Cycles are nominally 28 days. That's a mean. The tails of the curve are where this can happen, and for instance, while my Moderna doses were supposed to be 4 weeks apart, my followup was scheduled at 24 days by the clinician. No, I'm not menstruating... but you can see how the timing might work.
How would a women receive 2 doses in one and the same cycle? Think about it….
Cycles are nominally 28 days. That's a mean. The tails of the curve are where this can happen, and for instance, while my Moderna doses were supposed to be 4 weeks apart, my followup was scheduled at 24 days by the clinician. No, I'm not menstruating... but you can see how the timing might work.
So, an example of not following the minimum manufacture recommendations; Moderna = 4 weeks (28 days); PfizerBioNTech 3 weeks (21 days).