: Viruses mutate as they replicate in host cells, producing thousands of mutations that evolution then acts upon as the virus spreads through a population.

  • RNA viruses — HIV, influenza and coronaviruses, for example — tend to mutate faster than DNA ones.
  • But unlike other RNA viruses, coronaviruses have proof-reading capabilities that allow them to catch errors that arise as the virus copies itself.

Where it stands: There are no direct measurements of the raw mutation rate of SARS-CoV-2, but it is likely less than influenza and HIV viruses, says Rafael Sanjuán, who studies virus evolution at the University of Valencia in Spain.

  • As the novel coronavirus spreads around the world, researchers are tracking the changes that are occurring — a reflection of the virus' spontaneous mutations that are shaped by natural selection and other forces.
  • One example: A mutation appears to be recurring at different times and in clusters of people, suggesting it isn't random and increases the fitness of the virus, according to unpublished data that is itself evolving.
  • Yes, but: "Fitness doesn't necessarily mean a virus is more lethal," says Phoebe Lostroh, a molecular biologist at Colorado College and program director at the National Science Foundation.

The big picture: There are tradeoffs between how fast a virus replicates, how efficiently it is transmitted and how lethal it is.

  • The original SARS virus behind the outbreak in 2003 replicated low in the respiratory system whereas SARS-CoV-2 replicates in the upper tract — meaning it can be transmitted more easily through coughing and the symptoms are less severe, letting the virus sneak under the radar in many cases.
  • The Ebola virus had a less than 50% fatality rate in the 2014 epidemic compared to 90% for all previous (and smaller) outbreaks, perhaps because mutations in the virus allowed it to be transmitted more efficiently but also made it less lethal, Sanjuán says.

Keep in mind: Technology has kept up with changes in influenza and other viruses by monitoring mutations, and there are strategies to target multiple regions of a virus with drugs and vaccines.

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