{
  "slug": "unprotected-sex-sti",
  "question": "What are the odds of catching an STI from a single unprotected sexual encounter with a new partner?",
  "category": "health",
  "no_reliable_estimate": false,
  "perceived": {
    "description": "Public perception of STI risk from a single encounter is shaped almost entirely by HIV awareness campaigns, which anchored the idea that unprotected sex is an acute mortal threat. Most adults substantially overestimate the per-act probability of HIV from heterosexual vaginal sex and substantially underestimate the per-act probability of HPV, which is the dominant contributor to composite STI acquisition risk. No rigorous population survey quantifies perceived per-encounter composite STI probability across pathogens, so this is marked as editorial intuition.\n",
    "rough_estimate": "Most adults perceive a 'significant' risk per encounter, heavily weighted toward HIV and heavily underweighting HPV",
    "kind": "intuition"
  },
  "native": {
    "display": "~1 in 11 per unprotected vaginal encounter with a random US adult partner (any STI)",
    "numerator": 1,
    "denominator": 11,
    "unit": "per unprotected vaginal sex act",
    "population": "Heterosexual US adults aged 18-59, random partner draw from general population"
  },
  "normalized": {
    "lifetime_us_adult": 0.65,
    "display": "~2 in 3 lifetime (10 new partners, unprotected vaginal)",
    "log_value": -0.187,
    "assumptions": "Scope is activity_specific_lifetime. The per-encounter composite probability of acquiring at least one new STI from a single act of unprotected vaginal sex with a random US adult partner is estimated at ~9% (1 in 11), calculated as 1 - product(1 - prevalence_i * transmissibility_i) across six major pathogens: HPV (~42.5% prevalence * ~20% per-act ≈ 8.5%), chlamydia (~1.8% * 4.5% ≈ 0.081%), gonorrhea (~0.3% * 20% ≈ 0.06%), HSV-2 (~12% * 0.15% ≈ 0.018%), HIV (~0.36% * 0.08% ≈ 0.00029%), syphilis (~0.05% * 5% ≈ 0.0025%). HPV dominates the composite, contributing ~94% of the total risk. The lifetime figure assumes 10 new partners over an adult sexual lifetime (roughly median for US adults per NHSLS/GSS data) with one unprotected vaginal encounter each: 1 - (1 - 0.09)^10 ≈ 0.61. Rounded up to ~0.65 to account for repeated encounters within partnerships and minor pathogen contributions not individually modeled (e.g., Mycoplasma genitalium, trichomoniasis). This is consistent with the CDC's estimate that ~80% of sexually active adults acquire HPV at some point, which alone implies a lifetime STI acquisition probability well above 0.5.\n",
    "uncertainty": {
      "low": 0.4,
      "high": 0.85
    },
    "scope": "activity_specific_lifetime"
  },
  "sources": [
    {
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC6195215/",
      "title": "Estimating per-act HIV transmission risk: a systematic review",
      "publisher": "AIDS / Patel et al. (CDC)",
      "source_type": "peer_reviewed",
      "statistic": "Per-act HIV transmission risk: receptive vaginal 8 per 10,000 (95% CI 6-11), insertive vaginal 4 per 10,000 (95% CI 1-14), receptive anal 138 per 10,000 (95% CI 102-186), insertive anal 11 per 10,000 (95% CI 4-28)",
      "excerpt": "\"Sexual-exposure risks ranged from low for oral sex to 138 (CI 102-186) per 10,000 exposures for receptive anal intercourse. [...] Receptive vaginal intercourse: 8 (95% CI 6-11) per 10,000 exposures. Insertive vaginal intercourse: 4 (95% CI 1-14) per 10,000 exposures.\"\n",
      "source_date": "2014-06-19",
      "source_accessed": "2026-04-17",
      "archive_url": "http://web.archive.org/web/20260406062635/https://pmc.ncbi.nlm.nih.gov/articles/PMC6195215/",
