If you were first in line to get your COVID-19 vaccination, you might soon need to be first in line to receive a booster shot. The Delta variant is putting the current COVID-19 vaccine to the test and some countries are already thinking beyond the first two shots by requiring booster shots from international travelers.

Croatia is the first country to set a booster shot requirement for international travelers. The Balkan nations asks that travelers provide a “certificate that they have received two doses of vaccine used in the EU Member States (Pfizer, Moderna, AstraZeneca, Gamaleya, Sinopharm) not older than 270 days, or a certificate that they have received one dose of the vaccine if the vaccine is administered in a single dose (Janssen/Johnson&Johnson) not older than 270 days, provided that 14 days have passed since they received the single dose,” according to the Croatian government website.

That means in Croatia, your vaccine card is only valid for nine months before you’re expected to receive a booster shot. If your vaccine has expired, you’ll need to take a PCR test or rapid antigen test upon arrival, and you’ll have to self-isolate until you get a negative result. If for whatever reason, you are unable to get tested, you’ll have to quarantine for 10 days.

Similarly, Austria is toughening up its entry requirement for those who are vaccinated. While fully-vaccinated travelers from the US (among other countries) are allowed to enter Austria upon presentation of a vaccine certificate, the country’s Official Travel Portal explains that “The vaccine certificate is valid for 270 days after the 2nd dose for 2-dose-vaccines. For vaccines that require only one dose, it is valid from the 22nd day for 270 days from vaccination.” After the 270th day, a booster shot will be needed.

As the Delta variant continues to spread, this could become the trend in other countries. The Biden administration has already announced its plans to begin vaccine booster doses to Americans who received their vaccine at least eight months earlier.