Vaccines that work against SARS-CoV-2 have helped change the course of the pandemic by reducing illness and hospital admissions. But Chris Stokel-Walker asks what we know about their impact on preventing transmission. The range of vaccines developed in record time by pharmaceutical companies and research laboratories have helped quell the worst effects of SARS-CoV-2. But much of the focus of research has been on effectiveness in preventing infection, illness, and hospital admission. What is less well measured is the impact of vaccination on preventing onward transmission. Most papers to date (notably, many are preprints and have yet to be peer reviewed) indicate vaccines are holding up against admission to hospital and mortality, says Linda Bauld, professor of public health at the University of Edinburgh, “but not so much against transmission.” The first weekly covid-19 vaccine surveillance report for 20221 from the UK Health Security Agency (UKHSA) was more positive than Bauld’s assessment—but didn’t say outright that covid-19 vaccines prevent transmission. “Several studies have provided evidence that vaccines are effective at preventing infection,” it states, “Uninfected people cannot transmit; therefore, the vaccines are also effective at preventing transmission.” A study2 of covid-19 transmission within English households using data gathered in early 2021 found that even a single dose of a covid-19 vaccine reduced the …