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I recently adopted a female cat who had been a stray. After a few weeks, I noticed signs of pregnancy and have confirmed that the cat is pregnant with 6 kittens. ~2 weeks into gestation before the pregnancy was known, she received the FVRCP vaccine, which is not recommended during pregnancy due to the possibility of cerebellar hypoplasia occurring in kittens. The vet has looked at an ultrasound as well as X-rays and believes the kittens are healthy, but hasn't given any sort of likelihood that the vaccine will cause problems in the kittens.

What is the likelihood that a cat vaccinated early in pregnancy for Panleukopenia will have kittens affected by cerebellar hypoplasia? Is there any research published about probability of birth defects, or is it assumed that any kitten born from a vaccinated mother will almost certainly have abnormalities?

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The University of Wisconsin-Madison School of Veterinary Medicine has a Guidebook on Feline Panleukopenia for animal shelters:

https://www.uwsheltermedicine.com/library/guidebooks/feline-panleukopenia/vaccination

Quoting some excerpts here in case of link rot:

For pregnant cats expected to carry kittens to term, balance the risk of inducing abortion or birth defects (reportedly very uncommon with currently available vaccines) against the risk of death of mom and kittens from virulent disease. Additionally, vaccinating the queen for the respiratory viruses contained in the FVRP vaccine can confer some protection to the kittens by generating maternal antibodies. In most cases, the benefits outweigh the risk of vaccination. However, for pregnant cats seized as part of a legal case or cats in a shelter where the risk of panleukopenia is extremely low, a killed vaccine may be preferable.

This seems to indicate that the risk is generally low for modified live panleukopenia vaccines, and lower still for killed vaccines.

From their vaccination recommendation:

Vaccinate pregnant cats unless part of a legal case or where the risk is extremely low.

From their vaccination warnings:

Vaccination with MLV FVRCP in pregnant cats may cause birth defects or abortion. However, this has rarely been documented. This risk must be balanced against the risk of panleukopenia in the shelter in deciding whether or not to vaccinate pregnant cats.

Emphasis mine in all quotes. The linked resource also has links to material of the AAFP Vaccination Guidelines, which might yield further information, or studies.

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