You only need to get one dose of the Johnson & Johnson (Janssen) vaccine to be fully vaccinated. While the Johnson & Johnson (Janssen) vaccine is a single-dose vaccine, CDC recommends that people 18 and older receive a booster dose if at least 2 months have passed since getting the vaccine. A booster dose is another dose of a vaccine that is given to someone who built enough protection after vaccination, but then that protection decreased over time (this is called waning immunity). You should get a booster if you are 18 or older and at least 2 months have passed since your single-dose Johnson & Johnson vaccine. For primary and booster vaccination, people should receive an age-appropriate mRNA vaccine (Pfizer or Moderna). The Johnson & Johnson vaccine is authorized for anyone age 18 and older if no other approved vaccine is available. See additional recommendations for people who are moderately to severely immunocompromised.

The CDC has authorized a second booster shot using an mRNA vaccine for those who are:  

• 12 and older with weakened immune systems and received their initial booster at least 4 months ago
• 18 and older and received the Johnson & Johnson vaccine for their primary series and received a Johnson & Johnson booster dose at least 4 months ago
• 50 and older and received their initial booster dose at least 4 months ago