By Betty Yu

SAN FRANCISCO (CBS SF) — On Friday, the Centers of Disease Control and Prevention warned that fully vaccinated Americans can transmit the delta variant as easily as those who are unvaccinated, and it is more transmissible than the common cold.

A new CDC report examined a cluster of COVID cases in the Massachusetts resort community of Provincetown, all traced back to celebrations in early July.

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Of the 469 positive cases, 74% were fully vaccinated.

“I believe what the Provincetown study showed us is that these vaccines are highly effective in preventing severe disease and death,” said UCSF Infectious Disease Expert Dr. Monica Gandhi. “The second part of the study that what it showed us is that it is true that the delta variant is different than the alpha variant, and there are higher viral loads in the noses than there ever was with the alpha variant. Those higher viral loads are occurring even when you’re vaccinated.”

Dr. Gandhi said if you’re vaccinated, you’re unlikely to be as infectious for as long as someone who is unvaccinated, because your immunity should fight the virus.

About 35,000 vaccinated people per week are coming down with what’s called breakthrough infections.

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But that’s only a tiny fraction, considering that’s out of 164 million people whoa re fully vaccinated, or .02%

“There are more mild breakthrough infections in the case of the delta variant. That is why you should be masking indoors even with vaccinated people, until the delta variant passes,” added Dr. Gandhi.

Scientists say the vaccine is still our best weapon to fight the pandemic. In fact, the CDC says 97% of those hospitalized with covid have not been vaccinated.

San Francisco’s indoor masking mandate is expected next week.

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“I know that I have been vaccinated and I have taken all the precautions, but it would really suck because of how much more contagious this version is, to not take a simple precaution,” said Dennis, a San Francisco resident who declined to give his last name. “So I’ve already started wearing masks indoors again in most places.”