A safe vaccine?

My vet used a killed vaccine (no the modified live vacine) for my cat. Few weeks later we fond out she was pregnant. My vet thinks she will be OK because the killed vaccine is a low risk tipe of vaccine. Do you know more abot this type of vaccine?
Update: Dear embee,Thank you for the advice, but I'm not someone else who didn't spay the cat. I'm someone who RESCUED another cat from the street. We took het to the vet the same day we found her. She was already pregnant.
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