Public Health Wales is looking for healthy volunteers aged between 18 and 84 to take part.
The trial is evaluating an investigational Covid-19 vaccine to make sure it works to protect against the infection, and is safe for humans.
People will either be given an investigational vaccine or placebo.
Select patients will get a seasonal influenza vaccine and participants will be given reimbursement for reasonable travel expenses.
There will be two visits for injections – one on the first day, and one three weeks after.
Trial-related tests include physical examinations, Covid-19 testing, and blood and urine samples.
Those chosen will participate for up to 56 weeks and have six visits at a clinic, but may choose to leave the study at any time.
Anyone interested can take a questionnaire at www.covid19trial-uk.com to see if they qualify.
Covid-19 results from infection with the virus SARS-CoV-2, which can severely affect the respiratory tract.
Common symptoms include coughing, fever, shortness of breath, and sudden loss of taste or smell.
Researchers are actively trying to develop ways to prevent the spread of coronavirus, with many different trials ongoing.