Why Viruses Mutate, Explained by an Infectious Disease Expert


You’ve probably heard a lot about COVID-19 variants. Infectious Disease Expert Dr. Leyla Best, UnityPoint Health, identifies what you should know about virus changes, especially during the COVID-19 pandemic.

How Do Viruses Evolve So Quickly?

Viruses aren’t living things. They need a host to survive – like the cells in your body. Once a virus enters your body, it reproduces and spreads. The more a virus circulates in a population of people, the more it can change. All viruses mutate but not always at the same rate.

“The rate of change varies from virus to virus. Some change very fast, such as the influenza virus. That’s why we get a new flu vaccine every year,” Dr. Best says.

Because viruses are always changing, it's very important to stay up-to-date on all vaccines and booster shots.

Schedule a COVID-19 Vaccine or Booster 

What's the Difference Between Mutations, Variants and Strains?

  • Mutation. When a virus replicates, and the end copy has differences (in DNA or RNA), those differences are mutations.
  • Variant. When you accumulate enough mutations, you get a variant.
  • Strain. When you can prove a variant truly has new biologic capabilities, then you can call it a strain.

“With COVID-19, the changes to the virus are currently called variants. More research is needed to determine if any of the variants can be called a strain. In the spring of 2021, the World Health Organization (WHO) created a new system to name COVID-19 variants using Greek letters. This avoids the use of locations, which can be stigmatizing to a country. The CDC has identified five variants of concern so far, but two of them are currently circulating – delta and omicron,” Dr. Best says.

The CDC says no variants of high consequence have been identified in the United States at this time.

What is the Delta Variant of COVID-19?

The delta variant of COVID-19 was first identified in India in December of 2020. The CDC says delta is more than two times more contagious than other COVID-19 variants. 

“Self-reported data from the U.K. identify cold-like symptoms, including headache, runny nose and a sore throat are more common with the delta variant than the more traditional COVID-19 symptoms of loss of smell, shortness of breath, fever and cough,” Dr. Best says.

Researchers say the vaccines remain our biggest tool against warding off the COVID-19 variants, including delta. If you received an mRNA vaccine (Pfizer or Moderna), make sure you’ve received both doses and a booster dose, so you’re fully protected.

"Every time the virus jumps to a new person, its chance of mutation increases. We must use the tools we have – especially the COVID-19 vaccine – to help reduce the spread. Decreasing the number of infections in a community is the best way to prevent new variants from developing,” Dr. Best says.

What is the Omicron Variant of COVID-19?

South Africa researchers reported the omicron variant in late November 2021. The first U.S. case was identified just a week later. The CDC is working with experts around the globe to learn about omicron, but there’s not a lot of firm data yet. 

“It’s suspected the omicron variant does spread more easily than the original COVID-19 virus. That’s evident with how many people you probably know who got the virus around the 2021 December holidays. How it spreads compared to the delta variant is still unknown,” Dr. Best says.

The CDC expects anyone with omicron can spread the virus to others, even if they’re vaccinated or don’t have symptoms. It’s still best to make sure you’re up to date on your COVID-19 vaccines and booster to protect against severe illness, hospitalization and death.

“Patients infected with omicron are presenting with runny nose, cough and sore throat. Loss of taste and smell is not one of the common symptoms of omicron,” Dr. Best says.

Why is it Important to Focus on the Impact of the Virus’ Change?

“What matters is the impact the changes have on the virus itself. So, some viruses might have a few differences – a few mutations – but there are no noticeable changes to the virus. Sometimes viruses can have mutations that give the virus an advantage, whether that’s a better attachment to cells or the ability to replicate faster. Mutations can also result in disadvantages for the virus, lowering the ability to attach to cells or taking longer to reproduce,” Dr. Best says.

The important things for scientists to identify about changing viruses, like the virus that causes COVID-19, is how the change impacts people, if the vaccines still work, if treatment options are effective and if tests can still identify the active virus.

“For example, the omicron variant has been found to have 34 mutations in its spike protein. This has caused a decreased response to some monoclonal antibodies, which has been a treatment for COVID-19 infected patients. That’s problematic, because the treatment has helped reduce past hospitalization and death for those at high-risk for severe disease from COVID-19,” Dr. Best says.

Scientists are keeping a close eye on all variants to determine if the vaccines, or the treatments, are less effective. At this point, it appears current PCR testing and rapid testing can detect COVID-19 variants.

What Causes Viruses to Mutate?

Virus changes are associated with three things. First, sometimes a change in a virus is a pure error.

“A good analogy about virus changes is that it’s like copying a manuscript and, at some point, you're going to have a typo,” Dr. Best says.

Another reason a virus might change is because of pressure from select cells in the body.

“This hypothesis emerged regarding some of the COVID-19 variants. It states if a virus infects a person who doesn’t have a very strong immune system, for example, someone with cancer, then the body isn’t able to clear the virus very well. The can then say, ‘Hey, how are you going to attack me and make changes based on that?’,” Dr. Best says.

Variants may also emerge because people are immune to the older versions of the virus. That’s more likely to happen with viruses having higher mutation rates — like influenza.

“Any virus will keep trying to change, so it can continue to spread. With all vaccines, the more quickly people get vaccinated, the better. The slower vaccination happens, the higher the chance of having mutations in the virus and the appearance of more variants."

In order to keep viruses in check, everyone must do their part by getting vaccinated, and scientists must work together, around the world, to track emerging variants.