Shares of Inovio Pharmaceuticals Inc. /zigman2/quotes/202993817/composite INO -5.95% tumbled 10.8% in premarket trading on Tuesday after the company said its COVID-19 vaccine candidate was considered safe and well-tolerated in all 40 of the participants in a Phase 1 clinical trial. However, the company did not disclose data about the participants' antibody response, though it said 94% of 36 trial participants showed "overall immunological response rates based on preliminary data assessing humoral (binding and neutralizing) and T cell immune responses." (Three trial participants tested positive for COVID-19 and one left the study for non-clinical reasons.) There were some adverse events, though none were serious, that primarily consisted of redness on the skin where the experimental vaccine was administered. Inovio also said it has added older participants to the early-stage trial; the original 40 are between the ages of 18 and 50 years old. It also plans to start a Phase 2/3 study this summer. Inovio is one of a handful of companies in the U.S. studying whether their experimental vaccines can prevent infection with the coronavirus. Shares of Inovio have soared 860.3% year-to-date. The S&P 500 /zigman2/quotes/210599714/realtime SPX -2.37% declined 5.5% since the start of the year.