Felton Music Hall owner Thomas Cussins couldn’t wait to get vaccinated last spring against COVID-19, traveling to Fresno for the jab that offered hope of vanquishing the virus that had silenced the mellow rock and reggae beats in his new Santa Cruz Mountains venue less than a year after it opened.

There was plenty of pent-up demand for the Felton Music Hall’s live music in June when it began hosting concerts outdoors at nearby Roaring Camp and inside the hall. But less than two months later, it’s closed again through at least Aug. 5 over an outbreak following a July 18 concert in the hall.

“We all felt good that with the vaccine we were kind of covered so it’s just heartbreaking that we’re back here in this situation,” Cussins said. “It was so tough to be shut down since March of last year, and to finally be able to reopen and bring live music back and for this to happen.”

The outbreak comes amid a growing recognition among national health authorities and experts that the delta variant of the virus that devastated India in the spring and is now dominant across the U.S. poses a monumental threat to the effort to end the pandemic through mass vaccination.

Before Tuesday, many California health officials already had recommended masks indoors even for the vaccinated. But Santa Cruz County’s came a day too late.

On Sunday July 18, the Grateful Shred, a Los Angeles-based band that covers the eclectic, trippy rock music of the iconic 1960s-era Grateful Dead, played inside the Felton Music Hall after performing the day before outdoors at Roaring Camp. The place was packed, with a total of 291 inside, including the six band members.

The following Saturday, Cussins received a report that someone who attended had tested positive for COVID-19, and the band posted on its Instagram page that “a number of people have tested positive for Covid who attended our California shows this past weekend.”

“Nearly all of the band and crew have also tested positive and are at home recovering with their families,” the band continued in its post. “If you were at any of these events, please get a test, and if you’re feeling sick, stay home.”

Jason Hoppin, a spokesman for Santa Cruz County, said Tuesday the county has confirmed four cases locally from the outbreak, a number that Cussins said does not include the band and crew. Hoppin said that county health officials “expect that number to go higher.” It was unclear whether the local cases involved vaccinated people.

Hoppin said county health officials just became aware of the situation and “are actively investigating” and the “scope is still being determined.”

“Breakthrough cases are expected and symptoms are typically minor, if present at all,” Hoppin said. “We appreciate the operators’ advocacy for vaccinations and precautionary masking, especially among younger adults where vaccination rates tend to lag other age groups.”

The music hall emailed all those who had attended the Grateful Shred show and said they should get tested, and put a notice on its Facebook page, indicating that one staff member may have been infected as well. Staff are awaiting results of more accurate COVID-19 tests, and will have to show they are negative before returning to work, Cussins said. No infections have been linked to the concerts outdoors, where the virus doesn’t easily spread, he said. When the music hall reopens, he said, everyone will have to have a mask.

Cussins said he’s not sure of the vaccination status of all his staff but that his hall won’t offer more shifts to them unless they’re immunized.

“We just want to really just put out a plea everybody to get vaccinated,” Cussins said. “This delta variant is no joke. It’s so important.”

It was unclear from the band’s post whether the ill band members had been vaccinated, and the band did not respond to questions through its agent. But the post suggested at least some of them were immunized.

“Apparently the vaccine does not prevent transmission,” the band’s Instagram post said. “But fortunately it does seem to really help with reducing sickness and hospitalization. If possible, please vax up and be sure to wear a mask whenever you are inside or close to others.”

But Instagram posts in response to the band’s announcement indicated that some fans who were vaccinated also have tested positive after attending its concerts in other towns, including Ojai and Los Angeles just days before the fateful Felton show.

“Had fever, chills, and flu like symptoms until this this Thursday, but all better now except I still have no smell,” posted one fan who goes by “gratefulstardust” and said he attended the two prior shows in Los Angeles and Ojai and tested positive despite being vaccinated. “Get vaxxed and be safe everyone!”

Fans on social media mostly seemed understanding of the situation.

“I figured that’s where I picked it up,” one who goes by “mr.shuffles” posted. “Grateful Spread! Thanks for the heads up. Hope everyone feels better.”

Lauren Ahearn Krikke, a music hall patron in Felton who attended a concert last Friday, said she wouldn’t have gone knowing there had been an outbreak the weekend before. Although she’s vaccinated, she says she now wishes she had worn a mask, which she routinely wears to stores, and is now going to get tested for COVID-19. But she doesn’t blame the music hall, which she said she loves.

“I’ve never stopped wearing a mask indoors, but I did choose to attend and not wear one last Friday,” Krikke said. “Maybe false sense of security? Maybe just wanted to have a good time!”