While this is true, it looks entirely different when looking into the whole context of the vaccination campaign. I am a student in Germany and like many others currently enrolled in universities or for those still in school, politics didn't seem to care at all about us. The official STIKO recommendation nowadays (after they changed it twice) is that it should only be given to people older than 60 (or after consultation with ones general practitioner and if he agrees; in my experience, most don't for young patients).
After young people got (imho rightfully) outraged by a variety of factors, Minister of Health Jens Spahn suggested that young people who did not have the possibility to get an mRNA shot at the time, should get vaccinated with the AstraZeneca vaccine on their own risk and against the official STIKO recommendation.
So taking into account that government officials recommended millions of young people to get the vaccine at their own risk, Merkel (and others!) just "following STIKO guidelines" seems like a weak justification.
Nonetheless, the whole communication during the vaccination campaign, especially regarding the AZ vaccine and the change in direction, went really bad and to me, its no surprise that the acceptance of the AZ vaccine went downhill
But yeah, I can sympathize - I'm over 40, but still in the lowest vaccination priority group, and was also left wondering when my turn to get a vaccine would finally come, while the press was hyping "vaccine scarcity" - my turn came faster than I had expected BTW. Now, in only a few weeks, they have gone from that to hand-wringing about "vaccine skepticism" and what can be done to get more people vaccinated.
This is a classic example of putting a "spin" on a certain information. I'm not aware of the situation in Italy, but in Germany the current official recommendation is for everyone who got a first dose of AstraZeneca to get an mRNA shot (either Biontech/Pfizer or Moderna) as the second dose to increase the efficacy. So Merkel is not "opting" for anything personally, just following the recommendation...