 Shares of Acturus Therapeutics Holdings Inc. ARCT, -1.43 were down 2.3 in trading on Wednesday after the company said two doses of its experimental COVID-19 vaccine had an efficacy rate of 55 against infection. The Phase 3 clinical trial evaluated the vaccine in 19,000 people in Vietnam; it also found that the investigational shot had an efficacy rate of 95 against severe disease. 