Your Guide to Modern Sexual Health explains why the layered approach works. This article explains what the layers are — each one, what it covers, what it doesn't, and where to go for the full detail.
No single layer covers everything. No single layer needs to. The stack works because each one compensates for the gaps in the others.
Layer 1 — PrEP
What it does: Prevents HIV infection in the person taking it. If HIV-negative and sexually active, this is the foundation of your prevention strategy.
Effectiveness: >99% for daily dosing. ~86% for on-demand 2-1-1 (TDF/FTC only). Non-inferior for injectable cabotegravir.
What it doesn't cover: Gonorrhoea, chlamydia, syphilis, hepatitis, HPV, Mpox — anything other than HIV.
Who it's for: Anyone HIV-negative who is sexually active and wants to reduce HIV risk. There's no threshold of "enough sex" to qualify. If you want it, it's for you.
If you're HIV-positive, your equivalent of this layer is maintaining an undetectable viral load through treatment — see Layer 2.
→ Full guide: PrEP Mechanics: Daily, On-Demand & Injectable
Layer 2 — U=U (Treatment as Prevention)
What it does: If you or a partner is HIV-positive and has an undetectable viral load (<50 copies/mL on treatment), the risk of HIV transmission through sex is zero. Not "very low" — zero. This is settled science.
Effectiveness: 100% for HIV transmission. Zero transmissions across tens of thousands of condomless sex acts in the PARTNER and PARTNER2 studies.
What it doesn't cover: Any STI other than HIV.
Who it's for: HIV-positive people on treatment, and their partners. Understanding U=U changes the risk calculation entirely for serodiscordant couples — and removes a significant source of stigma and anxiety.
→ Full guide: U=U: Undetectable Equals Untransmittable
Layer 3 — Vaccines
What it does: One-time (or short-series) protection against specific viral infections that no other layer in this stack covers.
| Vaccine | Protects Against |
|---|---|
| HPV (Gardasil 9) | Anal cancer, throat cancer, genital warts |
| Hepatitis A | Liver infection via oral-anal contact |
| Hepatitis B | Liver infection via blood/semen |
| Mpox (Imvanex/Jynneos) | Mpox (monkeypox) |
| Meningitis MenACWY + MenB | Bacterial meningitis |
Effectiveness: 90–95% for HPV (strains not yet encountered), near-100% for Hep A/B after full course, significant reduction in Mpox severity and transmission.
What it doesn't cover: HIV, bacterial STIs (gonorrhoea, chlamydia, syphilis), hepatitis C.
Who it's for: Everyone. These are one-time investments. The HPV and hepatitis vaccines in particular are things most people haven't completed and should.
→ Full guide: The Vaccine Checklist
Layer 4 — Regular Testing
What it does: Catches what the other layers don't prevent. Confirms the layers that are working are working. Allows early treatment, which reduces transmission and prevents complications.
The standard: Every 3 months if sexually active. Every test must include throat swabs, rectal swabs, urine, and blood — the "three-site" rule. Urine-only testing misses the majority of infections in gay men.
What it catches: HIV, syphilis, gonorrhoea, chlamydia (all three sites), hepatitis B and C, and increasingly Mpox depending on local outbreak patterns.
Why it matters beyond yourself: If you test positive for a bacterial STI, early treatment means you're infectious for days rather than months. That directly protects the people you have sex with.
→ Full guide: The Testing Protocol
Layer 5 — Condoms
What it does: The only layer that provides physical barrier protection against fluids and skin-to-skin contact. The only layer that simultaneously reduces risk for HIV and every bacterial STI in a single action.
Effectiveness: ~85% real-world effectiveness for HIV (accounting for inconsistent use and breakage). High effectiveness for gonorrhoea and chlamydia when used correctly.
What it doesn't cover: HPV (skin-to-skin beyond the covered area), herpes (similar), Mpox (similar). Vaccines are the answer for HPV and Mpox.
Reality check: Consistent, correct condom use is the most protective single behaviour in this stack. It's also the layer most people use inconsistently. That's why the other layers exist — not to replace condoms, but to maintain protection when condoms aren't used.
→ Full guide: Advanced Condom Mechanics
Layer 6 — DoxyPEP
What it does: A dose of doxycycline (200mg) taken within 24–72 hours after unprotected sex to prevent syphilis, chlamydia, and gonorrhoea from establishing.
Effectiveness: ~87–88% reduction in syphilis and chlamydia. ~55% reduction in gonorrhoea.
What it doesn't cover: HIV (that's PrEP and PEP), viral STIs, or infections caught before this exposure.
The caveat: DoxyPEP doesn't replace quarterly testing — gonorrhoea in particular can break through, and resistance is an active concern. Use it as an additional layer, not a substitute for testing.
→ Full guide: DoxyPEP: The Morning After Pill for Bacteria
Layer 7 — PEP
What it does: A 28-day course of antiretroviral drugs started after a potential HIV exposure, preventing the virus from establishing infection. The emergency brake.
Effectiveness: ~95%+ if started promptly (within 24 hours). Effectiveness drops significantly after 48 hours. Not available after 72 hours.
What it doesn't cover: Any STI other than HIV. Side effects are significant (nausea, fatigue, diarrhea) — this is not a comfortable experience.
When to use it: Condom broke and not on PrEP. Unprotected sex with someone of unknown or positive status and not on PrEP. Sexual assault.
PEP is a safety net, not a routine prevention method. If you've needed it more than once, that's a signal your exposure pattern would be better served by proactive PrEP.
→ Full guide: PEP: The Emergency Brake
Layer 8 — Communication
What it does: Enables informed decisions before and during sex. Knowing a partner's testing status, HIV status, and treatment status changes the risk calculation for every other layer in this stack.
The key conversations:
- "When did you last get tested, and what did you test for?"
- "Are you on PrEP?" / "Are you positive and on treatment — and are you undetectable?"
- "Do you want to use condoms?"
Why it's in the stack: A partner who is HIV-positive and undetectable eliminates HIV risk (U=U). A partner who tested clean last week and has had no partners since is different from a partner with unknown status. Communication doesn't remove risk — it allows you to allocate your other layers intelligently.
→ Full guide: Talking to Partners About Status
How the Stack Combines
No single layer is required. The stack is designed so that multiple partial protections create robust overall protection:
| Situation | HIV risk | Bacterial STI risk |
|---|---|---|
| No layers | Moderate–high | High |
| PrEP only | Near-zero | Unchanged |
| PrEP + testing every 3 months | Near-zero | Caught early, treated fast |
| PrEP + vaccines + testing | Near-zero | Viral STIs prevented; bacterial caught early |
| U=U partner + testing | Zero (HIV) | Caught early, treated fast |
| Condoms + testing | Very low | Low |
| Full stack active | Near-zero across the board | Low–minimal |
The point isn't perfection. It's that each layer you add meaningfully shifts the odds — and the combination of several imperfect layers outperforms any single perfect one.
Related:
- > Your Guide to Modern Sexual Health — the mindset behind the stack
- > The STI Landscape: What You Need to Know — every infection, every transmission route
- > HIV in 2026: The Facts Without the Fear — the full HIV primer
- > What to Do If You Think You Have an STI — when something goes wrong despite the stack