Men who have sex with men (MSM) starting pre-exposure prophylaxis (PrEP) in King County, Washington, which includes Seattle, have sexually transmitted infection (STI) diagnosis rates 20 times greater than the overall local MSM population, aidsmap reports. Evidence as to how starting PrEP may have affected STI rates among this group of men is inconclusive.

Researchers analyzed data on 218 HIV-negative MSM who started Truvada (tenofovir dixoproxil fumarate/emtricitabine) as HIV prevention at a Seattle PrEP clinic between September 2014 and June 2016 and completed a baseline behavioral questionnaire.

Findings were presented at the 2017 Conference on Retroviruses and Opportunistic Infections (CROI) in Seattle.

A total of 108 of the men completed at least one follow-up visit (they were scheduled every three months). Seventy-six of them made the baseline visit as well as three quarterly follow-ups. The men had an average age of 30.6 years.

At each visit the study participants reported on their recent sexual behavior. The researchers collected STI diagnosis data on the participants through a Washington State public health records system.

Compared with records of other HIV-negative MSM in King County, the men in the PrEP study had dramatically higher diagnosis rates. The chlamydia, gonorrhea and syphilis diagnosis rates were a respective 22.6, 20.7 and 4.5 per cumulative 1,000 years among all HIV-negative MSM compared with a respective 539.3, 462.6 and 97.4 per cumulative 1,000 years among the men in the PrEP study. The respective per-cumulative-1,000-years rates of rectal gonorrhea and chlamydia were 14.8 and 8.8 among all HIV-negative MSM and 454.1 and 260.2 among the men in the PrEP study. For symptomatic STIs, specifically urethral gonorrhea and symptomatic syphilis, the respective rates were 6.9 and 3.1 for all HIV-negative MSM and 101.2 and 55.7 for the men in the study.

Reporting on condom use during the previous three months upon entering the PrEP study and a respective three and nine months after starting PrEP, a respective 6 percent, 10 percent and 8 percent reported never using condoms at these points in time.

The respective proportion reporting never using condoms for receptive sex at the study’s outset and while on PrEP was 2 percent and 4 percent for sex with HIV-positive partners and 4 percent and 8 percent for sex with HIV-negative partners. The corresponding proportion reporting insertive condomless sex was 2 percent and 6 percent with HIV-positive partners and did not change for HIV-negative partners.

Between a year prior to starting PrEP and the time they started PrEP, the men’s STI diagnosis rates rose significantly. During the three months leading into a year prior to their start on PrEP, a respective 6.5 percent, 10.2 percent and 10.2 percent of the men were diagnosed with chlamydia, gonorrhea and syphilis. The corresponding diagnosis rates during the three months leading into the men’s starting PrEP were 16 percent, 20.4 percent and 12 percent. Between these two time points, the rectal chlamydia rate increased from 4.6 percent and 14.8 percent while the rectal gonorrhea rate increased from 9.3 percent to 13.9 percent.

STI rates moved in varying directions while men were taking PrEP. The diagnosis rates that increased included chlamydia (16.7 percent to 22.2 percent), in particular rectal chlamydia (14.8 percent to 19.4 percent) and urethral gonorrhea (1.9 percent to 5.6 percent). Rates of gonorrhea essentially held steady (20.4 percent to 19.4 percent) and held exactly steady for rectal gonorrhea (13.9 percent). Syphilis rates, on the other hand, fell (12 percent to 6.5 percent) including symptomatic syphilis (7.4 percent to 3.7 percent).

Researchers look to symptomatic STIs to see whether changing diagnosis rates can provide clues as to whether higher testing rates are driving increased diagnosis rates of STIs that tend to be asymptomatic. The fact that the urethral gonorrhea and symptomatic syphilis diagnosis rates went in opposite directions provides conflicting answers for this particular study.

To read the aidsmap article, click here.

To read the King County study abstract, click here.