As US states lift more coronavirus restrictions, experts are worried people who aren't fully vaccinated could contribute to further spread of the virus.
The Delta variant, first reported in India, accounts for nearly 10 per cent of coronavirus cases in the US, according to the Centres for Disease Control (CDC).
With concerns it could become the dominant strain soon, medical experts are underscoring the importance of vaccination.
"I'm worried about those who are unvaccinated," US Surgeon General Vivek Murthy told CNN, noting the Delta variant "is rapidly increasing here in the United States."
The CDC has determined the Delta variant is a "variant of concern," a designation given to strains of the virus that scientists believe are more transmissible or can cause more severe disease.
The Delta variant "appears to be significantly more transmissible than even the Alpha variant or the UK variant, which is now dominant in the United States," Dr Murthy told CNN.
"The second reason it's concerning is that there is some data to indicate that it may in fact also be more dangerous, may cause more severe illness.
"That still needs to be understood more clearly, but these are two important concerns and they explain in part … why this is become the dominant variant in the UK, where over 90 per cent of cases are the Delta variant," Dr Murthy said.
The good news is that vaccines appear to be effective against the Delta variant.
A new study by Public Health England found that two doses of a coronavirus vaccine is "highly effective against hospitalisation" caused by the variant.
The study found the Pfizer/BioNTech vaccine is 96 per cent effective against hospitalisation after two doses.
Dr Murthy said there isn't enough data to indicate the effectiveness of Johnson & Johnson's one-dose vaccine in regard to the Delta variant, but the vaccine has shown it can help prevent hospitalisations and deaths when people are infected with other strains.
"The key is get vaccinated, get both doses," Dr Murthy said.
As of Wednesday, 44.1 per cent of the total US population was fully vaccinated while 52.7 per cent has received at least one dose of a vaccine, according to the CDC.
This comes on the heels of the US surpassing 600,000 deaths since the coronavirus pandemic began, according to data compiled by Johns Hopkins University. That means about one in every 550 people in the US has died from the virus.