Tourists act as herd members for the purpose of calculating numbers of potential disease vectors, but as they do not stay and get vaccinated, they do not increase the size of the herd.
This makes their impact far larger than if they were members of the herd, as they increase the vector surface as if they were members of the herd, but do not increase the size of the herd for the herd immunity calculation.
They can drop off their diseases and leave.
Again, it doesn't matter if you are vaccinated or not. It also doesn't matter where you are from. If you are there, you are part of the herd for that time. The disease doesn't care who you are. It only cares if you are immune or not.
If you are there and you are not immune, you can spread the disease.
If you are there and you are immune, you won't spread it and possibly act as a wall to protect others.
Your origin doesn't come into play here.
It isn't the origin alone of the tourists that matters (although it is of critical import that they are potential sources of non-native virulence), it is their impermanence. All of them are potential attack vectors for disease. Each of them might communicate a disease to immunocompromised herd members, instituting epidemic, and then leave, no longer contributing to the herd - except as a vector.
Since epidemic isn't simply an event that happens at an instant, it isn't the total number of tourists who are present at any one time that is the number of attack vectors weakening the herd that is relevant, but rather the total number of tourists that impact the herd that count for that purpose.
Were the tourists to stay for long periods of time, the rate of immunocompetence of the tourists would impact the development of epidemic. Since they do not, it is the increase in attack vectors alone that comprises their impact on herd immunity.
In the event of epidemic, only those tourists present at the time, a calculation impossible to make absent more information, or blind estimations, would be included in the herd.
I concede this does modify my original statement.
Thanks!