How necessary is the need for a gap or 12 or so weeks between vaccinations?

Here in Finland they are working on a gap of exactly 12 weeks....the appointment time for the 2nd vaccination is for the same day and same time as the 1st vaccination.     But on another forum someone in USA mentioned that her husband was getting the second vaccination just three weeks after the first one.    I am wondering if that is OK, or is the rush to get everyone their 2nd vaccination (and so more freedom of movement) going to result in a lower % of cover?

ps.  As I will be getting my first vaccination this coming Wednesday, then the 12 weeks until the second, and then 2 more weeks until the body responds, I still have to be extra careful until the end of July!

Here in the USA at least Virginia my husband they scheduled the 2nd Pifzer vaccine exactly three weeks from the first. Due to his age and uncontrolled  diabetes and heart disease. I got mine first in Jan and then nine weeks later because they tested me before giving the first shot to see if I had antibodies. Which I did.so they told me to wait so others could get their first dose due to shortages at the time.  I had gotten mine earlier because I had agreed to be part of a study and I am a teacher

I believe 3 weeks is the minimum and 12 the maximum.   Using the maximum time gives you the chance to vaccinate greater numbers with at least 1 dose earlier.  It is believed to cut the death rate earlier.  

Thanks for that answer Techno.  I feel a lot better, and most internet friends seem to have had a shorter time between doses.

ps.  the needles are a lot sharper/thinner than they used to be....I could hardly feel it go in.  (although I can feel the arm now the next day)  

pps  I was given the AstraZeneca vaccine.  According to the information sheet I was handed so far over 47 million AZ doses have been given world wide.