3 ms·

Thanks for explaining. The first reason makes sense. The second doesn't, though. I understand that if an entire generation wasn't immunized, then that genera

by endtime 18y ago

Thanks for explaining. The first reason makes sense. The second doesn't, though. I understand that if an entire generation wasn't immunized, then that generation is susceptible when they are old (as at any age), but I don't see why, in the general case, older people would be more vulnerable.

timr 18y ago
If you only immunize 50% of each generation against a virus, then you slow the transmission rate of the virus, but you also allow the number of susceptible individuals to grow over time. Eventually, the number of susceptible individuals reaches a critical threshold, and outbreaks occur. Because this process takes years, by the time the outbreaks begin, the average age of the un-immunized population has increased. On the other hand, if nobody is immunized, most people will acquire the virus at a young age, become naturally immune (or die), and the average age of incidence will be low. Here's a link to a slideshow that explains the concept in greater detail (I've started it at a slide that discusses the Greek rubella outbreak, but if you want the mathematical models, go back to slide 1): http://www.pitt.edu/~super1/lecture/lec1181/023.htm http://www.pitt.edu/~super1/lecture/lec1181/023.htm