Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, warned Friday that a highly contagious variant of the novel coronavirus that was first detected in India could soon become the dominant strain in the United States.
"I think that that's probably going to be the case," Walensky told ABC News in an interview on "Good Morning America."
After being identified in India in October, the so-called delta variant has since been reported in more than 80 countries around the globe, according to the World Health Organization. Meanwhile, the strain has already been detected in at least 41 U.S. states, according to the CDC.
The WHO declared delta a "variant of concern" last month, and the CDC upgraded its classification of the strain this week from "variant of interest" to a "variant of concern." WHO officials said variants of concern have shown to spread more easily than others.
"When these viruses mutate, they do so with some advantage to the virus. In this case, it is more transmissible," Walensky said. "It's more transmissible than the alpha variant, or the U.K. variant, that we have here. We saw that quickly become the dominant strain in a period of one or two months, and I anticipate that is going to be what happens with the delta strain here."
The CDC is concerned about the delta variant mutating to a point where it evades the existing COVID-19 vaccines, according to Walensky.
"That's really what we're actively trying to prevent, which is why we're really encouraging people to get vaccinated," she said. "I will say, as worrisome as this delta strain is with regard to its hyper-transmissibility, our vaccines work. Right now, they are working and they require actually two doses or to be fully vaccinated to work. So I would encourage all Americans to get your first shot and when you're for your second, get your second shot and you'll be protected against this delta variant."
Dr. Ashish Jha, dean of the Brown University School of Public Health in Providence, said the delta variant "is far more contagious than any variant we have seen throughout this entire pandemic."
"It also appears to be a little bit more deadly for people who get infected," Jha told ABC News in an interview Wednesday on "GMA."
"And what we are seeing is, while our vaccines seem to generally hold up," he added, "we're seeing a few more breakthrough infections."
Since the start of the pandemic, the U.S. has reported more than 33.5 million confirmed cases of COVID-19 and over 600,000 deaths from the disease, according to data collected by Johns Hopkins University.
Although the daily number of newly confirmed COVID-19 infections continues to fall nationwide, there are some parts of the country that continue to struggle with pockets of outbreaks, particularly areas with low vaccination rates. COVID-19 data from the CDC and the U.S. Department of Health and Human Services show that five states -- Alabama, Arkansas, Missouri, Oklahoma and Utah -- have seen notable increases of 37% or more in their seven-day case average over the last two weeks. The rates of fully vaccinated residents in all five of those states are lower than the national average.
Meanwhile, there are reports of heart inflammation in young people who have been vaccinated against COVID-19. More than 300 confirmed cases of myocarditis, an inflammation of the heart muscle, or pericarditis, an inflammation of the sac-like structure surrounding the heart, have been reported across the U.S. in people under 30 who got the Pfizer or Moderna vaccines, according to the CDC.
The CDC says most patients who received care responded well to medicine and rest, and they quickly felt better. Health officials are still investigating whether there is a link to COVID-19 vaccination. A CDC advisory panel is set to convene in an emergency meeting next week to discuss the issue.
"We certainly are looking at this carefully," Walensky told ABC News. "And we are going to be eager to look at the data during that meeting."
The CDC still recommends everyone over 12 get immunized against COVID-19 but says anyone experiencing chest pain, shortness of breath or rapid heartbeat within a week of getting the shot should seek medical attention.
Walensky noted that the risk of getting these "mild" conditions, which she described as "really quite rare," are "overwhelmed by the benefit of getting vaccinated."
"I have three kids and all of mine are vaccinated," she said. "The really most important thing that you need to do is be comfortable as a parent with your choice in making this decision."
More than 175 million people in the U.S. have received at least one dose of a COVID-19 vaccine, including over 147 million -- 44.5% of the population -- who are fully vaccinated, according to the CDC.
"If you make an informed decision where you listen to the science around the vaccines, the safety of the vaccines, the overwhelming data we have on the safety of vaccines and how effective they are at preventing severe disease and sickness in your children, I think you'll come down the way I did and vaccinate your children," Walensky said.
ABC News' Arielle Mitropoulos contributed to this report.