Source: https://www.aafp.org/dam/AAFP/documents/patient_care/public_health/COVID19-Vaccine-FAQs.pdf and https://khn.org/news/article/when-your-chance-for-a-covid-shot-comes-dont-worry-about-the-numbers/ and https://www.washingtonpost.com/health/2021/02/28/johnson-and-johnson-covid-vaccine/ and https://www.cdc.gov/vaccines/covid-19/info-by-product/clinical-considerations.html
Three vaccines are currently licensed: One developed by Pfizer/BioNTech, one by Moderna and one by Johnson & Johnson. Having different types of vaccines available for use, especially ones with different storage and handling requirements and dosing recommendations, can offer more options and flexibility for jurisdictions and vaccine providers.
The Pfizer and Moderna vaccines are mRNA vaccines that have a piece of mRNA specific for the SARS-CoV-2 spike protein. They have similar efficacy and safety profiles.
The main differences between the Pfizer and Moderna vaccines include the ages of individuals eligible to get the vaccines, the length of time between doses, the requirements for storage and the preparation of the vaccine.
The factors above will be different for other vaccines that may be authorized soon. One important difference is that some vaccines may only require one dose.
Johnson & Johnson is an adenovirus vector vaccine.
In clinical trials, people reported fewer and milder side effects from the Johnson & Johnson vaccine.
The Johnson & Johnson vaccine is easier to administer as well—it only requires one dose and can be stored in regular refrigerators. Unused doses have a longer shelf life as well. Use of the Johnson & Johnson vaccine was paused in April while federal health officials investigated a small number of blood clots reported after vaccination. A panel of experts decided that the benefits of the vaccine outweighed the risks and cleared it for use again.