HIV and other STIs: how they raise risk and why to test for both

How other STIs affect HIV risk

CDC’s review of the research finds that STIs raise HIV risk in both directions. An STI in the partner without HIV makes HIV easier to get, and an STI in the partner with HIV makes it easier to pass on. For STIs that cause sores or ulcers, such as syphilis and herpes, CDC estimates that each effect more than doubles the risk. CDC also notes that the exact size for each STI and group is unclear, because the studies are observational.

The calculator follows that: when either partner has another STI, it multiplies the per-act risk by 2.

Here is what that looks like for one act of vaginal sex without a condom, a woman with a man who has HIV and isn’t on treatment:

• No STI: 0.08% (about 1 in 1,200)
• With an STI: 0.16% (about 1 in 620)

And for receptive anal sex with a man who has HIV and isn’t on treatment:

• No STI: 1.4% (about 1 in 72)
• With an STI: 2.8% (about 1 in 36)

An STI is also a warning sign in its own right. CDC’s treatment guidelines describe an STI diagnosis as a marker for HIV risk, especially primary or secondary syphilis.

You may not know you have one

Many STIs cause no symptoms, or only mild ones, so people can have an infection without knowing it. That’s why the calculator asks about STIs, and why “I feel fine” isn’t a reason to skip testing. If you’re not sure whether you or a partner has an STI, it’s safest to assume the risk could be higher and to get tested.

Test for both, at the same time

CDC recommends an HIV test for everyone who is checked for STIs and isn’t already known to have HIV. In practice, ask for both whenever you get tested.

A few things to keep in mind:

• Timing is different. Each STI has its own timing for testing. A clinic can tell you when to test for each. See the HIV window period.
• Testing for HIV alone misses the rest. A negative HIV test doesn’t tell you anything about syphilis, gonorrhea or chlamydia, and the reverse is true too.
• STIs are treatable. CDC notes that all STIs are treatable with medicine, and some can be cured completely.
• Partners matter. If you’re diagnosed with an STI, tell your partners so they can get tested too.

In the US, gettested.cdc.gov lists free and low-cost places to test for HIV and other STIs.

PrEP, condoms and doxy-PEP each do different jobs

PrEP (pre-exposure prophylaxis) protects against HIV. Taken as prescribed, it lowers the risk from sex by about 99% for anal sex and about 90% for vaginal sex. But, as CDC points out, it doesn’t prevent other STIs.

Condoms lower HIV risk by about 80% in real-world use, and can also help prevent certain other STIs.

Doxy-PEP is a newer option for bacterial STIs. It means taking a dose of the antibiotic doxycycline within 72 hours after sex. CDC says it reduces the risk of syphilis and chlamydia, and of gonorrhea in some studies. CDC recommends that providers discuss it with gay, bisexual and other men who have sex with men, and with transgender women, who have had at least one bacterial STI in the past 12 months. CDC makes no recommendation yet for cisgender women.

Doxy-PEP is not HIV prevention. Alongside it, CDC recommends HIV testing and talking about HIV PrEP with people who could benefit. If you use doxy-PEP, you still need a separate plan for HIV. More in PrEP and PEP.

What to do now

1. If the exposure was in the last 72 hours, ask about HIV PEP.
2. Get tested for HIV and other STIs together, even if you feel fine.
3. If you have an STI, get it treated and tell your partners.
4. Include the STI when you use the calculator, for a more accurate estimate.
5. Think about ongoing protection: PrEP for HIV, condoms for HIV and other STIs, and doxy-PEP if a clinician says it suits you.