      "calculation_notes": "Per-act receptive vaginal HIV risk = 8/10,000 = 0.08%. Combined with US adult HIV prevalence of ~0.36% (1.2M PLWH / 330M): per-encounter HIV risk from random partner = 0.0008 * 0.0036 = 0.0000029, or about 1 in 345,000. Even the higher receptive-anal figure of 1.38% * 0.36% prevalence = 0.005%, or ~1 in 20,000 per act with a random partner. HIV is a negligible contributor to the composite per-encounter risk in the general heterosexual US population.\n",
      "independence_note": "CDC-authored systematic review of per-act HIV transmission studies; partially overlaps with Boily 2009 in included primary studies but uses a different analytic framework. The two meta-analyses converge on similar per-act estimates.\n"
    },
    {
      "url": "https://www.cdc.gov/sti-statistics/annual/index.html",
      "title": "Sexually Transmitted Infections Surveillance, 2024 (Provisional)",
      "publisher": "US Centers for Disease Control and Prevention",
      "source_type": "govt_report",
      "statistic": "In 2024 (provisional), over 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported: 1,515,985 chlamydia, 543,409 gonorrhea, 190,242 syphilis",
      "excerpt": "\"There were still more than 2.2 million reported STIs in 2024. [...] Chlamydia cases declined for the second year in a row, down 8% since 2023. Gonorrhea cases declined for the third year in a row, down 10% since 2023. Primary and secondary syphilis cases declined for the second year in a row, down 22% since 2023.\"\n",
      "source_date": "2025-09-24",
      "source_accessed": "2026-04-24",
      "archive_url": "https://web.archive.org/web/20260424000000*/https://www.cdc.gov/sti-statistics/annual/index.html",
      "calculation_notes": "Reported cases understate true prevalence because most chlamydia and many gonorrhea infections are asymptomatic. Chlamydia point prevalence estimated at ~1.8% of adults (reported ~1.5M cases/year in 2024 provisional data, but screening captures only a fraction of infections; CDC estimates ~4M total infections/year against ~220M adults 18+). Gonorrhea point prevalence ~0.3% (reported ~543K cases/year, shorter duration of infection). Syphilis point prevalence ~0.05% (~190K all-stages cases but most are latent/non-infectious). These prevalence estimates feed into the composite calculation. The 2024 provisional data shows declines across all three bacterial STIs vs 2023.\n",
      "independence_note": "National surveillance data from mandatory reporting; independent of the per-act transmission studies (Patel, Boily) and the prevalence surveys (McQuillan HPV, McQuillan HSV-2). Provides the incidence and case-count denominators for bacterial STI prevalence estimation.\n"
    },
    {
      "url": "https://pubmed.ncbi.nlm.nih.gov/28463105/",
      "title": "Prevalence of HPV in Adults Aged 18-69: United States, 2011-2014",
      "publisher": "NCHS Data Brief No. 280 / McQuillan et al.",
      "source_type": "govt_report",
      "statistic": "During 2013-2014, any genital HPV prevalence among adults aged 18-59 was 42.5% overall (45.2% men, 39.9% women)",
      "excerpt": "\"During 2013-2014, prevalence of any genital HPV for adults aged 18-59 was 42.5% (45.2% among men and 39.9% among women). Prevalence of high-risk genital HPV was 22.7%.\"\n",
      "source_date": "2017-04-01",
      "source_accessed": "2026-04-17",
      "archive_url": "https://web.archive.org/web/20260503095041/https://pubmed.ncbi.nlm.nih.gov/28463105/",
      "calculation_notes": "HPV point prevalence of 42.5% among US adults 18-59 is the single largest contributor to per-encounter STI risk. Per-partnership HPV transmission probability is ~20% within the first few months (Burchell et al. 2006), implying a per-act rate on the order of ~20% for a new partner encounter (most transmission occurs rapidly after exposure). 0.425 * 0.20 = 0.085, or ~8.5% per encounter, which alone accounts for >94% of the composite STI risk.\n",
      "independence_note": "NHANES-based population prevalence survey; independent of the per-act transmission studies and the bacterial STI surveillance data. Shares the McQuillan author with the HSV-2 data brief below but uses a different NHANES cycle and pathogen assay.\n"
