L22 - Sex, germs and vaccines Flashcards
Epidemiological Triangle of STI
1) Host: Human
2) Etiological Agents: STI agents
3) Environment
Etiological agents of STIs
Bacterial: Syphilis, gonorrhea, chlamydia
Viral: HPV, HBV, HIV, HSV
Protozoal: trichomoniasis
Environmental factor of STI
Transmission dynamics of STIs subject to continuous change of high level of clustering driven by:
- spatial location
- race
- culture
- religious belief
- sexual activity level
- socio-economic status
- education level
Levels of effect of STI
(in order of number of people affected)
1) Individual
2) Sexual partners and unborn child
3) Population
Risk of sexual practise and infection
Higher risk of sexual practices - infection less asymptomatic
STI transmission dynamics
- infectiousness may persist for extended periods due to natural history and asymptomatic infections (most STI rarely generate a solidly protective immune response)
- Sexual behaviour heterogenous within populations
- Core group -> bull’s eye for preventive program
- Not only sexually active persons are affected by STIs (mother-to-child, blood transfusion)
Core Group examples
- CSW: Commercial Sex Workers
- IDU: Injecting Drug Users
- MSM: Men who have sex with men
STI epidemic determinants
1) Sexual Structure
2) Societal Determinants
3) Interventions
Sexual structure
- Age of sex debut
- Sexual orientation and attitudes
- Number of sex partners (past year, lifetime)
- Sex practice (e.g. vaginal, oral, anal)
- Concurrency of sexual partnerships
- Pattern of serial monogamy
- Frequency of sex without condoms
- Frequency of casual and paid sex
- Size and distribution of high risk/core groups
- Sexual mixing between groups odf different attributes
Societal determinants of STI determinants
Stigma
culture
level of poverty
income
socio-economic status
literacy and education
employment
status of women
volume of mobility and migration
Interventions of STI epidemic
1) Diagnostic tools
2) Medical Treatments
3) Screening programs
4) Education and awareness programs
5) Prevention programs (e.g. partner notification, use of condoms)
While routes of transmission and preventive measures of STIs sufficiently understood, why is preventive control not effective?
STIs remain somewhat of a social taboo
Elimination of STIs in revolutionary China
- Training of paraprofessional and public health personnel, e.g. barefoor doctors
- Mass screening and treatment under the cooperative health insurance system: surveys -> diagnostics -> treatment
- Propaganda that targets STIs instead of patients; culprit identified as western civilization and capitalism; elimination of STI considered patriotic
- Complete elimination of prostitution (incarceration, reeducation and improvement of women status)
- Place chief emphasis of prevention: immunity through knowledge
100% Condom Campaign (Thailand)
Enforce condom use in commercial sex (main source of heterosexual HIV infection)
- government supplied as many condoms to sex establishments as needed
- Non-compliant sex establishments sanctioned
- Mass media campaign to stress that men should use condoms with prostitutes
Sexual Mixing
Sexual mixing is strongly associated with social structure and values (see sexual structure)
Can be classified into three types:
1) Assortative (like with like)
2) Proportionate (random)
3) Disassortative (dislike)
It is usually assortative for most attributes (age, sexual activeness, race, socio-economical statis); in the case of clubbing, while apparently proportionate, it is still assortative as the clubbing population/group is characteristic
Sexual network
- Few empirical studies due to practical and ethical difficulties
- Most studies focus on networks in the context of disease transmission
- Not just an individual’s number of partners that determines the risk of STI acquisition, but their partners’ partners and beyond, i.e. the entire sexual network
HPV and Cervical Cancer
- Cervical cancer is caused by persistent infection of high-risk HPV (type 16, 18 account for 70% of cervical cancers)
- HPV infection is most common STI worldwide
- Most HPV infection is harmless and clears within 12 months
- Incubation time is years to decades
- HPV type 6, 11 accounts over 90% of genital warts
- Weak natural immunity (common reinfection)
Available commercial HPV vaccines
1) Gardasil (Merck): >90% efficacious against type 6, 11, 16, 18
2) Cervarix (GSK): >90% efficacious against type 16, 18
Herd immunity
Part of vaccine effectiveness - vaccinating an individual indirectly reduces the risk of infection of his contacts, contacts’ contacts and so on, ultimately protecting the whole population
(Australian free Gardasil vaccination for 12-26yr women; significant decline of genital warts among young females and young heterosexual males)
Health economics
Problem: healthcare system do not have resources to every clinically effective intervention to all persons who can benefit
Resolution: health services should prioritize clinical practices that yield a relatively high health gain per dollar spent over those that give a smaller health gain per dollar spent (prioritize interventions that are the most cost-effective).
Cost-effectiveness Analysis (CEA) Overview
- WHO recommend CEA of vaccination considered before national programs
- CEA require expertise in infectious disease epidemiology (assess effectiveness) and health economics (asset costs)
CEA calculations
Cost
- medical costs
- productivity loss
Health outcome
- Life-year
- Quality-adjusted life year (QALY)
- Disability-adjusted Life year (DALY)
- (herd immunity in case of vaccine)
ICER (Incremental cost-effectiveness ratio)
Difference in cost / difference in health outcome
Analysis
An intervention is cost-effective if its ICER is smaller than the cost-effectiveness threshold (see graph)
