Here are a few alternative titles, playing with different angles:

Focusing on the risk:

• Flu Shot Urgency: Don’t Let the Flu Ruin Your European Summer or Cruise
• Holiday Health: Why You Need a Flu Shot Before Your European Trip or Cruise
• European Vacation or Cruise on the Horizon? Protect Yourself with a Flu Shot

Focusing on prevention:

• Travel Smart: Get a Flu Shot Before Your European Summer Adventure or Cruise
• Keep Your Holiday Healthy: The Importance of Flu Shots for European Travel
• Flu Shots for Travelers: Enjoy Your Euro Summer or Cruise Without Getting Sick

More direct and informative:

• Flu Shots and Travel: Why You Need One Before Your European Summer or Cruise
• Planning a Trip to Europe or a Cruise? Don’t Forget Your Flu Shot!

A bit more catchy:

• Sun, Sea, and Sanity: Why a Flu Shot is Your Best Travel Accessory
• Ready for Your Dream Trip? A Flu Shot is Your Ticket to a Healthy Holiday

The best choice will depend on the specific tone and target audience of the article.