"Serial Monogamy": A New STI Risk Factor?

A New Study Finds that the Risk of Sexually Transmitted Infection Persists for Months after a Relationship Ends

This week, sexually-transmitted infection rates are on the rise.

Should we worry about sexually transmitted infection rates among serial monogamists?  That’s the assertion in this study, appearing in JAMA Network Open.

This study is chock-full of tantalizing statistical tidbits, which I’ll get to in a second, but I think the conclusions are a bit premature.  Let’s walk through the analysis.

OK, we know from multiple prior studies that individuals with multiple sexual partners at the same time – the technical term here is “concurrent” partners – are at higher risk of STIs.

But what about the serial monogamists? The individuals who have a sexual relationship with one partner at a time, often separated by months of sexual inactivity? According to this study, this group (I’m looking at you, Taylor Swift) is not off the hook.

Researchers from the UK used the National Survey of Sexual Attitudes and Lifestyles to identify 8,867 sexually active individuals between the ages of 16 to 44.  The survey asked a variety of questions about sexual preference, sexual activities, sexual partners and – critically – sexually transmitted infections.

Digging into this paper reveals some really interesting trends.

First, take a look at this figure, which plots when people start a relationship with a new partner relative to the end of a relationship with the old partner.

Scientist 1: Can we somehow graphically convey the concept of “rebound booty”?

Scientist 2: Hold my beer.

On the X-axis, we have the time gap between partners. Time 0 – in the middle there - means that the very month you ended a relationship with partner A, you started one with partner B. Sorry partner A. Note the spike at time 0. On average, the gap between partner A and B was about 2 months in this study.

But there is obviously a range. Negative time gaps imply, well, that you started the relationship with partner B before you ended it with partner A.

You’d think that the longer you had a concurrent partner, the higher your risk of STI, but that is not what was found.

Full disclosure - this data was not in the primary manuscript. I sort of back-calculated the raw percentages from the reported odds ratios. But percentages are easier to understand than odds ratios so I hope you’ll forgive me.

In fact, as you can see in this chart, the risk of STI persisted a few months into a partner gap. The STI risk starts to really trend down after about a 6 month gap in men and a 4 month gap in women.

The argument the authors make is that this persistent risk of STI even among the serially monogamous means that this crowd isn’t safe, writing:

In English, they attribute their main finding to the long infectious periods of STIs like chlamydia.

But there’s a real problem with this interpretation. Do you see it? Because, really, my risk of a new STI should have nothing to do with how long it has been since I’ve been in a sexual relationship, it should be related to how long it’s been since my new partner has been in a sexual relationship – a data point not captured in this analysis.

My alternative conjecture (that it’s the new partner that really matters) is supported by another finding in the study. When asked, 40% of men and women stated that their most recent partner probably or definitely had sex with someone else during the relationship. And sure enough, the individuals who were suspicious about their partners fidelity had a higher rate of STI. Now that’s what I call biologic plausibility.

In the end, when counseling patients, it’s not that we need to worry about serial monogamists. We need to worry about everyone. We need to remind all of our patients that, just like in this study, we often don’t have as much data as we’d like on our sexual partners. So - practice safe sex, partners should get tested for STIs before entering a new relationship, and – if you meet anyone named partner A, save yourself some heartache and swipe left.

This commentary originally appeared on medscape.com.