Marin County health officials unveiled a new coronavirus data dashboard on Monday to help residents keep track of variant activity in the region, following a sharp rise in infections driven by the highly contagious delta variant.
Delta, thought to be up to twice as contagious as the original coronavirus, made up about 70% of cases over the past 30 days that underwent genomic sequencing in the county, according to data collected by the health department.
Marin County reported 17 new infections on Friday — the highest single-day total since mid-April, according to Chronicle data. The rate of new daily cases has been steadily climbing since the state reopened June 15, to about 3.5 cases per 100,000 residents this week, from a low of fewer than 1 case per 100,000.
“If you look back, you can see there’s obviously a concerning trend toward more variants of concern starting in late March,” said Dr. Matt Willis, the Marin County health officer. “We thought it was important for the community to see that we are seeing more and more variants that are more infectious.”
Even with 92% of all county residents receiving at least one dose of vaccine, county officials said they have seen a steady uptick in cases and hospitalizations over the past month.
Twelve people have been hospitalized with COVID-19 in Marin County since June 1. All but one were not vaccinated. The county has seen a major drop-off in deaths; the two most recent fatalities, reported in May, were people who had not been vaccinated.
The new dashboard traces the progression of emerging variants back to Jan. 1, and it makes clear the rise of the delta variant since it was first identified in the county in April. In three months it’s climbed from making up 20% of cases sequenced to more than 80% last week.
The county is currently doing genomic sequencing, the process used to identify variants, on about half of cases.
Officials said they have also detected the so-called delta-plus variant of the virus, which may be even more easily transmissible than delta, though the mutation is not as well understood as delta.
“There’s a reason to watch this closely. We are all learning that the behavior of these variants warrants watching the impact on the population,” Willis said. Genomic sequencing, and a public dashboard tracking variants, is “the best way we’re going to know what this variant is doing in this community.”
“It’s important for the public to follow along with us,” he said. “There may be changes in policy that come out of this. We might need to rethink our policies around facial coverings and a possible third dose of vaccine.”
He added that the rising cases attributed to delta validates the state’s decision last week to require masks in K-12 classrooms in the fall, contrary to guidance from the Centers for Disease Control and Prevention that said vaccinated students and staff do not need to wear them.
Nationally, the delta variant makes up more than half of all cases, and it’s leading to surges in infections, along with climbing hospitalizations, in many regions. Places with very low vaccination rates are being especially hard hit.
The variant is thought to make up roughly 40% of cases in California.
“This variant has legs that go beyond previous variants,” Willis said.
Aidin Vaziri is a San Francisco Chronicle staff writer. Email: avaziri@sfchronicle.com