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-　The study, which has　not yet been peer-reviewed,　looked at　175 people aged between 80 and 99　...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)