Do viruses always evolve to be milder?

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Editor’s note: As what we know about COVID-19 evolves, so could the information in this story. Find our most recent COVID-19 articles here and learn the latest in COVID-19 prevention at the Centers for Disease Control and Prevention. Some photos and videos on this site were filmed prior to the COVID-19 outbreak or may not reflect current physical distancing and/or masking guidelines.


Is this the beginning of the end of the COVID-19 pandemic? Many of us wonder.

Viruses don’t always change in a predictable way.

While some viruses have evolved to cause less serious illness over time, others gain in strength as they constantly change and adapt, causing more illness and death.

Some viruses change to be milder; others more harmful

A coronavirus that started out deadly in the early 1800s and triggered a pandemic at that time now only causes the common cold. Typically, when a virus jumps from infecting an animal to infecting a different species such as humans, the virus becomes more virulent, or quickly spreading. But as that virus adapts to its new host, it normally becomes less harmful. That’s because the virus needs to co-exist in its (live) host to continue to replicate.

But viruses such as the one that triggers Ebola have mutated to cause even more serious illness. The virus that causes Ebola was harmless to the bats that were infected with it before the virus changed enough to be able to infect people — and proved to be fatal in humans. Since the first human outbreak of Ebola in Africa in 1976, the virus has been causing more serious disease levels and death across the continent.

As they replicate and spread infection, viruses constantly change. The omicron variant may still be evolving, and other variants can emerge that can be as problematic as omicron — or not.

Omicron stands out genetically

It’s still uncertain where omicron originated. Omicron’s genetic makeup is more similar to the earlier versions of the virus than the delta variant, the dominant variant identified just before omicron emerged. You might have thought that omicron came directly from the delta variant, but it didn’t. While omicron contains mutations similar to the earlier variants of the virus, many are new. Those genetic changes to the virus also affect how it functions. That includes gaining the ability to partly overcome vaccines designed to stop it.

Mutations, or changes in the genetic makeup of omicron, are happening constantly as the virus makes copies of itself and spreads. New variants are created all the time, especially among people whose immune systems have been compromised, such as cancer patients, who can provide viruses an easy gateway. From there, the virus can spread to people with fully functioning immune systems. Once it spreads enough, a new variant can eventually overtake another variant to become the dominant strain.

Could omicron combine with other variants or other coronaviruses to form another variant?

Possibly. And could that variant cause more illness than the current dominant omicron variant? It might. As scientists, we have to be open-minded and prepared for the worst. The recent discovery that the virus causing COVID-19 shifted from people to white-tailed deer and hamsters poses additional concerns. Will the virus continue to circulate among animals, potentially mutate, and then jump back over to humans? We don’t know.

Meanwhile, the more people who get vaccinated, the less likely it is that a new variant will emerge, spread widely, and cause significant illness. If enough people worldwide become either vaccinated or infected with omicron, then the pool of people susceptible to COVID-19 will decrease significantly, giving the virus fewer opportunities to emerge and spread.

Defending against the next variant

Not all will be vaccinated or immune, whether because they didn’t get vaccinated and boosted, they have health conditions that suppress their immunity, or they’re very young and have under-developed immune systems. So there will always be residual pockets of susceptibility where the virus can circulate and continue to evolve. That’s true globally, especially in regions with high infection rates and low vaccination rates.

Even though omicron has proven it can partially evade vaccines, those vaccines remain the best defense we have right now as new vaccines are being developed. The ideal next vaccines will block the virus from infecting the upper respiratory tract to prevent the virus from spreading so easily when we breathe. Ideally, these vaccines also would protect against multiple variants of the SARS-CoV-2 virus that causes COVID-19 as well as other emerging coronaviruses.

Getting vaccinated and boosted gives us our best chance of controlling the COVID-19 pandemic

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