Covid-19 cases are on the rise in Marin County once again, as they are in all 50 states, fueled by the spread of the more contagious Delta variant. Last Friday, Marin joined six other Bay Area counties in recommending that all residents, including vaccinated people, wear masks in indoor public settings as an “added layer of protection for unvaccinated residents.” The surge began after the June 15 reopening date and accelerated after July 4. Three weeks ago, Marin had just 45 active cases, a number that has more than quadrupled since. The county is now averaging around 20 new cases a day. The resurgence demonstrates the heightened transmissibility of the Delta variant, which now makes up most of the new cases in Marin. Dr. Matt Willis, the county’s public health officer, cautioned the Board of Supervisors on Tuesday that the surge isn’t as concerning as previous waves. “It doesn’t mean the same thing, in terms of the threat to our community,” he said, adding later: “If you’re infected, and you’re vaccinated, you’re not going to end up in the hospital.” Of eligible Marin residents, 85 percent are fully vaccinated, the highest rate in California, and those people have significant protection. With more than 200 active cases in the county, there are only three people hospitalized with Covid. With such a high vaccination rate in Marin, Dr. Willis said it is “remarkable” how much the latest surge is being driven by unvaccinated individuals, who he said are living in their own “deep purple” tier. The case rate among the unvaccinated is almost five times as high as the rate among the vaccinated. Breakthrough cases do happen: 177 Marin residents have tested positive for Covid after being vaccinated. Three have been hospitalized, but none have died from the virus in Marin. “It’s too early to say whether or not we’re going to need a booster,” Dr. Willis said, adding that data on breakthrough cases will ultimately contribute to the government’s decision to authorize one.