Overview
This lecture uses the Public Health Model (problem to response, via Who?/Where?/When?, Why?, intervention development, and policy dissemination) as its organising framework to cover STI surveillance and notification in Aotearoa New Zealand, the descriptive epidemiology of the major notifiable STIs, the drivers of their population patterns, and the individual- and population-level interventions used to reduce their impact, worked through in detail via a cervical cancer/HPV case study and a summary of current government action plans.
The Public Health Model
A four-stage flow, prompted at the outset by “What?”, running along an axis from Problem to Response:
- Defining & measuring the problem (prompted by Who?/Where?/When?)
- Describing causes & consequences (prompted by Why?)
- Developing & evaluating interventions
- Disseminating effective policy & practice (prompted by “What can we do about it?”)
STI Surveillance and Notification in Aotearoa New Zealand
Sources of surveillance information:
- Clinic data for HPV, HSV and NSU (non-specific urethritis), from Sexual Health Clinics (broken down by age, sex, ethnicity, DHB) and from FPA and Youth Health Clinics
- Laboratory data for chlamydia and gonorrhoea diagnoses; since 2013 this includes data on who was tested, with positive tests linked to the person’s NHI
- Notification data: gonorrhoea and syphilis (and HIV) are notifiable, requiring clinical notification from all providers for every case
- Research studies and surveys
Notification is a legal requirement for health professionals to notify the Medical Officer of Health, and applies beyond infectious diseases generally. It serves surveillance (national-level trend monitoring; identifying higher-risk areas and population groups to inform service design and delivery) and public health action (Public Health units investigate cases and outbreaks and act to control spread where necessary, though most notifications need only a minimal response).
Notifiable STIs:
- HIV, AIDS, gonorrhoea and syphilis: notifiable without identifying information
- Hepatitis A, B and C: notifiable to the Ministry of Health
STIs and Their Complications
- Syphilis: congenital syphilis, tertiary syphilis
- Gonorrhoea and chlamydia: pelvic inflammatory disease, infertility, neonatal conjunctivitis
- HPV (Human Papilloma Virus): genital warts; cervical, anal and oropharyngeal cancer
- Herpes Simplex Virus: genital herpes
- Hepatitis A, B and C: chronic hepatitis, cirrhosis, hepatocellular carcinoma
- Non-specific urethritis
Descriptive Epidemiology of STIs
When: rolling 12-month rates from 2016 Q4 to 2025 Q1 show a shared pattern across all three notifiable STIs, a pre-pandemic peak, a trough around 2022 Q1, then a rebound.
- Gonorrhoea (rate per 100,000): rose from ~85 to a peak of ~150 (2019 Q4-2020 Q3), fell to ~115 (2022 Q1), rose again to ~145 (2024 Q2), settling around 135-140 by 2025 Q1.
- Chlamydia (rate per 100,000): stable at ~620-650 to 2019 Q4, declined to a trough of ~420 (2022 Q1), then partially recovered to ~500-520 (2023 Q3-2025 Q1).
- Syphilis (rolling 12-month case counts): rose from ~330 to a peak of ~700-720 (2019 Q4-2020 Q1), fell to ~430 (2022 Q1), rose to a peak of ~780 (2024 Q2), settling around 700 by 2025 Q1.
Where: choropleth maps of gonorrhoea rates, chlamydia rates and syphilis case counts by district all show the same pattern, higher rates/counts in the North Island, and particularly the upper North Island, than in the South Island.
Who:
- Gonorrhoea disproportionately affects males, especially men who have sex with men (MSM), and people aged 20-24
- Chlamydia is most reported among females and people aged 20-24
- Syphilis case numbers are highest in males, people aged 30-39, and the NZ European/Other ethnic group; about 55% of cases are MSM
- Māori and Pacific peoples continue to be inequitably affected by STIs
Drivers of STI Patterns and Sexual Networks
Why the population patterns arise:
- Prevalence drives incidence
- Limited access to healthcare, including timely testing and treatment
- Limited access to culturally appropriate sexual health services
- Social stigma
- Lack of awareness of risk among healthcare professionals and the public
Sexual network structure also shapes transmission potential. The Jefferson High adolescent sexual network study showed one large interconnected “giant component” cluster linking many individuals, alongside several smaller separate clusters, chains of 2-10 people, and isolated pairs/small groups (of 63, 12, 9 and 2 people) not connected to the larger network, illustrating that transmission potential varies enormously with network structure even within one population.
