This paper summarizes the main knowledge on mRNA vaccines in September 2021. The only contraindication for a 1st dose of vaccine is an allergy to one of the components of the vaccine, but a specialized consultation is possible for an eventual split vaccination under medical supervision. Serious side effects are rare and consist mainly of myocarditis, shingles and appendicitis, but the risk/benefit ratio is always favorable for vaccination. Efficacy against severe COVID-19 is > 90 % after 6 months, and this against all variants. It is recommended to vaccinate pregnant women. A 3rd dose is not recommended at this time, except for immunosuppressed individuals without detectable antibodies after 2 doses. Vaccine mixing is possible, including with a viral vector vaccine.