Covid-19 cases surge nationwide as vaccination rates plummet from spring highs - CNN

"If you do get a breakthrough infection -- which itself will be unusual -- it will be more likely to be mild or asymptomatic."

Murthy said he is concerned the Delta variant is spreading quickly among unvaccinated people, who account for the vast majority of coronavirus hospitalizations and deaths.

"The thing that's making this possible is the fact that we are dealing with the most transmissible version of Covid-19 that we've seen to date," he said.

Immunocompromised Americans remain at risk

Those who are immunocompromised and vaccinated against Covid-19 may want to continue wearing masks, according to Murthy, and vaccinated people living in high transmission areas and those with young children may want to mask up as well.

"Those are all circumstances where people may make the decision to actually go the extra mile, be cautious and wear masks, especially in indoor settings," Murthy told CNN.

Immunocompromised people may be at greater risk for Covid-19 themselves, Murthy noted, while others may want to mask up to protect the people around them.

"In the case of parents who have kids at home, they may decide that, 'Even if there's a really small chance I could pass it on to my kids, I want to be cautious because I'm living in an area with a lot of virus circulating,'" he said.

Meanwhile, research is being conducted to determine whether an additional boost with vaccines can be provided for the immunocompromised.

A study in the US found 44% of hospitalized breakthrough cases were immunocompromised people, while a study in Israel found a rate of 40%.

"Emerging data show there is an enhanced antibody response after an additional dose of mRNA COVID-19 vaccine in some immunocompromised people," the CDC said in a statement to CNN on Friday.

"While early data show some potential benefit to administering an additional dose, more evidence is needed to determine safety and effectiveness in immunocompromised people."

Caution urged at festival as cases rise in Florida

Even for the community at-large who are not immunocompromised, the Delta variant is still putting people at considerable risk for infection, as evident in the state of Florida.

The state reported 73,199 new Covid-19 cases this past week, which is up from 45,603 the previous week, according to data from the Florida Department of Health (DOH).

And the number of Covid-19 vaccine doses administered dropped in the past 10 weeks, from more than 769,000 doses administered the week of May 14 to around 245,000 this week.

Miami-Dade has the highest number of new cases among all Florida counties this week, according to DOH data, and officials are calling for caution during summer gatherings.

Miami-Dade County Mayor Daniella Levine Cava on Saturday urged concertgoers at the three-day Rolling Loud Festival to help "slow the spread by practicing safe behavior."

The mayor said anyone who isn't vaccinated can do so at the county's pop-up vaccine site set up at the music festival.

The vaccine site was set up in conjunction with the state Department of Health and will offer both Pfizer and Johnson & Johnson vaccines until midnight Sunday, according to the county's website.