While Bay Area health officials don't plan to reimplement COVID-19 guidelines for masking and social distancing for now, their concern about the virus' delta variant is increasing for unvaccinated people.

The variant, first detected in India and now in more than 80 countries, currently represents about 36% of California's new coronavirus cases and is likely to become the country's dominant strain as the year progresses. As of Thursday, July 1, Santa Clara County reported 65 cases of the delta variant.

The main concern, according to health experts, is that the delta variant is believed to be roughly 60% more transmissible than the alpha variant, also known as the UK or B.1.1.7 variant, which is currently the most common source of new infections in the U.S.

While state public health officials and the Centers for Disease Control and Prevention have both stood by their current guidance allowing fully vaccinated people to forego a face covering in most settings, the rise of the delta variant is giving some officials pause.

That pause includes the World Health Organization and the Los Angeles County Department of Public Health, both of which have advised but stopped short of mandating that fully vaccinated people should once again don their masks indoors, particularly in places where the vaccination status of others is not readily apparent.

"This is something we've been closely following since April," Marin County Deputy Public Health Officer Dr. Lisa Santora said of the variant. "We had a very proactive approach towards whole genome sequencing."

Marin County has the distinction of the highest vaccination rate in the 11-county greater Bay Area; 91% of residents age 12 and up have received at least one dose and 83.5% have completed their vaccination series.

The proportion of delta variant cases to Marin County's total number of new cases is one of the highest in the state, which Santora said is somewhat misleading since the county currently analyzes each new confirmed case of the virus to determine which variant caused the infection.

"There's probably a lot more delta activity in counties the size of Los Angeles County, where they have higher absolute numbers of unvaccinated persons compared to Marin County," she said.

Marin County's population as of 2019 was just shy of 260,000 while Los Angeles County was estimated at roughly 10.04 million, according to data from the U.S. Census Bureau.

Marin County health officials investigated a cluster of delta variant cases in late April and early May, but Santora said the cluster was almost immediately extinguished as it simply ran out of unvaccinated hosts to infect.

"Eventually, in Marin County, it doesn't take long for (unvaccinated) people to interact with many more vaccinated people," Santora said. "When you're working in a county the size of Los Angeles County, they don't have that luxury."

Initial studies have found that the two available vaccines that use messenger RNA to prime the body's immune system — those manufactured by Moderna and Pfizer-BioNTech — are upwards of 80% effective against the delta variant once a vaccinated person is two weeks past their second of two shots.

On Friday, Johnson & Johnson said the one-dose vaccine developed by its pharmaceutical subsidiary, Janssen, is also effective against the variant.

The effectiveness of the two-dose vaccines plummets, however, when a person has only received their first of two shots, according to health experts.

Even so, Bay Area health officials have used the delta variant to underscore the importance of getting vaccinated, not just to avoid transmitting the virus but to significantly reduce the chance of becoming seriously ill or dying.

"While we are not changing our (guidance) at this time, we are concerned about ongoing transmission in our county, especially amongst the unvaccinated population," Contra Costa Health Services said in a statement.

Some 72% of Contra Costa County residents age 12 and up are fully vaccinated as of Thursday, with 78.2 percent having received at least one dose.

Meanwhile, the divide between the county's average number of new cases per day per 100,000 residents over the last seven days was stark as of Tuesday: 0.5 for vaccinated residents, 8.2 for unvaccinated residents.

On Friday, state health officials added to that chorus, urging the state's residents to get vaccinated if they've yet to do so and noting that they expect the state's proportion of delta variant cases to continue rising.

"We are closely monitoring the spread of COVID-19 and its variants across our state," California Department of Public Health Director and State Public Health Officer Dr. Tomas Aragon said in a statement. "COVID-19 has not gone away. If you are not vaccinated, you are still at risk."

Breakthrough cases among vaccinated residents across the state have also been far and few between since vaccinations began in mid-December.

According to state data, only 584 of the more than 20 million vaccinated people in California have contracted the virus and required hospitalization, a rate of just 0.003%.

Marin County has identified 76 breakthrough cases to date, Santora said, but none of them have become so ill as to need to be hospitalized.

"A vaccine does not provide a forcefield," she said. "If you're exposed to someone who's coughing with COVID-19, that virus is going to get into your system and start replicating. Your body is ready to respond ... but if you're vaccinated, that's going to happen much more quickly."