It seems that young females (comparing age 15 vs age 24) are 20x more likely to get chlamydia from a single unprotected sexual encounter with an infected male. What are the reasons for this? Are there other diseases with such extreme age-dependence?



2 Answers 2


The "Description of the analyzed data set" section seems suspicious to me:

In a nutshell, young women between the ages of 14 and 17 years old were identified and recruited by the project.


At each follow-up visit during the study period, enrolled participants underwent STI testing and treatment. Also at each follow-up visit, the participants received follow-up interviews, in which they were queried about the number of unprotected coital events since previous visit. Most of the participations did not visit the clinic every quarter and left the project before its completion.

For our analysis, we collected 1173 quarterly test results from the first 200 participants who were never infected and have completed at least two follow-up visits. Their average enrollment age was 15 years with standard deviation of 1.1 years.

It means that data is biased as for some reason most of girls were not able to participate in the study.

And I'm not sure about the correctness of your interpretation. I'd interpret it as "females are 20x less likely get chlamydia infection after being treated and warned about STD" as there were relatively continuous followed-up 15-17 y/o females and no older females. And I guess it was one of the goals of this "Young Women’s Project".

  • $\begingroup$ I'd like to simply ignore this paper too, but I'd also like to believe that the authors would have enough integrity to recognize and stop the error first! I had thought there were other sources showing age-dependence, but now can't find them... $\endgroup$
    – bobuhito
    Jun 14, 2016 at 13:39

In this case the authors appear to imply that the mechanism is primarily the accumulation of partially protective immunity resulting from earlier infection.

From Table 1, we observed that 1) [p-bar] monotonically decreases as age increases; 2) [p-bar] decreases as q increases. [...]

The first observation support the concept that some degree of protective immunity against reinfection develops after first-time infection, although it appears to be partial at best [53, 54]. The second observation supports the concept that acquired protective immunity may restrict chlamydia replication in older persons [55].

There are quite a lot of diseases (infectious and non-infectious) with strong age-dependence - sometimes positively correlated, sometime negatively. For example, most severe disease associated with Plasmodium infection (such as cerebral malaria and severe malarial anaemia) occurs in infants and young children (source), while with chicken pox (varicella zoster virus) infection, adults and teenagers (or very young children) are more likely to develop severe disease and complications (source).


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