It is completely different. For example, if I have been vaccinated by a health official, I will be given the credential about it (and remain to me alone), and also be validated on the decentralized ledger which can be verified through a form that will be given me with a specific cryptographic key which will serves as prove. The form with the specific cryptographic key key will be used to verify if truly I am recorded on the blockchain as one of the people that has been vaccinated. If I want to manipulate in this case, before I can manipulate anything, I will have to make a deal with a health official that do vaccinate people to help me validate a form with a cryptographic key on blockchain. Without this, no way to manipulate, but not compared to physical documents that I can make the fake ones from anywhere which is a lot easier.
1. The ledger will clearly not be decentralized (who would store and mine that and for what incentive?!). That's why it's not better than an ordinary database (it's actually worse).
2. If needed the API that accesses the DB can give you whatever keys and proof they implement, from something basic to cryptographic keys too.
3. I was referring to crooked officials manipulating the data. Either a DB entry is altered or added, either a blockchain entry (unalterable) is added, somebody will get a fake "passport".
All in all, while blockchain is - in theory - not changeable (depends on implementation though, it can be done bad too), as long as somebody can add new records it's not really better than a DB. Also since it's not decentralized, also a hacker can do deeper change (double spend), but indeed, that's more difficult. And since it's cheap to just add a new record, it doesn't make sense to hassle.
So I don't see a real advantage of the (again, centralized) blockchain for this use case.
This is a way our credentials (data) can be safe, it will remain with us than on a third party database, only the form will be on blockchain which has no credential of us. Unlike physical credentials in which blockchain is not used, there must be a database for collecting our vaccination data report. And the positive thing about this is that, it is the method suggested to be used, which will help protect peoples data from third party, although, still at its earliest age.
You know that databases can also be encrypted, right? You know that whoever can access the software creating the blockchain can also read/understand the info, so it's not a difference from the DB, right?