Prevention and Intervention
Individual-level prevention of infection:
- Vaccines: HPV, Hepatitis B, Hepatitis A
- Barrier methods: condoms, dental dams
- Pharmacological prophylaxis: pre-exposure (PrEP) or post-exposure (PEP)
- A combination of strategies, tailored to the individual, should be used
Population-level (public health) intervention, via policy/regulation supporting:
- Testing, treatment and safe sex: access to prompt diagnosis and treatment to break the chain of transmission; cost and availability of barrier methods and PrEP/PEP; targeted, culturally appropriate, accessible programmes and services; equitable access to antenatal care for pregnant people, including syphilis screening; alcohol harm reduction
- Measures to address the social determinants of health
- Surveillance
- Vaccination where possible
- Screening where appropriate
Case Study: Cervical Cancer Prevention
Persistent infection with certain HPV types is the main cause of cervical cancer; the WHO has a global strategy to accelerate the elimination of cervical cancer as a public health problem.
Primary prevention: HPV vaccination. A NZ study using the national cervical screening and vaccination registers (Sykes et al. 2025) found vaccination was associated with a reduction of more than two-thirds in cervical cancer, and that vaccinated people were about one-third less likely to have cervical precancer changes requiring treatment.
Secondary prevention: cervical screening. NZ cervical cancer incidence (age-standardised rate per 100,000 women) fluctuated between roughly 10 and 16 from 1950 to about 1990, then declined fairly steadily from about 12-13 down to roughly 7 by 2002, a decline coinciding with the introduction of organised cervical screening around 1990. HPV testing replaced cytology as the primary method of cervical screening in NZ on 12 September 2023, and provides 60-70% more protection against invasive cervical cancer than cytology screening. Screening can now be done via an 8-step patient self-test swab.
Screening Programme Criteria
NZ criteria for assessing screening programmes (National Advisory Committee on Health and Disability, 2003):
- The condition is a suitable candidate for screening
- There is a suitable test
- There is an effective and accessible treatment/intervention for the condition once detected early
- There is high-quality evidence, ideally from randomised controlled trials, that the programme reduces mortality or morbidity
- Potential benefit outweighs potential physical and psychological harm (from the test, diagnostic procedures and treatment)
- The health care system can support the whole screening pathway (diagnosis, follow-up, programme evaluation)
- Social and ethical issues are considered
- Cost-benefit issues are considered
The lecture applies criteria 2 and 4 to HPV screening specifically:
- Criterion 2 (suitable test): HPV testing is more sensitive (90-95%) than cytology for low-grade lesions, though its specificity is lower; a negative HPV test provides better reassurance
- Criterion 4 (high-quality evidence): Ronco et al. (Lancet 2014), following up four European randomised controlled trials, demonstrated the efficacy of HPV-based screening in preventing invasive cervical cancer
Government Policy Plans
Current Aotearoa New Zealand action plans referenced: the National Syphilis Action Plan (2019, aimed at stopping the syphilis epidemic), the National Hepatitis C Action Plan for Aotearoa New Zealand (2020-2030), a blood-borne infection strategy (2023-2030), and the National HIV Action Plan for Aotearoa New Zealand (2023-2030, aiming for elimination of HIV transmission and healthy, stigma-free lives for people living with HIV).
The title of the blood-borne infection strategy document (2023-2030) was partially obscured by an overlapping document cover in the source slide and could only be transcribed as far as legible.
Self-test
- Describe the four stages of the Public Health Model in order, and the question that drives each stage.
