Why Do Viruses Like COVID-19 and the Flu Mutate Rapidly and What Does it Mean for Vaccines?

Have you ever wondered why you have to get a flu shot every year, but some vaccines are one-and-done? It all has to do with how and how fast viruses evolve — and that depends on mutation rates.

Viruses are tiny microbes, even smaller than bacteria, made up of either RNA or DNA enveloped in a coat of protein, according to the National Human Genome Institute. Viruses can reproduce, sometimes quite rapidly, but they can’t do it without a host.

For a virus to make more viruses, it must infect a cell and then use that cell to make copies of itself. Basically, a virus hijacks a cell and forces it to reproduce the virus. The original cell usually dies, but the viruses it has churned out go on to infect other cells, according to the Memorial Sloan Kettering Cancer Center.

The body’s immune system does its best to keep these viruses in check. But some viruses evolve quickly, making it difficult for the immune system to keep up. One reason viruses evolve so quickly is that they mutate rapidly.

Most RNA viruses — the flu virus or COVID-19, for example — mutate especially fast. High mutation rates increase the amount of genetic variation in the population. This means there’s a greater chance that a variant will evolve and make the virus spread faster or make the host sicker. That’s also why it’s difficult to make vaccines for rapidly evolving viruses. The goalposts keep moving.

Read More: COVID-19: A Basic Guide to Different Vaccine Types and How They Work

Why RNA Viruses Mutate Quickly

But why do RNA viruses mutate so fast? At least part of the reason is that RNA viruses, unlike DNA viruses, don’t check their work. Almost all DNA polymerases have proofreading mechanisms. (Polymerases are enzymes that help assemble new strands of DNA and RNA.)

These proofreaders can detect copying errors. If they find a mistake, they can fix it. However, almost all RNA polymerases lack that ability, explains Adam Lauring, a scientist at the University of Michigan who studies viral evolution. That’s one reason you have to get an influenza vaccination every year.

Flu is caused by an RNA virus that evolves so fast that this year’s strain is not enough like last year’s for the immune system to recognize it.

Why COVID Mutates So Fast

However, there’s one family of RNA viruses in which the members do have a proofreading mechanism: The coronaviruses. Yep, that’s the family that SARS-CoV-2, the virus that causes COVID-19, is a part of.

“Their mutation rates, believe it or not, are about tenfold lower than for other RNA viruses,” says Lauring. “They’re not quite as good as DNA viruses, but they can actually fix their mistakes to a certain degree.”

Although SARS-CoV-2 has lower mutation rates than other RNA viruses, it still evolves rapidly, just like the flu virus. If SARS-CoV-2 has a proofreading mechanism, why does it evolve as quickly as influenza, which doesn’t have one? To understand this, we need to think not of the biochemical mutation rate — how many mutations occur per unit time when copying the genome — but of how the virus has accumulated mutations in the population over time, says Lauring.

Since the pandemic began, more than 700 million people have been infected with SARS-CoV-2, according to the World Health Organization. So even though the virus makes fewer mistakes than other RNA viruses, in such a large population, those mutations add up quickly.

How Measles Mutates

While we are understanding the flu and COVID, what about measles? Measles is also caused by an RNA virus, yet despite its high mutation rate, the virus isn’t that much different from what it was decades ago. The measles vaccine in use today is the same strain that was introduced in the 1960s, and it’s still just as effective. In most cases, a measles vaccination covers you for life.

Now we know the measles virus mutates rapidly, but doesn’t evolve quickly. Why? A study published in the journal Cell Reports Medicine in April 2021 proposed an answer to that question. The researchers found that it has to do with how the measles virus evolves.

Even though the virus spawns a lot of mutations, it’s virtually impossible that those mutations will be able to evade the immune system and, at the same time, remain capable of replicating. It mutates, but those mutations do it no good when it comes to making us sick.

Lauring explains this to his students with this analogy: If there were a mutation that gave a giraffe three legs, that giraffe wouldn’t live very long; a predator would pick it off quickly. And that’s how a lot of mutations work, including those of the measles virus. They don’t provide anything that will make the virus better at what it does. And sometimes mutations, like those that create a three-legged giraffe, make it easier for the immune system to do its job.

This article is not offering medical advice and should be used for informational purposes only.

Read More: Vaccines, Not Supplements, Remain the Best Way to Fight Measles

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