CNN  — 

With so many areas in the United States witnessing low vaccination rates, the spread of the Delta variant of the coronavirus will make it that much harder for the country to reach herd immunity, a top expert says.

“We don’t exactly know what the herd immunity percentage would be for Covid-19. It would be different for the Delta variant, and higher, because it is more transmissible,” Dr. Rachel Levine, assistant secretary of the US Department of Health and Human Services, told CNN’s Ana Cabrera on Friday.

Herd immunity is the point where a virus can no longer infect people because enough people already have it or are vaccinated against it.

Estimates vary on how much of the population needs to have immunity to reach that goal. Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, has said herd immunity might be reached if 70-85% of people are immune.

There is good news, per Levine, who said, “People who are vaccinated are protected against this Delta variant. And they’re extremely unlikely to get sick and it’s virtually impossible for them to require hospitalizations.”

As for those who remain unvaccinated, “The Delta variant poses a threat,” Levine said.

Variant accounting for greater percentage of cases in some states

The Delta variant, first seen in India, can infect people more easily and causes even more severe illnesses.

In California, where about half of the state’s 40 million population is vaccinated, Covid-19 cases are rising, with the Delta variant accounting for 36% of all new cases, according to officials.

“The most important thing we can do to stop the spread of COVID-19, and the variants, is ensure everyone who is eligible gets vaccinated,” said Dr. Tomás Aragón, California’s public health officer.

On Monday, Los Angeles County recommended that people wear masks in indoor public spaces even if vaccinated and, on Friday, the city and county of St. Louis, Missouri, did the same. Both jurisdictions made the recommendations because of the spread of the variant.

“As we monitor the Delta variant, we are seeing that it’s spreading fast, and data shows it is more infectious and impacting younger segments of the population,” said Dr. Fredrick Echols, the acting director of the City of St. Louis Department of Health, in a news release.

“Missouri is in a really bad place right now,” said Dr. Megan Ranney, an emergency physician and CNN medical analyst. She said the state is seeing a “dramatic rise” in Covid-19 cases, similar to the rise seeing last summer.

“You have places in Missouri where only 10%, 15%, 20% of their residents are fully vaccinated. This new Delta (variant) is going to spread like wildfire there,” she said. “They have no choice but to put mask mandates in place while they also try to get vaccines in.”

Regarding Los Angeles County’s mask mandate, Ranney said that “in the face of this surge in cases, putting an indoor mask mandate back in place is not the worst thing in the world.”

Such a mandate will send a signal to people that they should wear a mask if they’re not vaccinated. But the problem is, she noted, that people who aren’t vaccinated are also least likely to wear a mask.

CNN analyst Dr. Leana Wen told CNN Friday the states with the lowest vaccination rates are seeing an even higher percentage of the Delta variant in new cases.

“In the five states with the highest rate of Covid-19 spread, the Delta variant is more than 50% of the cases there. Arkansas is 50%. Utah 60%. Missouri is 70%.” Wen said. “I think we have to keep in mind, too, the statistics of who is getting ill. By and large, it is unvaccinated people.”

People cross Las Vegas Boulevard on Friday, July 2. Tourists are streaming into the city again, and gambling revenue has hit an all-time high. But that progress is threatened: Nevada this week saw the highest rate of new Covid-19 cases in the country.

Where US states stand in vaccinations

As of Saturday, 47.3% of the total US population was fully vaccinated, according to data from the US Centers for Disease Control and Prevention. Over the past week, an average of 685,472 people became fully vaccinated per day.

There are 19 states that have fully vaccinated more than half their residents. They are California, Colorado, Connecticut, Delaware, Hawaii, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia and Washington, as well as Washington, DC.

Those states plus Illinois have also reached the Biden administration’s goal to vaccinate 70% of adults with at least one dose by July 4. The entire country is 3% shy of achieving the goal, with 67% of US adults having at least one dose.

Experts agree that the best protection against the Delta variant is vaccination.

Two weeks after the second dose, the Pfizer/BioNTech vaccine is 88% effective against symptomatic infections caused by the Delta variant, Fauci said. Those who received only one dose have less protection.

Moderna’s vaccine was found in lab experiments to work against new variants including the Delta strain, the company said this week. Serum samples from people who received two shots of the Moderna vaccine showed neutralizing activity against the variants, Moderna said in a pre-print study that hasn’t yet been peer-reviewed.

Johnson & Johnson said its one-shot coronavirus vaccine provides immunity that lasts at least eight months, and it appears to provide protection against the Delta variant.

Travel guidance from the WHO

Fauci offered a glimmer of good news ahead of the Fourth of July holiday: Americans can celebrate with the proper precautions, he said.

“That is, if you were vaccinated, you have a high degree of protection. If you are not, you should wear a mask, and you should think very seriously about getting vaccinated,” he said.

However, US Surgeon General Dr. Vivek Murthy said he was most worried about people who are unvaccinated headed into the holiday weekend.

“If you are not vaccinated and if you’re going to be traveling and seeing others from various households, gathering indoors and if you’re not masked, then there is a significant risk that the virus will continue to spread,” he told CNN on Friday.

People are traveling in ever-increasing numbers: The Transportation Security Administration said it screened 2.19 million people at airports across the country on Friday. That’s the highest number since the pandemic started and higher than the same day in 2019, prior to the pandemic.

AAA expects 47.7 million people will travel by road or air between July 1 and 5, a 40% increase compared to the July 4 holiday last year.

Meanwhile, the World Health Organization’s new interim international travel guidance released Friday takes into account vaccination status as well as variants of concern and variants of interest.

The organization recommends that a ” risk-based approach” should be followed when Covid-19 measures are implemented.

“This approach should consider the risk posed by travel for the importation and exportation of cases in the context of the evolving epidemiology, including the emergence and circulation of virus variants of concern; the expansion of the COVID-19 vaccination roll-out; and lessons learned while responding to the pandemic, including on the early detection and management of cases and the application of public health and social measures,” WHO said in a statement.

WHO also pointed out that proof of vaccination should not be required as a condition to enter or leave a country and that national authorities could consider individualized approaches when implementing testing or quarantine as a condition of entry if people have immunity from vaccination or previously confirmed infection.

CNN’s Maggie Fox, Carma Hassan, Nadia Kounang, Deidre McPhillips, Cheri Mossburg and Naomi Thomas contributed to this report.