HIV

HIV in STIsim is modeled with CD4-based disease progression through acute, latent, and late-stage phases, with ART treatment effects.

Class: sti.HIV | Alias: 'hiv' | Base class: BaseSTI

States and transitions

                        ┌─────────────┐
                        │ Susceptible │
                        └──────┬──────┘
                               │ infection
                               ▼
                        ┌─────────────┐
                        │    Acute    │  CD4 declines from ~800 to ~500
                        │  (~3 mo)   │  Transmissibility: 6x baseline
                        └──────┬──────┘
                               │
                               ▼
                        ┌─────────────┐
                        │   Latent    │  CD4 stable at ~500
                        │  (~10 yr)  │  Transmissibility: 1x baseline
                        └──────┬──────┘
                               │
                    ┌──────────┴──────────┐
                    │                     │
                    ▼                     ▼
             ┌─────────────┐       ┌─────────────┐
             │   Falling   │       │   On ART    │  CD4 reconstitutes
             │   (~3 yr)   │       │   (~3 yr)   │  Transmissibility: 0.04x
             │ 8x baseline │       └──────┬──────┘
             └──────┬──────┘              │ ART dropout
                    │                     ▼
                    │              ┌─────────────┐
                    │              │  Post-ART   │  CD4 declines linearly
                    │              └──────┬──────┘
                    │                     │
                    ▼                     ▼
             ┌─────────────────────────────────┐
             │          AIDS death             │
             │       (CD4 reaches 0)           │
             └─────────────────────────────────┘

Parameters

Natural history

Parameter	Default	Description
`cd4_start`	normal(800, 50)	Initial CD4 count at infection
`cd4_latent`	normal(500, 50)	CD4 count during latent phase
`dur_acute`	lognorm(3 mo, 1 mo)	Duration of acute infection
`dur_latent`	lognorm(10 yr, 3 yr)	Duration of latent infection (untreated)
`dur_falling`	lognorm(3 yr, 1 yr)	Duration of late-stage CD4 decline
`include_aids_deaths`	True	Whether to include AIDS mortality

Transmission

Parameter	Default	Description
`beta_m2f`	0.05	Per-act male-to-female transmission probability
`rel_beta_f2m`	0.5	Female-to-male transmission relative to male-to-female
`beta_m2c`	0.025/mo	Prenatal mother-to-child transmission probability
`beta_breastfeed`	0.005/mo	Postnatal (breastfeeding) transmission probability
`rel_trans_acute`	normal(6, 0.5)	Relative transmissibility during acute phase
`rel_trans_falling`	normal(8, 0.5)	Relative transmissibility during late stage
`eff_condom`	0.9	Condom efficacy for reducing transmission

Initialization

Parameter	Default	Description
`init_prev`	0.05	Initial prevalence
`init_diagnosed`	0.0	Proportion initially diagnosed
`dist_ti_init_infected`	uniform(-120, -5)	Time of initial infection (months before start)

Treatment (ART)

Parameter	Default	Description
`art_efficacy`	0.96	ART efficacy at reducing transmission
`time_to_art_efficacy`	6 months	Time to reach full ART efficacy (linear ramp)
`art_cd4_growth`	0.1	Logistic growth rate for CD4 reconstitution on ART
`dur_on_art`	lognorm(3 yr, 1.5 yr)	Duration on ART before dropout

Care seeking

Parameter	Default	Description
`care_seeking`	normal(1, 0.5)	Relative care-seeking behavior (per agent)
`maternal_care_scale`	2	Multiplicative increase in care seeking during pregnancy

Results

HIV produces the standard BaseSTI results (new_infections, prevalence, incidence, etc.) plus HIV-specific results:

HIV-specific results

Result	Description
`new_deaths`	HIV/AIDS deaths this timestep
`cum_deaths`	Cumulative HIV/AIDS deaths
`new_diagnoses`	Newly diagnosed this timestep
`cum_diagnoses`	Cumulative diagnoses
`new_agents_on_art`	Agents starting ART this timestep
`p_on_art`	Proportion of infected agents on ART
`prevalence_15_49`	HIV prevalence among 15-49 year olds
`n_on_art_pregnant`	Number of pregnant women on ART (only when pregnancy is in the sim)
`p_diagnosed_pregnant`	Proportion of HIV+ pregnant women who are diagnosed (only when pregnancy is in the sim)

Transmission route results

All STI diseases (including HIV) track infections by transmission route:

Result	Description
`new_infections`	Total new infections (sexual + MTCT)
`new_infections_sex`	New infections via sexual transmission
`new_infections_mtct`	New infections via mother-to-child transmission

These are always consistent: new_infections_sex + new_infections_mtct == new_infections.

When pregnancy is modeled, prenatal and postnatal MTCT are tracked separately:

Result	Description
`new_infections_prenatal`	New infections via prenatal (in utero) transmission
`new_infections_postnatal`	New infections via postnatal (breastfeeding) transmission

These satisfy: new_infections_prenatal + new_infections_postnatal == new_infections_mtct.

Accessing MTCT results:

sim.run()

# Total MTCT infections over the simulation
total_mtct = sim.results.hiv.new_infections_mtct.sum()

# Time series
plt.plot(sim.t.yearvec, sim.results.hiv.new_infections_mtct)

PMTCT (prevention of mother-to-child transmission)

STIsim models PMTCT through three mechanisms:

1. ANC testing

ANC (antenatal care) testing identifies HIV-positive pregnant women so they can start ART. This is implemented as an HIVTest with pregnancy-based eligibility, the same pattern used for FSW-targeted testing:

import stisim as sti

# Test undiagnosed pregnant women in first trimester
anc_test = sti.HIVTest(
    test_prob_data=0.9,
    dt_scale=False,
    name='anc_test',
    eligibility=lambda sim: sim.demographics.pregnancy.tri1_uids[
        ~sim.diseases.hiv.diagnosed[sim.demographics.pregnancy.tri1_uids]
    ],
)

ANC testing is not included in hivsim.Sim defaults (which use a single general-population HIVTest). Add it explicitly when modeling targeted testing pathways. The p_diagnosed_pregnant result tracks what proportion of HIV+ pregnant women have been diagnosed, which measures the effectiveness of the ANC testing pathway.

2. ART retention during pregnancy

Pregnant women on ART are less likely to drop out thanks to the maternal_care_scale parameter (default 2), which doubles care-seeking behavior during pregnancy. This makes it much less likely that pregnant women stop ART, keeping them on treatment through delivery and breastfeeding.

3. Prenatal protection (MaternalNet)

When a pregnant woman is on ART, her unborn infant's susceptibility is reduced by the pmtct_efficacy parameter on the ART intervention (default 0.96). This is applied each timestep via the MaternalNet.

4. Postnatal protection (BreastfeedingNet)

When a breastfeeding mother is on ART, her infant's susceptibility is similarly reduced by pmtct_efficacy via the BreastfeedingNet.

For both prenatal and postnatal transmission, total protection compounds two effects: the infant's reduced susceptibility (rel_sus, from pmtct_efficacy) and the mother's reduced transmissibility (rel_trans, from art_efficacy).

Configuring PMTCT

import stisim as sti

# Custom PMTCT efficacy
art = sti.ART(pmtct_efficacy=0.98)

# Complete protection (previous default behavior)
art = sti.ART(pmtct_efficacy=1.0)