"As the Delta variant comes in you are at very high risk," said San Francisco Public Health Director, Dr. Grant Colfax. "This is not a good time to not be vaccinated."
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UCSF epidemiologist Dr. George Rutherford is concerned about large unvaccinated groups gathering and potentially spreading the highly transmissible strain.
"I think as a state it's quite possible we can backtrack if we're not careful," Rutherford said. "It's probably 30 to 40 percent more transmissible than the Alpha and U.K. variant that we've been worried about for so long."
The state's masking guidance recommends even vaccinated people stay masked when attending mega-events -- which include 5,000 or more people indoors or 10,000 people outdoors.
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"I'm deeply concerned about this Fourth of July holiday," said Dr. Lisa Santora, Deputy Health Officer for Marin County. "I would not be surprised if Marin County goes from 3 cases a day to 20 cases of COVID a day."
Santora says more than 90 percent of the county's eligible population has been vaccinated with at least one dose. But, she says it only takes one unvaccinated pocket to trigger a serious outbreak.
"Not everyone is following the rules," she said. "People who are not vaccinated are not wearing masks and that's what is increasing the risk of transmission in our community."
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Santora also recommends families gathering for large fireworks celebrations with children 12 and under who aren't yet eligible for the vaccine to all remain masked.
As for those who are eligible and haven't yet -- it's not too late.
"The most important message ahead of Fourth of July is to get vaccinated today," Santora said. "You won't have full protection, but you will have some."
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