ERS35 Sex, Germs And Vaccines Flashcards
STIs epidemiology triangle
Transmission dynamics of STI is interaction between 3 components:
- ***Etiological agent (transmissibility, replication rate, antigenicity, resistance, virulence, genetic diversity)
- Bacterial: Syphilis, Gonorrhoea, Chlamydia
- Viral: HPV, HBV, HIV, HSV
- Protozoal: Trichomoniasis - ***Host (pathogenesis, natural history, risk factors, immunity, severity)
- ***Environment (transmission settings, reservoirs, seasonality, population density, socio-economic factors)
Transmission dynamics of STIs are subject to continuous changes with ***high level of clustering driven by spatial location, race, culture, religious belief, sexual activity level, socio-economic status, education level etc.
Observations about STI epidemiology
- Most STIs rarely generate a solidly protective immune response
- ***Re-infection common after recovery - Infectiousness may ***persist for extended periods
- ∵ Natural history + Asymptomatic infections
- e.g. HPV infected individuals are asymptomatic —> spread to sexual partners —> most prevalent STI
- more symptomatic —> require more risky sexual practices for transmission to be maintained —> less prevalent - Sexual behaviour is ***heterogeneous within / across populations
- some only have 1 sexual partner, some have many e.g. commercial sex workers - ***Core group for transmission
- individuals with many sexual partners e.g. Commercial sex workers
- Target groups for prevention programme
—> ∵ high prevalence, infections spread
- Bridging population (幫手傳開去General population): e.g. Clients of sex workers - Misconception
- only sexually active and promiscuous people are affected (Mother-to-child, Blood transfusion e.g. HIV)
Adverse effects and Interventions of STI
1. Individual level Effects: - Infertility - Cervical cancer (HPV) - Genital tract inflammation —> ↑ risk of HIV acquisition
Interventions:
- ***Counselling to ↓ high risk sexual behaviour
- Condoms
- ***Antibiotics
- Vaccines
- Sexual partner / Children
Effects:
- STI vertical transmission —> Congenital Syphilis, Neonatal HSV encephalitis
Interventions:
- ***Partner notification
- ***Antenatal Syphilis screening
- Antibiotics / Antivirals to prevent sexual transmission to Sexual partner / Children
- Population level
Effects:
- Epidemic
- Exacerbation of HIV (∵ ↑ Community transmission of STI —> ↑ Genital tract inflammation —> ↑ risk of HIV acquisition e.g. in Africa)
Intervention: - ***Population screening - ***Vaccination programme - ***Primary prevention programme —> Aim to ↓ population level transmission of STI
Level of evidence in preventing / treating STI
1. Individual level (↓ morbidity in individual) Level 1 (Empirical result from >=1 RCT) - Pathogen-specific Diagnostic test - Antimicrobial medications - Vaccines
Level 2 (Empirical result from >=1 well-designed observational studies e.g. Cohort, Case-control)
- Counselling
- Condoms
- Partnership level (↓ morbidity / mortality in both partners)
- Partner notification
- Pathogen-specific Antenatal screening —> effective in preventing vertical transmission - Population level (↓ transmission of infection in population)
- Periodic presumptive treatment
- HPV vaccination programme (not only protected vaccinated people but also general population)
***Determinants of STI epidemics
- ***Sexual structure of population
- size, distribution of high-risk / core groups
- sexual mixing between groups of different attributes (e.g. age, average sexual behaviour, patterns of concurrency, serial monogamy) - ***Societal factors
- stigma, culture, socio-economic status, levels of poverty, income / status equality, literacy, education, employment, women status, volume of mobility, migration - ***Interventions available / implemented
- diagnostic tools, medical treatments, prevention programmes (e.g. partner notification, use of condoms), screening programs, education + awareness programs - STIs and HIV transmission
- esp. those cause genital ulceration —> ↑ risk of HIV acquisition + transmission (e.g. HSV-2)
- Treatment ↓ shedding of HIV in genital secretions and plasma
Elimination of STIs in China
Menpower + Health care system + Financial support
—> Rapid case finding + Treatment
During 1950-70:
- Training of paraprofessional + public health personnel specific to STI
- Mass screening and treatment: Surveys —> Diagnostics —> Treatment; Cooperative health insurance
- Propaganda: Anti-Western civilisation —> Elimination of STI is patriotic “Target STI, not the patients”
- Complete elimination of prostitution (Incarceration, Re-education, Improvement of women status)
- Chief emphasis on prevention: “Immunity through knowledge”
STI have returned since Chinese economic reforms in 1980s
—> ∵ Economic, Political, Social changes
Other successful examples
100% condom campaign in Thailand
Sexual structure
Average Sexual mixing behaviour at population level alone
—> not sufficient to explain STI transmission!!!
