At your first two visits, you will receive 2 injections of either the study vaccine or a placebo, which is an injection without the active ingredient. These visits will last up to 2 hours. At some study visits, we will take samples of your blood and saliva. Your phone visits will last about 10-20 minutes. You will also be asked to fill out an electronic diary to record any side effects you experience after your injection and to monitor for COVID-19 symptoms.