The Delta surge did not treat all of San Francisco equally, and new data shows just how significantly the neighborhood-by-neighborhood impact changed between the surge last winter and the one driven by the newer variant after much of the city was already vaccinated. The data points to a key factor in Delta's spread, which was younger people out socializing more just as the variant began spreading in earnest.

For some SF residents, especially people working in nightlife this summer, it seemed like the surge in breakthrough infections was a lot more prevalent than experts were saying across the country. And now, a new data analysis by the Chronicle of neighborhood case rates across two time periods — December 15, 2020 to February 15, 2021, and July 1 to September 18, 2021 — finds that your likelihood of getting a COVID infection this summer in the Castro and Haight-Ashbury neighborhoods was far greater than it was last winter.

Also, neighborhoods that were hardest hit during last winter's surge, like Visitacion Valley, had far fewer infections when Delta came around.

The calculations by the Chronicle are based on per-capita daily case rates during each surge, based on each neighborhood's population. And they found that in the Castro, the average daily probability of getting COVID between July and mid-September was 73% higher than it was in the winter surge. In the Haight, it was 56% higher, and in Hayes Valley, 32% higher.

Meanwhile, the Excelsior saw a 62% drop in daily probability of infection between the two surges.

By way of explanation, UCSF infectious disease specialist Dr. Peter Chin-Hong points to the fact that neighborhoods like Visitacion Valley and the Excelsior are home to more essential workers who were among the first infected and most exposed in the winter surge. But the Delta surge affected more affluent, younger San Franciscans, and those who were out partying this summer despite some restrictions.

Dr. Chin-Hong notes to the Chronicle that bars in the Castro and elsewhere were "jam packed" many nights despite the city reimposing mask rules in late July.

Also, earlier data showed that SF residents aged 21 to 34 were less likely to be vaccinated, though it's not clear if that was the case in these neighborhoods in particular.

"You have the conflagration of feeling invincible, plus lower vaccination (rates), plus going out and about in indoor settings with a lot of people," Dr. Chin-Hong says.

This explanation doesn't explain why relative case rates didn't rise this summer in places like the Marina or North Beach, where bars were also pretty packed. But perhaps that's because more people go out there who go home to other neighborhoods or live outside the city?

These figures also belie the fact that COVID cases remained prevalent in some parts of the city, like the Bayview, through both surges. Case maps from the city's Department of Public Health show how new cases in the last two months have continued to be most prevalent in the Bayview/Hunters Point, with SoMa coming in second ahead of the Castro. The Bayview saw 825 recorded cases between July 24 and September 22 — or 218 new cases per 10,000 residents — while the Castro saw 369 cases — or 164 per 10,000 residents.

Comparatively, a map of total cumulative cases in the city shows how the Bayview remains the hardest hit neighborhood overall, followed by the Tenderloin, Visitacion Valley, the Mission, and the Excelsior.

What this could mean as we enter the winter holiday period is anyone's guess — and hopefully vaccination rates continue to rise among the young and reluctant.

Across the Bay Area, daily new cases have been leveling off for several weeks now, and hospitalizations stemming from the Delta surge continue to fall steadily.

San Francisco's seven-day rolling average of new daily cases has plateaued recently around 100 — far higher than the low of 10 we saw in mid-June, though it may continue trending downward.

Photo: Max Templeton