If I’ve gotten the first dose of the Moderna or Pfizer vaccine is there any flexibility in the 21 or 28 day waiting period to receive the second dose? And, if I missed the time window do I have to start all over again?

Timing of Your Second Shot

The timing between your first and second shots depends on which vaccine you received.

If you received the Pfizer-BioNTech COVID-19 Vaccine, you should get your second shot 3 weeks (or 21 days) after your first.

If you received the Moderna COVID-19 Vaccine, you should get your second shot 4 weeks (or 28 days) after your first.

You should get your second shot as close to the recommended 3-week or 4-week interval as possible. However, your second dose may be given up to 6 weeks (42 days) after the first dose, if necessary. You should not get the second dose early. ​There is currently limited information on the effectiveness of receiving your second shot earlier than recommended or later than 6 weeks after the first shot.

However, if you do receive your second shot of COVID-19 vaccine earlier or later than recommended, you do not have to restart the vaccine series. This guidance might be updated as more information becomes available.

Source: CDC