CDC官网文献写得很清楚，推荐间隔21/28天，实在没办法最多允许42天：The recommended interval between doses is　21 days　for Pfizer-BioNTech and　28 days　for Moderna; however,　up to 42 days　between doses is permissible　when a delay is unavoidable.　(https://www.cdc.gov/mmwr/volumes/70/wr/mm7011e2.htm)

但你可能看到过类似这种标题的报道：“Delayed second Pfizer Covid-19 shot produces more antibodies: Study” (https://www.straitstimes.com/world/europe/delayed-second-pfizer-covid-19-shot-produces-more-antibodies-study)，“学术研究”好像言之凿凿，但如果点进去仔细看内容：

- ... After Britain moved to extend the interval between doses,　Pfizer and vaccine partner BioNTech said there was　no data to back up the move. However, Pfizer has said that　public health considerations　outside of the clinical trials　might be taken into consideration.

-　The study, which has　not yet been peer-reviewed,　looked at　175 people aged between 80 and 99　...　Antibodies are one part of the immune system, and vaccines also generate T cells.　The　peak T cell responses were higher in the group　with a 3 week interval between doses, and　the authors warned against drawing conclusions on how protected individuals were based on which dosing schedule they received.

读完了文章感觉标题下得有点问题？Nope. 因为所以，科学道理——几天后，小王宣布了拖长两针间隔以便更多人能打到第一针的政策。(https://www.businesstimes.com.sg/government-economy/singapore-to-delay-second-dose-of-covid-19-vaccine-to-get-shots-into-more-arms)