Lecture 12. Public Health 3: Sexually Transmitted Infections (STI)

Question 1

How many diagnosed cases of STIs were there in England in 2019?

Answer 1

466,742

Question 2

What is the current STI causing the most concern?

Answer 2

Chlamydia (45-50% of total cases)

Question 3

When was vaccination against HPV introduced?

Answer 3

2008

Question 4

What disease was virtually eliminated in the UK through antibiotic use but has seen a resurgence in gay men?

Answer 4

Syphilis (8010 cases in 2019 with 7189 of those being men)

Question 5

Where are STIs diagnosed?

Answer 5

Genitourinary medicine (GUM) clinics

Question 6

How is prevalence calculated?

Answer 6

Incidence x duration

Question 7

What was the prevalence of chlamydia in females aged 16-24 in 2010-2012?

Answer 7

~3.2%

Question 8

Of those with chlamydia, what percentage are asymptomatic?

Answer 8

70% women
50% men

Question 9

What are the two aims of the National Chlamydia screening program?

Answer 9

Surveillance data (incidence in those people who report and are accurately diagnosed)
Treatment of infected individuals = control

Question 10

What is the relationship between genital chlamydial infection, pelvic inflammatory disease (PID) and the sequelae of PID?

Answer 10

10-40% of untreated genital chlamydial infections result in PID
PID can result in infertility (in ~20% of cases) and ectopic pregnancy (in ~47%)

Question 11

What are the risk factors of STIs?

Answer 11

STI have a lower limit to the “intimacy” of contact required for transmission (‘flu can be transmitted via sexual contact)
Risk of acquiring STI is almost exclusively determined by the rate of sexual partner change (Not necessary by the number of sexual contacts within a partnership)
Choice of partners (Men are ~4yrs older than their female partners, IQ of non-casual sexual partners is highly correlated)

Question 12

What age groups are the biggest risk of getting an STI in males and females?

Answer 12

Male: 25-34 (4.7%)
Female: 16-24 (6.7%)

Question 13

How can sexual behaviour be measured up to 1985 and why was this stopped?

Answer 13

Interview/questionnaire
Up to 1985 all were biased, and concentrated on attitudes & motivations

Question 14

What must be accounted for when measuring sexual behaviour through random, large-scale surveys?

Answer 14

They are subject to non-disclosure
Definitions of sexual acts are not commonly agreed (e.g in all studies the number of partners claimed by men exceeds those claimed by women)

Question 15

What is NATSAL?

Answer 15

The National Survey of Sexual Attitudes and Lifestyles
Rolled out every 10 years (started in the 90s)

Question 16

What data is shown from NATSAL 3 (2010-12)?

Answer 16

Average age of first intercourse had decreased, but average age of first child had increased
The number of people having large numbers of sexual partners over a five year time span has increased

Question 17

Why do reported results ≠ actual results?

Answer 17

Self-report is a poor predictor of recent sexual behaviour
(e.g large number of 16 year olds claim large amount of sexual patterns or women having PSA when claiming they didn’t have sex)

Question 18

What are prostate-specific antigen (PSA levels)?

Answer 18

Measure of exposure to semen in the past 2 days

Question 19

What is the distribution of sexual activity within the UK?

Answer 19

Most people have no/few partners, few have very many (about 20% of people have 80% of sexual partnerships)

Question 20

Which is higher in the distribution sexual activity, variance (v) or mean (m)?

Answer 20

v > > m

Question 21

What is the average variance and mean number of sexual partners of homosexuals within the UK?

Answer 21

v ≈ 5 per year
m ≈ 1 per year

Question 22

What is R₀ proportional to?

Answer 22

(m² + v)/m = (m + v/m)

Question 23

What is the core group?

Answer 23

10+ sexual partners
Small proportion have disproportionate effect on epidemic
Basic reproduction number should include the variability in behaviour
Most likely to become infected and most likely to infect

Question 24

What is an assortative sexual behaviour?

Answer 24

People with many sexual partners are more likely to have partners who have many sexual partners

Question 25

What is a random sexual behaviour?

Answer 25

People have sexual partners at random

Question 26

What is a disassortative sexual behaviour?

Answer 26

People with many sexual partners have partners who have few sexual partners

Question 27

What sexual behaviour increases R₀?

Answer 27

Assortative

Question 28

What is concurrency?

Answer 28

Sexual partnerships in parallel bigger risk than sexual partnerships in series

Question 29

What does human papillomavirus (HPV) cause?

Answer 29

Genital warts and cervical, anal, penile & oropharyngeal cancers

Question 30

Is the HPV vaccine against all types of HPV?

Answer 30

No, only some

Question 31

Why did did the UK switch from the bivalent Cervarix to Garadasil HPV vaccine?

Answer 31

Garadasil protects against the four main types of HPV (6, 11, 16 and 18) and is more cost-effective

Question 32

If all females are successfully vaccinated against HPV, do males need to get a HPV vaccine?

Answer 32

If all females were successfully vaccinated, then heterosexual males would not contract HPV. However given that the benefits of the programme would be seen over an extended period of time, it agreed to apply a different rate to assessing the vaccine which meant that a gender neutral programme was deemed to be cost-effective.