My intact female cat (about 3 years old) is almost due for re-vaccinations, and I'm worried she's going to be in heat soon. I've read somewhere the dogs in heat shouldn't get re-vaccinated but that seemed to mostly be due to the stress caused by being in heat. But I've also read that estrogen can have immune-boosting properties and estrogen increases during estrus and so I'm worried that her chances for an adverse reaction might go up if I re-vaccinate her while in heat. I am looking for any advice (whether from research or experience) on whether her chances for an adverse reaction might go up or if I shouldn't be worried?

To be clear, I still plan on vaccinating her, I just might wait until she goes into heat and comes out of it before I make the appointment. And also, I have NO PLANS on ever spaying her (and yes I know the health risks that might come from that) so please refrain from any lecturing on spaying her in your answers.

  • I would talk to your vet about alternatives to spaying such as hormonal contraceptives. I'm pretty sure there are ones that stop the cat from going into heat.
    – Kai
    Commented Jul 11, 2023 at 17:10
  • 1
    I plan on bringing it up next time when I go to re-vaccinate, I was just wondering if anyone had experience on this so I can feel safe going ahead to make appointment now, rather than waiting. And I don't mind my cat being in heat and have even read bad things about hormonal contraceptives, so definitely avoiding those. I just usually try to time her cycles well and might've missed the mark on this one.
    – user25771
    Commented Jul 11, 2023 at 18:42

1 Answer 1


I don't think most vaccines care where an animal is in the oestrus cycle. There might be an issue with some medications if the cat was pregnant, but the immune system should be pretty independent of hormonal cycles. (When was the last time your own doctor asked you about your cycle before recommending a vaccination?)

If still in doubt, ask your vets; they're the experts. Pick up the phone and call them, if you aren't willing to wait until the day of the check-up; most I've known are glad to answer quick yes-no questions though they may take a few days getting back to us.

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