A primary rabies vaccination — meaning the first shot a pet has ever had, or the first one after any lapse in coverage — needs at least 21 full days to take effect before the pet is considered travel-eligible under EU rules. Some vaccine manufacturers recommend a longer 30-day window for full immunity to develop, so it's worth asking the vet to confirm the exact number for the specific vaccine used and have it written clearly on the certificate, since that date is what gets checked at the border, on public transport, and at airline check-in.
Timing interacts directly with the microchip rule: if the rabies shot was given before the microchip, EU rules treat it as if it never happened for travel purposes, and it has to be repeated after the chip goes in. This catches out a surprising number of long-term pet owners who assume their pet's regular annual vaccination already covers this — it doesn't, unless the chip was already in place when that shot was given.
Booster shots are more forgiving. If a pet has an unbroken vaccination history — no lapse between doses — a booster generally doesn't trigger a fresh 21-day wait; it's treated as continuous protection. But certificate validity periods vary by vaccine brand and by country (commonly one to three years), so always check the expiry date on the most recent certificate rather than assuming it's still current.
For US travellers specifically, USDA guidance notes that a rabies vaccination given in the United States is only considered a valid one-year primary vaccination for EU travel purposes, even if the vaccine itself is labelled as a three-year product — a US-specific quirk worth flagging to your vet ahead of the appointment.