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Hello, thanks for your question today! I am Dr. Drew, and I am a licensed veterinarian. I'll be happy to help you in any way I can.
It's fairly common for cats to become a bit lethargic after receiving a vaccine.
the vaccine stimulates the immune system, in order to produce protective antibodies,
which leads to a feeling of mild illness.
I would not expect that this is a true illness or a "flu" as you mentioned,
but rather a normal immune response to the vaccine.
You can try giving a dose of Diphenhydramine (12.5mg, every 12 hours), this is sold without a prescription.
this may help reduce some mild vaccine reactions.
If there's no improvement in Molly's condition by tomorrow, then a veterinary exam would be recommended.