∵ Strongly associated with social structure, values:
- Sexual activity highly dependent on age
- Sexual mixing is strongly associated with social structure
- No. of sex partners
- Sex practice
- Concurrency of sexual partnerships
- Frequency of sex without condoms
- Frequency of casual sex and paid sex
**Sexual mixing:
- **Assortative for most attributes (age, level of sexual activity, race, socio-economic status)
—> i.e. Like-with-like (差唔多年紀, 種族, 地位)
- ***Disassortative mixing: Sexual orientation (不同attribute起埋一齊)
- Proportionate mixing: Random
Sexual network
Core groups:
- Individuals whose sexual behaviour patterns, social / health-seeking behaviours within networks
—> contribute **disproportionately to transmission of STI
—> target these Core groups have **similar effects than targeting universal population, but more ***cost-effective
***Networks:
- Most people are sexually connected
—> STI can spread between people with no direct sexual relationships
—> transmit via their respective partners
Few studies of sexual network, even fewer studies that track network evolution
—> ∵ Practical and Ethical difficulties
Most studies focus on networks in context of “realised” disease transmission
—> i.e. Networks constructed after outbreak based on observed cases (i.e. Contact tracing)
—> biased, misleading
—> ∵ miss network structure in wider population (might be substantially different)
- **Chlamydia
- more asymptomatic
- spread easily
- more branching like network, larger degree of separation (i.e. larger distance)
- **Gonorrhoea
- symptomatic
- harder to spread unless high risk sexual behaviour
- more linear, less branching network, low degree of separation (i.e. short distance between 2 people in network)
Natsal: National Surveys of Sexual Attitudes and Lifestyles
- high risk sexual behaviour ↑ over past 20 years
- STI associated with **high risk behaviour, **young age, ***lower socio-economic status
Common sense but little documented
- Not just individuals’ no. of partners that determines risk of STI acquisition
- but also partner’s partners and beyond
- Sexual network matters!!!
HPV infection
HPV infection:
- most common STI worldwide
- ***Natural immunity is weak (i.e. reinfection is common)
- most HPV infection harmless, clears within 12 months
Cervical cancer:
- caused by persistent infection of ***high risk HPV (16, 18 for ~70%)
- incubation time: years - decades
—> hard to evaluate effectiveness of vaccination
Condyloma acuminata / Genital warts:
- >90%: HPV 6, 11
HPV vaccination
3 HPV vaccines
- Gardasil: >90% efficacious against 16, 18, 6, 11
- Cervarix: >90% efficacious against 16, 18
- Gardasil-9: >90% efficacious against 16, 18, 6, 11 + five other high risk types
WHO recommendation:
- Cost-effectiveness of vaccination in country / region should be considered before inclusion in national programs
Economic evaluation (e.g. cost-effectiveness analysis, CEA) of vaccination requires a lot of expertise:
- Infectious disease epidemiology for assessing effectiveness
- Health economics for assessing cost
Public health impact of vaccination
Herd immunity:
- Vaccinating an individual indirectly ↓ risk of infection of his contacts, his contacts’ contacts and beyond i.e. whole population
- Indirect protection
Example:
- Routine Quadrivalent HPV vaccination in Australia
—> protect not only young females but ALSO young males (with no vaccine uptake)
***Health economics
No health care system has enough resources to provide every clinically effective intervention to all people who could benefit
- Weigh benefits against cost
Health economists:
- Prioritise higher health gain per dollar spent > smaller health gain per dollar spent
- i.e. Choose most cost-effective interventions
—> Aim: Maximise population health
- **Cost effectiveness analysis:
1. Costs: Medical costs, Productivity loss etc.
2. Health outcome: Life-year, QALY, DALY, ICER
Different interventions compared using ICER (incremental cost-effectiveness ratio)
- ***Difference in cost / Difference in health outcome
Cost-effectiveness plane:
- X-axis: Health gained
- Y-axis: Additional cost
- Steeper slope: Higher ICER, Lower cost-effectiveness
Lower Right quadrant: Cost-saving (not just cost-effective) (favoured)
Upper Left quadrant: less effective, higher cost (not considered)
Upper Right quadrant: weigh ICER against Willingness to pay
- **Steps:
- Choose higher effect (越右越好)
- Choose lower ICER (slope唔斜)
- Compare ICER between ***successive pairs of options against Willingness to pay threshold
Rmb: An intervention is cost-effective if ICER «_space;**Willingness to pay threshold
—> **ICER slope不能夠斜過threshold
WHO Willingness to pay threshold: 1-3 GDP per capita / QALY
