[mdj930]

Originally Posted by Steve Carlton

Looks legit, but doesn't make a lot of sense, except for the effect of the immunity fading over time. Looks like the source isn't very reliable.

https://retractionwatch.com/2015/08/17/64-more-papers-retracted-for-fake-reviews-this-time-from-springer-journals/

Let me help you out Steve. First let's forget this springe-journal you researched I have no idea what that is. The paper was submitted to the European Journal of Epidemiology. that paper was submitted by S. V. Subramanian and Akhil Kumar. If you go to the article you can just click on the names and you are directed to who they are.
The references they used to write the article and authors are on the bottom of the article.
Now since all that was right in front of you and all you saw was springer link and missed all the important information.
I suggest you go back and read the paper with an open mind.
Researching the Springer journal after reading the article makes no sense.
Mike