A Trial of a Next Generation COVID-19 Vaccine Delivered by Inhaled Aerosol

The goal of this clinical trial is to study the safety of a new inhaled vaccine to prevent COVID infection and learn about the immune responses that are made in the lungs and the blood after vaccination. Participants will be randomized (like the toss of a coin) to receive the experimental vaccine or a placebo (a look-alike solution that contains no vaccine). To be in the study participants will have to have already had three doses of a messenger ribonucleic acid (mRNA) COVID vaccine and be generally healthy. Participants are given a single dose of the vaccine by breathing in a fine mist that goes directly into the lungs. During follow-up participants will: * visit the clinic for checkups and blood tests at 2, 4 and 8 weeks after vaccination * report their symptoms for 24 weeks after getting the vaccine. In some participants, the researchers will collect cells from the lung 4 weeks after vaccination (a test known as a bronchoscopy).