Syringes filled with the Pfizer-BioNTech Covid-19 vaccine. FREDERIC J. BROWN/AFP via Getty Images Last September, as Covid-19 vaccine candidates were rapidly advancing, Katherine Park and six of her fellow school nurses in St. Louis County, Mo., envisioned school-based vaccination sites as an extension of the district’s pandemic response plan, which they had been working on for months. They reached out to the local health department, letting it know the district had buildings for use and more than 30 school nurses who could jump in on administering shots. “Honestly, our health department here was kind of surprised that we even reached out to them,” said Park, who is also the interim director of health services at Parkway Schools, a public school district in western St. Louis County. “It’s almost like they had never really considered they could utilize us.”