    },
    {
      "url": "https://pubmed.ncbi.nlm.nih.gov/29442994/",
      "title": "Prevalence of Herpes Simplex Virus Type 1 and Type 2 in Persons Aged 14-49: United States, 2015-2016",
      "publisher": "NCHS Data Brief No. 304 / McQuillan et al.",
      "source_type": "govt_report",
      "statistic": "HSV-2 seroprevalence among persons aged 14-49 was 11.9% in 2015-2016, down from 18.0% in 1999-2000",
      "excerpt": "\"During 2015-2016, prevalence of HSV-1 was 47.8% and HSV-2 was 11.9% among persons aged 14-49. Age-adjusted prevalence of HSV-2 decreased linearly over time by 5.9 percentage points, from 18.0% in 1999-2000 to 12.1% in 2015-2016.\"\n",
      "source_date": "2018-02-01",
      "source_accessed": "2026-04-17",
      "archive_url": "http://web.archive.org/web/20260318224612/https://pubmed.ncbi.nlm.nih.gov/29442994/",
      "calculation_notes": "HSV-2 prevalence of ~12% combined with a per-act transmission probability of ~0.1-0.3% during asymptomatic shedding (Schiffer et al. 2014, J R Soc Interface; Wald and colleagues' longitudinal cohort data) gives a per-encounter risk from a random partner of ~0.12 * 0.0015 = 0.00018, or about 1 in 5,500. HSV-2 is a minor contributor to the single-encounter composite but compounds substantially over a lifetime of partnerships because shedding is intermittent and lifelong.\n",
      "independence_note": "NHANES-based seroprevalence survey; shares the McQuillan lead author with the HPV data brief above but uses different NHANES cycles (2015-2016 vs 2011-2014) and a different serological assay. Provides the HSV-2 prevalence input to the composite calculation.\n"
    },
    {
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC4467783/",
      "title": "Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies",
      "publisher": "The Lancet Infectious Diseases / Boily et al.",
      "source_type": "peer_reviewed",
      "statistic": "Pooled per-act female-to-male HIV transmission in high-income countries: 0.04% (95% CI 0.01-0.14%); male-to-female: 0.08% (95% CI 0.06-0.11%); receptive anal: 1.7% (95% CI 0.3-8.9%)",
      "excerpt": "\"Pooled female-to-male (0.04% per act [95% CI 0.01-0.14]) and male-to-female (0.08% per act [95% CI 0.06-0.11]) transmission estimates in high-income countries indicated a low per-act risk of infection in the absence of antiretrovirals. [...] The pooled receptive anal intercourse estimate was much higher (1.7% per act [95% CI 0.3-8.9]).\"\n",
      "source_date": "2009-02-01",
      "source_accessed": "2026-04-17",
      "archive_url": "http://web.archive.org/web/20260503084014/https://pmc.ncbi.nlm.nih.gov/articles/PMC4467783/",
      "calculation_notes": "Boily's meta-analysis corroborates Patel 2014 within overlapping confidence intervals. The high-income-country male-to-female rate of 0.08% is the figure used in the composite calculation for heterosexual vaginal sex. Multiplied by US adult HIV prevalence of 0.36%: 0.0008 * 0.0036 ≈ 2.9e-6 per encounter — confirming HIV's negligible contribution to the composite for population-blinded heterosexual encounters.\n",
      "independence_note": "Lancet Infectious Diseases meta-analysis of observational cohort studies; partially overlaps with Patel 2014 in included studies but uses different pooling methodology. The convergence of the two meta-analyses raises confidence in the per-act estimates.\n"
    },
    {
      "url": "https://www.cdc.gov/hiv-data/nhss/estimated-hiv-incidence-and-prevalence.html",
      "title": "Estimated HIV Incidence and Prevalence in the United States",
      "publisher": "US Centers for Disease Control and Prevention",
      "source_type": "govt_report",
      "statistic": "Estimated 1.2 million persons living with HIV in the United States at end of 2022; approximately 31,800 new infections in 2022",
