Aetiology and Risk Factors for STIs Flashcards
Ro
basic reproductive rate; Ro = BcD where B = transmission probability per sexual partnership; c = rate of partner change; D = duration of infection
Basic reproductive rate Ro reflects
number of secondary cases of an infection that arise from a primary case at t = 0
If one primary case leads to two secondary cases
epidemic; Ro > 1
If one primary case leads to half (on average)
disease disappears; Ro < 1
As infection prevalence rises, what happens to susceptible population and epidemic?
number of susceptible people falls (because more people are infected/fewer people are not) leading to a levelling of the epidemic
Steady state of an infection occurs when
new cases = cured cases/time
Ro is dependent on
transmission probability per sexual partnership B; duration of infection D; rate of partner change c
Spread and transmission of STIs depends on
reproductive rate Ro
What is the effective reproductive rate?
Rt; basic reproductive rate times the proportion of people susceptible
Rt
effective reproductive rate
Why are viral STIs more difficult to control than bacterial?
Duration of infection for viral STIs is lifelong; bacterial can usually be treated over a few days and they are no longer infectious
Probability of transmitting HIV per partnership (B) can be reduced by
reducing cofactors (gono, chlam, other STIs - more likely to transmit and receive HIV); decrease viral load; increase condom use; change type of sex
nPEP is available for
exposure to HIV within the last 72 hours
nPEP has to be taken for
1 mo
When someone presents with possible HIV exposure, risk of exposure is established by
type of sex (and cofactors eg insertive vs receptive), duration, symptoms of STI, rough (bleeding), viral load; likelihood that partner is HIV+
T/F nPEP is only available if the partner is HIV+
False; nPEP is available to those who have had sex with someone who is unknown to be HIV+
Rate of partner change ācā is made up of
mean number and SD
The non-core group reflects the
mean of the population (majority of people, few partners)
The core group reflects the
SD of the population (few people with many partners)
The non-core group is comprised of
most people, and they have few partners
The core group is comprised of
a few people who have many partners
What is assortative mixing?
High prevalence with high prevalence - core with core (Ro»_space; 1)
What is disassortative mixing?
High prevalence with low prevalence - core with non-core (Ro < 1)
How is the core group reflected in calculation of rate of partner change c?
SD^2; mean partners + (SD^2)/mean
T/F Introducing an STI intervention aimed at the non-core group is more efficacious than those aimed at the core group
False; the non-core group will stop having sex (fear of exposure) while the core group will continue to have sex with many partners - promoting assortative mixing and tf spread of STI in the core group
T/F an intervention that reduces the mean number of partners but does not reduce the SD will reduce the spread of STIs
False - this is targeting the non-core group (mean number of partners); need to take into account the SD and the spread in the core group of the community
Interventions that reduce the mean number of partners
will not have an effect on the number of STI cases overall because it promotes assortative mixing and spread within the core group
What does a quadravalent vaccine mean with regards to Gardasil?
it protects against HPV 6 & 11 (genital warts) and 16 & 18 (cervical cancer)
How can the susceptible group x be manipulated to reduce Rt?
increasing the proportion of vaccinated individuals reduces number susceptible and therefore decreases Rt even if Ro stays the same; can eventually change Ro
Vaccination against HPV has
reduced Rt and Ro of genital warts
What determines the number of STIs in different populations?
reproductive rate
What is the strongest determinant of reproductive rate?
Duration of infection D
If reproductive rate Ro = 1
infection is endemic in that population - each person who gets the disease passes it only to 1 other person on average
