You should wait at least 21 days before getting the second dose of the Pfizer vaccine or 28 days before getting the second dose of the Moderna vaccine. For both vaccines, you should wait no longer than 6 weeks (42 days) between doses. If more than 42 days have passed since your first dose, you should still get your second dose as soon as possible. The Centers for Disease Control and Prevention do not recommend getting a third dose if you fall outside of this time frame. For more detailed information, visit https://www.cdc.gov/vaccines/covid-19/info-by-product/clinical-considerations.html.