Guangdong CDC published in preprint a critically important study of the effectiveness of China’s inactivated vaccines against the B.1.617.2 (Delta) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) — a strain of the virus that causes coronavirus disease 2019 (COVID-19) and is circulating globally (1-2). The Delta variant managed to get through China’s strong international border quarantine protection and spark an outbreak in Guangdong Province in May–June 2021. While the outbreak was being stopped using guidance of the Protocol for Prevention and Control of COVID-19 (3), the investigators took advantage of provisions in the Protocol that identified close contacts of anyone infected with SARS-CoV-2, quarantined the close contacts, and tested them for infection while in managed quarantine. Guangdong CDC determined the vaccination status of these close contacts and which of the close contacts became infected, allowing an estimate of the effectiveness of the vaccines used in Guangdong against COVID-19 caused by the Delta variant. The study design was elegant, as the investigators were able to evaluate vaccine effectiveness (VE) in the population with the highest force of infection in the outbreak area, maximizing the possibility of making an accurate estimate of VE.