Evidence is growing that post-vaccination breakthrough cases like Johnston’s are not as rare as once previously thought, or perhaps hoped for. And the culprit appears to be the highly infectious delta variant that’s now dominating San Francisco and pretty much everywhere else in the United States.

To be clear: The vaccines are holding up when it comes to preventing the most dire outcomes, in particular hospitalization, intensive care requiring ventilation, and death. They’re also still very good at preventing infection. Vaccines remain the best protection against COVID-19 and are key to ending the pandemic.

But delta is proving to be the first variant to test the strength of the vaccines, especially when it comes to stopping transmission. And that, in turn, has led to disheartening setbacks in the public health response as cases rise faster now than at any other time in the pandemic and health officials consider new mask mandates and other measures to stop the spread of disease.

“It is COVID on steroids. In many ways this is a different virus than the virus we were dealing with earlier this year,” said Dr. Grant Colfax, director of the San Francisco Department of Public Health, in a news briefing on Friday during which he implored people who are not vaccinated to get the shots now.