another kind of puzzle

Over a month ago, my spouse and I scheduled a visit to my doctor's office for today.  The reason was to get our flu and RSV shots before we leave on our trip, but not too early in the season.  Our appointment was for 2:00pm.

At 10:30ish this morning my husband received a call informing us that they were out of the RSV shots but we could still get our flue shots.  So we presented ourselves at the office at the appointed time and were given the extra powerful flu vaccines for older adults.  We certainly qualified for that.  They suggested that we go to our local Walgreens for the RSV shots.

As we were leaving my spouse commented that we would probably go get those shots tomorrow if we could schedule them then.  "Oh no" we were told "You need to wait at least a week between the two shots."  "But" my wondering spouse replied "We were supposed to get them both today. Why must we wait a week?"

There didn't seem to be a logical answer to this.  It's just 'the rule.'  If you don't get them at the same time, you must wait a week between the two kinds of shots.

As you might expect I Googled this and can find no explanation about the waiting period. I did find plenty of information about why it's important to get both shots but nothing about the wait time.  

It's a mystery.  Maybe someone at Walgreens can explain it next week.