People who have not been vaccinated against COVID-19 not only risk their own health but are "variant factories," an infectious disease expert told CNN.

"Unvaccinated people are potential variant factories," Dr. William Schaffner, a professor in the Division of Infectious Diseases at Vanderbilt University Medical Center, told CNN.

"The more unvaccinated people there are, the more opportunities for the virus to multiply," he added.

Variants evolve in the body of a person who is infected with the coronavirus.

"When it does, it mutates, and it could throw off a variant mutation that is even more serious down the road," Schaffner said.

Viruses, including the coronavirus that causes COVID-19, endure mutations, and while some mutations may harm the virus, others could help it. For example, a mutation could help a virus be more transmissible.

When the virus spreads to another person, the mutation will replicate and also spread, and if it's successful and continues to spread, it becomes a variant.

Unvaccinated people provide the opportunity for the mutation to continue to replicate, CNN reported.

"As mutations come up in viruses, the ones that persist are the ones that make it easier for the virus to spread in the population," Andrew Pekosz, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health, told CNN.

"Every time the virus changes, that gives the virus a different platform to add more mutations. Now we have viruses that spread more efficiently."

Essentially if a virus can't spread, it can't mutate.

There are already several variants of the coronavirus, including four called "variants of concern" by the World Health Organization.

These four include Alpha, first discovered in the UK, Beta, first discovered in South Africa, Gamma, first discovered in Brazil, and Delta, first discovered in India, pose risks because they are either more transmissible, cause worse disease, or can evade immune protection.

These variants were also discovered in a period where cases of infection were high.

Public health experts are specifically already concerned about the risks the Delta variant poses even for those already vaccinated. While vaccines have been found to be effective against the variants, they may not be as effective against some. Two doses of Pfizer's vaccine for instance have been found to be 88 percent effective at preventing symptomatic cases of COVID-19 from the Delta variant.

In the US, however, experts are worried the variant can spread and cause breakthrough cases especially in communities with low vaccination rates. The fewer people that are vaccinated, the more chance there is for the virus to hop from one person to the next.

"Currently, approximately 1,000 counties in the United States have vaccination coverage of less than 30 percent. These communities, primarily in the Southeast and Midwest, are our most vulnerable. In some of these areas, we are already seeing increasing rates of disease," CDC Director Dr. Rochelle Walensky told a White House briefing Thursday.

"As the Delta variant continues to spread across the country, we expect to see increased transmissions in these communities, unless we can vaccinate more people now."

Experts are urging more Americans to get vaccinated to stop the spread.

This article was originally published by Business Insider.

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