I've read both his post and this one and feel that it's possible he's made another mistake in his interpretation of herd immunity. Herd immunity should not count the interlopers (tourists) regardless of their percentage. Herd immunity deals with the percentage of a fairly stable group that need to be immuno-competant, such as from a vaccine, previous exposure, etc..., so that an exposure to a pathogen does not gain a foothold. So if this is in the 80%, it is never meant to convey that no one in the "herd" will get sick. Obviously, if one of the 20% on non immuno- competent , they are at risk of getting infected. So herd immunity is not a zero illness issue. It is instead, a statistical issue where if there is exposure, it doesn't sweep through the "herd" or population, because even if one person is exposed, whether from a tourist or a bat, the people who interact and surround that person will not get sick and thus participate in the stifling of the infection. Variables that will change the ultimate exposure and thus illness include how far and often a sick person travels within a "herd" but at a certain number (such as the 83-94% presented) the infection will become self limiting. This herd immunity is a barrier whether there are 1 or 1 million tourists or 1 or 1 million bats that might expose the "herd" to infection. Lower numbers of immune individuals will allow for a more rapid and devastating spread through the population because more sick individuals represent an increased risk that they will expose other vulnerable victims. Herd immunity really has nothing specific to do with vaccinations, other than this is a way we can affect herd immunity. Immunity levels in a population from other causes, such as sublethal exposure will result in the same protection of it's immune naive members through statistics... The disease causing agent is simply less likely to come in contact with vulnerable people if it is surrounded by people who can't continue the transfer.
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100% agreed here
Another 100% agreement here!
I didn't want to argue about something we can't prove as easily when I just could show him that even with counting tourists as well, it would work.
Showing a clear logic mistake seemed easier than debatting over another thing to me.
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!