      "excerpt": "\"An estimated 1.2 million persons in the United States were living with diagnosed and undiagnosed HIV at the end of 2022. [...] Approximately 31,800 people acquired HIV in the United States in 2022.\"\n",
      "source_date": "2024-05-01",
      "source_accessed": "2026-04-17",
      "archive_url": "http://web.archive.org/web/20260502021355/https://www.cdc.gov/hiv-data/nhss/estimated-hiv-incidence-and-prevalence.html",
      "calculation_notes": "1.2 million PLWH / 330 million US population = 0.36% adult prevalence. This is the population prevalence figure used to convert Patel/Boily per-act-given-infected-partner rates into per-encounter-with-random-partner rates. Note that prevalence is heavily concentrated among MSM (estimated ~15% prevalence among MSM vs ~0.1% among heterosexual adults), so the population-blinded 0.36% substantially overstates heterosexual encounter risk and understates MSM encounter risk.\n",
      "independence_note": "National HIV Surveillance System data; independent of the per-act transmission meta-analyses and the NHANES prevalence surveys. Provides the population prevalence denominator used to convert conditional per-act rates into unconditional per-encounter-with-random-partner rates.\n"
    }
  ],
  "comparison_anchors": [
    {
      "label": "Lung cancer (lifetime, US adult)",
      "lifetime_us_adult": 0.062
    },
    {
      "label": "Car crash death (lifetime, US adult)",
      "lifetime_us_adult": 0.0108
    },
    {
      "label": "Skiing serious injury (per 20-day season)",
      "lifetime_us_adult": 0.0392
    }
  ],
  "regional_breakdown": [
    {
      "region": "Unprotected vaginal, heterosexual, US adult — any STI composite",
      "probability": 0.09,
      "notes": "Per-encounter. HPV dominates (~8.5% contribution). Headline figure."
    },
    {
      "region": "Same encounter with consistent condom use",
      "probability": 0.04,
      "notes": "Condoms ~50-60% effective against HPV (skin-to-skin pathogen), ~80-90% against bacterial STIs and HIV. Composite roughly halved."
    },
    {
      "region": "Receptive anal, MSM, major US metro — any STI",
      "probability": 0.25,
      "notes": "HIV prevalence ~15% among MSM; gonorrhea/syphilis prevalence 5-10x general population. Per-act anal gonorrhea transmission ~30-50%. Composite far higher."
    },
    {
      "region": "With PrEP — HIV-specific risk reduction",
      "probability": 1e-7,
      "notes": "PrEP reduces HIV acquisition risk by ~99% (Grant et al. 2010, iPrEx). Applies only to HIV; no effect on other STIs."
    },
    {
      "region": "Serodiscordant couple, known HIV+ partner, no ART — receptive vaginal",
      "probability": 0.0008,
      "notes": "Patel 2014 per-act receptive vaginal: 8/10,000. With ART achieving viral suppression: effectively 0 (Cohen et al. 2016, HPTN 052)."
    },
    {
      "region": "HPV per-encounter (random US adult partner, unprotected vaginal)",
      "probability": 0.085,
      "notes": "42.5% prevalence × ~20% per-encounter transmission. Dominant pathogen in composite."
    },
    {
      "region": "Chlamydia per-encounter (random US adult partner, unprotected vaginal)",
      "probability": 0.00081,
      "notes": "~1.8% prevalence × ~4.5% per-act male-to-female transmission."
    },
    {
      "region": "Gonorrhea per-encounter (random US adult partner, unprotected vaginal)",
      "probability": 0.0006,
      "notes": "~0.3% point prevalence × ~20% per-act male-to-female transmission."
    },
    {
      "region": "HIV per-encounter (random US adult partner, unprotected vaginal)",
      "probability": 0.0000029,
      "notes": "~0.36% prevalence × 0.08% per-act male-to-female. About 1 in 345,000."
    },
    {
      "region": "HSV-2 per-encounter (random US adult partner, unprotected vaginal)",
      "probability": 0.00018,
      "notes": "~12% seroprevalence × ~0.15% per-act during asymptomatic shedding. About 1 in 5,500."
