Aetiology and Risk Factors for STIs

Question 1

Ro

Answer 1

basic reproductive rate; Ro = BcD where B = transmission probability per sexual partnership; c = rate of partner change; D = duration of infection

Question 2

Basic reproductive rate Ro reflects

Answer 2

number of secondary cases of an infection that arise from a primary case at t = 0

Question 3

If one primary case leads to two secondary cases

Answer 3

epidemic; Ro > 1

Question 4

If one primary case leads to half (on average)

Answer 4

disease disappears; Ro < 1

Question 5

As infection prevalence rises, what happens to susceptible population and epidemic?

Answer 5

number of susceptible people falls (because more people are infected/fewer people are not) leading to a levelling of the epidemic

Question 6

Steady state of an infection occurs when

Answer 6

new cases = cured cases/time

Question 7

Ro is dependent on

Answer 7

transmission probability per sexual partnership B; duration of infection D; rate of partner change c

Question 8

Spread and transmission of STIs depends on

Answer 8

reproductive rate Ro

Question 9

What is the effective reproductive rate?

Answer 9

Rt; basic reproductive rate times the proportion of people susceptible

Question 10

Rt

Answer 10

effective reproductive rate

Question 11

Why are viral STIs more difficult to control than bacterial?

Answer 11

Duration of infection for viral STIs is lifelong; bacterial can usually be treated over a few days and they are no longer infectious

Question 12

Probability of transmitting HIV per partnership (B) can be reduced by

Answer 12

reducing cofactors (gono, chlam, other STIs - more likely to transmit and receive HIV); decrease viral load; increase condom use; change type of sex

Question 13

nPEP is available for

Answer 13

exposure to HIV within the last 72 hours

Question 14

nPEP has to be taken for

Answer 14

1 mo

Question 15

When someone presents with possible HIV exposure, risk of exposure is established by

Answer 15

type of sex (and cofactors eg insertive vs receptive), duration, symptoms of STI, rough (bleeding), viral load; likelihood that partner is HIV+

Question 16

T/F nPEP is only available if the partner is HIV+

Answer 16

False; nPEP is available to those who have had sex with someone who is unknown to be HIV+

Question 17

Rate of partner change ‘c’ is made up of

Answer 17

mean number and SD

Question 18

The non-core group reflects the

Answer 18

mean of the population (majority of people, few partners)

Question 19

The core group reflects the

Answer 19

SD of the population (few people with many partners)

Question 20

The non-core group is comprised of

Answer 20

most people, and they have few partners

Question 21

The core group is comprised of

Answer 21

a few people who have many partners

Question 22

What is assortative mixing?

Answer 22

High prevalence with high prevalence - core with core (Ro&raquo_space; 1)

Question 23

What is disassortative mixing?

Answer 23

High prevalence with low prevalence - core with non-core (Ro < 1)

Question 24

How is the core group reflected in calculation of rate of partner change c?

Answer 24

SD^2; mean partners + (SD^2)/mean

Question 25

T/F Introducing an STI intervention aimed at the non-core group is more efficacious than those aimed at the core group

Answer 25

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

Question 26

T/F an intervention that reduces the mean number of partners but does not reduce the SD will reduce the spread of STIs

Answer 26

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

Question 27

Interventions that reduce the mean number of partners

Answer 27

will not have an effect on the number of STI cases overall because it promotes assortative mixing and spread within the core group

Question 28

What does a quadravalent vaccine mean with regards to Gardasil?

Answer 28

it protects against HPV 6 & 11 (genital warts) and 16 & 18 (cervical cancer)

Question 29

How can the susceptible group x be manipulated to reduce Rt?

Answer 29

increasing the proportion of vaccinated individuals reduces number susceptible and therefore decreases Rt even if Ro stays the same; can eventually change Ro

Question 30

Vaccination against HPV has

Answer 30

reduced Rt and Ro of genital warts

Question 31

What determines the number of STIs in different populations?

Answer 31

reproductive rate

Question 32

What is the strongest determinant of reproductive rate?

Answer 32

Duration of infection D

Question 33

If reproductive rate Ro = 1

Answer 33

infection is endemic in that population - each person who gets the disease passes it only to 1 other person on average