Oh, interesting clarification re Indian English. Learn something new every day!
Yes, the original sentence is in passive voice. Its verb is actually a phrase: 'were getting vaccinated". Similar verb phrases could be 'were getting killed' or 'were getting married'. Complicating matters is the fact that some past-tense verbs (such as married) can also be used as adjectives.
E. g. I am a married woman.
Passive voice is when the subject of the sentence has something done to them. Active voice is when the subject of the sentence is doing the acting, even if it's to receive something.
I was driven to the airport (or even, 'I got driven to the airport').
I got a ride to the airport.
There are some slight shades of difference in meaning (after all, in the second sentence, my ride might have been on the back of someone's bicycle) between the two sentences. However, in the first one, the action is by someone else - my dad, a taxi driver, etc. In the second one, I am the actor, even though I am receiving something.
He gave me a book. <-- active voice. The subject is 'he'.
I was given a book by him. <-- passive voice. The subject is 'I', but the action is performed by someone else, the person mentioned in the predicate.
I received a book from him. <-- active voice. The subject is 'I', and the subject is performing the action.