Title: Chlamydia Infection, PID, and Infertility: Further Evidence from a Case-control Study
in China
Version: 3 Date: 10 Dec 2021
Reviewer's report:
Thank you for an interesting study focusing on the relationship between chlamydia and PID and
infertility.
The main finding of an increased risk of infertility following a past episode of PID is not novel
however the study provides updated evidence for this relationship through comparing infertile
cases to two control groups (pregnant women and non pregnant women attending obs gyn
services). As a case control study examining infertility that has a long timeframe from exposure
to outcome the main limitations are selection of controls and potential for recall bias. Some
additional detail about how these were addressed would be useful.
My comments for improving the manuscript are as follows.
• Case selection – could additional detail be provided about how the infertile cases were selected
and the settings where they were recruited from.
• Controls – the use of two control groups (pregnant and other women seeking obs/gynae care)
is a strength – however it is possible that some of the non pregnant controls could be infertile.
Could further detail be provided of selection of non-pregnant controls and how they differed
to cases.
• Could we be given detail about how the questionnaire was administered – was it online, paper
based, with a researcher.  What were the questions asked to ascertain past STIs / diseases
• The sentence on line 112 is unclear, could the authors clarify how asymptomatic and
symptomatic infection were defined.
• Line 128-130 Please clarify the explanation for the sensitivity analysis involving effects on
primary infertility and secondary infertility and why secondary infertility may contribute to
reproductive tract infections.  I am unclear why secondary infertility would contribute to a
reproductive tract infection.
• Please define primary and secondary infertility
• Sensitivity analysis - omission of infertility subtypes thought not to be associated with
chlamydia infection or PID is likely to bias the findings toward a positive association between
PID and chlamydia and infertility.  The sensitivity analysis drops some of the cases, but there
does not seem to be a similar adjustment to the controls.  Could further explanation of the
sensitivity analysis be provided to justify this approach.
Introduction:
• Considering this case control study is focusing on factors associated with the risk of infertility,
the sentence about poor obstetric outcomes “Quite a few studies have revealed a negative impact
of infertility on endometrial, myometrial, cervical, and placental alterations that underlie the
poor obstetric outcomes.” doesn’t add to the paper.
• Some of the tense should be altered to the present tense – eg  line 58 “Chlamydia trachomatis
was one of the most prevalent sexually transmitted microorganisms.”
• PID is a well recognised interim measure for monitoring the relationship between STIs and
more distant adverse outcomes, so I feel the sentence line 62 “And to some extent, PID represents
the link between chlamydia infection and infertility” understates where PID sits in the causal
pathway.
• Line 75 should be infertile not infertility women.
• Case selection – could additional detail be provided about how the infertile cases were selected
and the settings where they were recruited from.
• Could we be given detail about how the questionnaire was administered – was it online, paper
based, with a researcher.  What were the questions asked to ascertain past STIs / diseases
• Please describe how primary and secondary infertility were defined
• For the results some clarification as follows would be helpful.
• Line 148-150 and Figure S1  Why are tubal disease (34%) and endocrine disorder (20%) noted
rather than cervical, uterine, vaginal, ovarian factors (22%).  The infertility subtypes would be
better mentioned in the text of the results rather than a sup0plemntary figure.
• Line 152 – past chlamydia infection was measured by recall which may not be a reliable
measure.  Could chlamydia antibodies have been used as an objective measure.
• Fig 1 – please include numbers excluded due to male infertility and vaginal antibiotic use
• Table 2 and multivariable analysis – I find this somewhat confusing.  The results are reported
as that the infertile cases had a higher risk of past PID and past chlamydia, but the odds ratios
are reported for the controls (e.g. OR 2.51, 95%CI 1.48-4.26) which looks like non pregnant
women had higher rates of past PID.  Please alter the table so it clearly state which groups the
odds ratios refer to.
• Table 3 – similarly for table 3 it isn’t clear that the breakdown of primary and secondary
infertility is the cases.  The table reads as if the breakdown relates to the non-pregnant and
pregnant controls.  Please adjust.
• The numbers don’t always add up.  There are 510  non-pregnant and 510 pregnant women but
Table S1 refers to 596 non pregnant women and 596 pregnant women.  Please check all figures
and alter needed.
• Similarly for table S2, the heading has n=48, but there are 717 non-pregnant controls and 717
pregnant controls.  It is unclear where these numbers come from.
The discussion would be strengthened by discussion about
• the role of other pathogens in PID and PID where no pathogen is detected (up to 70% of PID
cases have no pathogen detected)
• whether the PID cases among cases and controls were diagnosed and treated PID cases, or
something that was recalled based on symptoms.
• The issues in PID diagnosis, potentially the recall of PID could be confused with investigation
of causes of abdominal pain or for infertility 0 thereby biasing recall of a PID diagnosis in cases.
• The policy / screening implications are not specifically supported by the findings.