BERKELEY (KPIX) — Health experts are concerned the explosions over the Fourth of July holiday weekend may be followed by a boom in COVID-19 cases.

The highly contagious Delta variant is sweeping across the nation, which now accounts for one in four coronavirus cases. The variant has become the dominant strain in five U.S. states, including California.

READ MORE: San Francisco Safeway Adds Barriers To Deter Rampant Shoplifting

“If you’re not vaccinated or not completely vaccinated you should be very, very concerned,” said University of California, Berkeley Prof. and infectious disease public health expert Dr. John Swartzberg. “This is a very, very transmissible virus, much more so than anything we’ve encountered. So if you’re not vaccinated there’s a high chance that you’re going to get infected with this virus.”

The Delta variant is to blame for a surge in cases in Missouri, making the state’s case rate the highest in the nation. The state also has low vaccination rates.

California reopened last month to fewer restrictions and got rid of its colored-tiers. Since then, COVID-19 cases have crept up in almost all nine Bay Area counties, including Alameda, San Francisco and Santa Clara counties. Swartzberg said it’s the price Californians will pay for reopening.

But he said the state’s high vaccination rate will protect Californians from a surge that we saw after last Fourth of July, when cases and deaths spiked dramatically.

“There are places in the country where we have low vaccination rates where we are going to see serious problems,” Swartzberg said. “Not in the Bay Area. Frankly, not in almost all of California. We’ll see cases, we’re going to see a rise in cases, there’s little question about that. We’re already seeing it, but we’re not going to see anything that is going to be anything approaching catastrophic.”

READ MORE: SF Supervisor Preston Proposes 10-Day Warning Period For Renters Facing Eviction

The holiday weekend saw families and friends gathering, and many for the first time without masks or social distancing.

Marissa Mowen traveled to Las Vegas, Nevada for Independence Day. She said she’s traveled since last year, and has very little concern over the Delta variant.

“We had a cabana two days in a row,” Mowen said. “I hopped off the plane, didn’t wear my mask, didn’t worry about anything. Everything was good and going back to normal.”

However, Cindy Moore said although she’s vaccinated she still takes precautions. Moore also traveled to Las Vegas for her birthday.

“Being vaccinated isn’t a 100% you’re not going to get something,” said Moore. “We’re always wearing our masks, always social distancing, always making sure that we use hand sanitizer.”

MORE NEWS: Redwood City Retail Theft Suspect Arrested; Stolen Home Depot Merchandise Recovered

“The most important thing we can do is get vaccinated, but on top of that if you’re indoors with people and you don’t know if they’re vaccinated or not, just wear a mask,” Swartzberg said. “That’s going to protect you considerably, and that added to your vaccination status, is really going to help.”