San Francisco health officials Friday said the highly infectious delta variant has led to a ten-fold increase in cases in the city since the beginning of June, including breakthrough cases among vaccinated individuals. But case rates among the unvaccinated and partly vaccinated were more than double and hospitalization rates eight times higher than among the fully vaccinated. Even places without substantial or high COVID-19 transmission, the study added, “might consider expanding prevention strategies.” That could include “masking in indoor public settings regardless of vaccination status, given the potential risk of infection during attendance at large public gatherings that include travelers from many areas with differing levels of transmission.” Despite the alarming news on transmission, public health experts on Friday stressed that the evidence is clear: Vaccines are still very effective at protecting people against the delta variant from severe illness or death.
“High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus,” CDC Director Dr. Rochelle Walensky said on Friday. “This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.” Calls for renewed universal masking took on new urgency Friday when the CDC released a new study on a massive COVID-19 outbreak in Cape Cod that showed similar “viral loads” in vaccinated and unvaccinated people infected by the delta variant.
The new study, a collaboration between the CDC and the Massachusetts Department of Public Health, sounds an even louder alarm. Event organizers and local health officials, it said, “should continually assess the need for additional measures, including limiting capacity at gatherings or event postponement, based on current rates of COVID-19 transmission, population vaccination coverage, and other factors.” The CDC earlier this week revised its mask guidance from mid-May that vaccinated people no longer need to wear masks in most settings and recommended those in areas of substantial or high infection rates wear them indoors. Bay Area health officials already had recommended but not ordered masks for all indoors, as the six-county region recorded 8,233 new cases in the last week — a 32% increase from the previous week and more cases than the peak of last summer’s surge.
While local health officials reported a 14-day average rate of zero cases per 100,000 people on July 3, that quickly swelled to 177 per 100,000 by July 17. Those infected, nearly half of whom were residents of the town, reported attending densely packed indoor and outdoor events at venues that included bars, restaurants, guest houses and rental homes. And 346 of them, or 74%, were fully vaccinated, as were four of the five people hospitalized, who ranged in age from 20 to 70. Two of the people who were hospitalized had underlying health conditions. No deaths have been reported. The new study examined 469 COVID-19 cases among Massachusetts residents associated with multiple summer events and large public gatherings July 3 to 17 in the Cape Cod resort of Provincetown.
“This failure to consider data objectively was a major cause of the premature lifting of mask recommendations in May and the subsequent two months of overoptimistic assessments of vaccine efficacy against disease from delta,” Lin said. Still, Dr. Michael Lin at Stanford University said the CDC should have recognized the delta threat earlier based on evidence from other countries where it was rapidly spreading.
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