CDC指南說：第二針應該第一針後在六周（42天）以內

https://www.cdc.gov/coronavirus/2019-ncov/vaccines/second-shot.html

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.