Propiolactone (β-propiolactone; 2-Oxetanone) is a viral chemical inactivator that causes the infectious inactivation of viruses. Propiolactone was co-incubated with SARS-CoV at a ratio of 1:1000 (v:v) and used as a bacteriostatic agent to formulate the BPL-inactivated influenza virus vaccine (Flu-BPL) [1][2].