Is it normal for my 5 month old to coo and babble only when she is by herself?

My 5 month old does coo and babble but only when she is by herself. When we talk to her, she makes eye contact and smiles but she doesn't respond back with coos and babbles. Is it normal? She used to be very talkative until 4 months old and then she stopped. She is not vaccinated so there is no chance of having any harm from them

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