A group of vaccinated beachgoers changed our knowledge of the delta variant.
Before Provincetown, health officials had been operating under the assumption that it was extraordinarily rare for a vaccinated person to become infected with the virus.
When a vaccinated person gets infected with delta -- called a "breakthrough infection" -- "the level of virus in their nasopharynx is about 1,000 times higher than with the alpha variant," Fauci said in an interview Wednesday with MSNBC.
"In recent days I have seen new scientific data from recent outbreak investigations, showing that the delta variant behaves uniquely differently from past strains of the virus that causes COVID 19," CDC Director Rochelle Walensky told reporters Tuesday announcing the new recommendations.
"Information on the delta variants from several states and other countries, indicate that in rare occasion some vaccinated people infected with a delta variant after vaccination may be contagious and spread the virus to others," she added.
"What we've learned â€¦ is that when we examine the rare or breakthrough infections and we look at the amount of virus in those people, it is pretty similar to the amount of virus in unvaccinated people," she said.
"These early data suggest that breakthrough delta infections are transmissible," the CDC said of the research in India.