    },
    {
      "region": "Syphilis per-encounter (random US adult partner, unprotected vaginal)",
      "probability": 0.000025,
      "notes": "~0.05% infectious-stage prevalence × ~5% per-act with infectious partner. About 1 in 40,000."
    }
  ],
  "personal_factor_multipliers": [
    {
      "factor": "MSM (men who have sex with men)",
      "multiplier": 3,
      "notes": "HIV prevalence ~15% vs ~0.1% heterosexual; gonorrhea/syphilis prevalence 5-10x. Composite per-encounter risk roughly 3x the heterosexual baseline, driven primarily by higher pathogen prevalence and anal-route transmissibility."
    },
    {
      "factor": "Age under 25",
      "multiplier": 1.5,
      "notes": "Chlamydia and gonorrhea rates 2-3x higher in 15-24 age group (CDC STI Surveillance 2023). HPV acquisition peaks in late teens/early 20s. Composite elevated by higher partner-pool prevalence."
    },
    {
      "factor": "Multiple new partners per year (>4)",
      "multiplier": 2,
      "notes": "Applies to cumulative annual risk, not per-act. More exposures compound, and high-partner-count individuals are drawn from a higher-prevalence subnetwork (assortative mixing)."
    },
    {
      "factor": "Partner from high-prevalence subgroup (e.g. MSM, sex worker, IV drug user)",
      "multiplier": 5,
      "notes": "Conditional on partner being from a high-prevalence sexual network, the 'prevalence' term in the per-encounter formula can be 10-50x the general population for HIV/syphilis/gonorrhea."
    },
    {
      "factor": "Consistent condom use",
      "multiplier": 0.45,
      "notes": "Condoms reduce HIV/gonorrhea/chlamydia by 80-90% per act, HPV by ~40-60%, HSV-2 by ~30%. Weighted across pathogens where HPV dominates, net composite reduction ~55%."
    },
    {
      "factor": "PrEP (for HIV component only)",
      "multiplier": 0.97,
      "notes": "Reduces HIV acquisition by ~99%. Since HIV is a negligible fraction of the composite per-encounter risk (~0.003% of total), the effect on the composite headline is minimal. The value of PrEP is in preventing the highest-consequence infection, not the most probable one."
    },
    {
      "factor": "Male circumcision",
      "multiplier": 0.7,
      "notes": "Three RCTs in sub-Saharan Africa showed ~60% reduction in female-to-male HIV transmission (Auvert 2005, Bailey 2007, Gray 2007). Also ~30% reduction in HPV and HSV-2 per meta-analyses (Albero 2012). Net composite reduction ~30%."
    },
    {
      "factor": "Partner on suppressive ART (HIV-specific)",
      "multiplier": 0.97,
      "notes": "Undetectable = untransmittable (Cohen et al. 2016 HPTN 052). Eliminates HIV risk from that partner. Negligible composite effect for same reason as PrEP — HIV is a tiny fraction of per-encounter composite."
    }
  ],
  "short_label": "Unprotected sex",
  "myth_framing": "calibrated",
  "outcome_severity": "moderate_harm",
  "exposure_pattern": "acute",
  "outcome_type": "recoverable_injury",
  "valence": "negative",
  "caveats": "These are per-act probabilities conditional on partner infection status weighted by population prevalence. The calculation assumes a truly random partner draw from the US adult population, which does not reflect reality: sexual networks exhibit strong assortative mixing by geography, age, race, sexual orientation, and risk behavior. A person's actual risk depends heavily on who they select as partners and vice versa. All prevalence figures are US-centric; global prevalence varies enormously (HIV prevalence in sub-Saharan Africa is ~25x the US rate; HPV prevalence is high everywhere). Per-act transmissibility estimates carry wide confidence intervals and vary by viral load, co-infections, mucosal integrity, and host genetics. The composite treats pathogen acquisitions as independent events, which is approximately but not exactly true (STI co-infection raises susceptibility to other STIs, particularly HIV). This is a probability calculation based on published epidemiological data, not medical advice.\n",
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    "scored_at": "2026-05-25",
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  "last_reviewed": "2026-04-17",
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