- List the data sources used for STI surveillance in NZ and what each source covers.
- Which STIs are notifiable in NZ, and what is the legal basis for notification?
- List the major complications associated with syphilis, gonorrhoea, chlamydia, HPV and HSV.
- Describe the trend in gonorrhoea, chlamydia and syphilis rates in NZ from 2016 to 2025, noting the pattern common to all three.
- Where in NZ are STI rates highest?
- Distinguish how gonorrhoea, chlamydia and syphilis differ in the populations they most affect.
- List the factors that drive the observed population patterns of STIs.
- Explain how sexual network structure, as illustrated by the Jefferson High study, affects STI transmission potential.
- List the individual-level strategies for preventing STI infection.
- List the public-health/population-level interventions used to reduce STI incidence and impact.
- Distinguish primary from secondary prevention of cervical cancer, giving an example of each from the lecture.
- What evidence supports HPV vaccination’s effect on cervical cancer and precancer rates in NZ?
- What change occurred in NZ cervical screening in September 2023, and what evidence supports HPV testing over cytology both as a “suitable test” and as “high-quality evidence” of reduced mortality/morbidity?
- Integrative: using the Public Health Model as a framework, describe how the lecture’s cervical cancer content maps onto each of the model’s four stages.
Answers
Reveal answers
- Defining & measuring the problem (Who?/Where?/When?) leads to describing causes & consequences (Why?), which leads to developing & evaluating interventions, which leads to disseminating effective policy & practice (What can we do about it?); the whole sequence is prompted at the start by “What?”.
- Clinic data (HPV, HSV, NSU) from Sexual Health Clinics and FPA/Youth Health Clinics; laboratory data for chlamydia and gonorrhoea (since 2013, linked to NHI); notification data for gonorrhoea, syphilis and HIV; research studies/surveys.
- HIV, AIDS, gonorrhoea and syphilis are notifiable without identifying information; hepatitis A, B and C are notifiable to the Ministry of Health. Notification is a legal requirement for health professionals to notify the Medical Officer of Health, not restricted to infectious diseases.
- Syphilis: congenital and tertiary syphilis. Gonorrhoea/chlamydia: pelvic inflammatory disease, infertility, neonatal conjunctivitis. HPV: genital warts, cervical/anal/oropharyngeal cancer. HSV: genital herpes. Hepatitis A/B/C: chronic hepatitis, cirrhosis, hepatocellular carcinoma.
- All three rose to a peak around 2019-2020, fell to a trough around 2022 Q1, then rose again toward 2024-2025 (gonorrhoea ~85 to ~150 to ~115 to ~145; chlamydia ~620-650 to ~420 to ~500-520; syphilis ~330 to ~700-720 to ~430 to ~700-780).
- Highest in the North Island, particularly the upper North Island, for gonorrhoea, chlamydia and syphilis alike.
- Gonorrhoea: mainly males, especially MSM, and 20-24 year olds. Chlamydia: mainly females and 20-24 year olds. Syphilis: mainly males, 30-39 year olds and the NZ European/Other ethnic group, with about 55% of cases MSM. Māori and Pacific peoples are inequitably affected by STIs generally.
- Prevalence driving incidence; limited access to healthcare (timely testing/treatment); limited access to culturally appropriate sexual health services; social stigma; lack of awareness of risk among healthcare professionals and the public.
- Network structure determines how far infection can spread: a network with one large interconnected “giant component” allows an infection to reach many linked individuals, while small isolated pairs or chains limit spread to those small groups; the Jefferson High study showed both patterns coexisting within one adolescent population.
- Vaccines (HPV, hepatitis B, hepatitis A); barrier methods (condoms, dental dams); pharmacological prophylaxis (PrEP pre-exposure, PEP post-exposure); a combination tailored to the individual should be used.
- Policy/regulation supporting testing, treatment and safe sex (prompt diagnosis/treatment, cost/availability of barrier methods and PrEP/PEP, targeted culturally appropriate accessible services, equitable antenatal care access including syphilis screening, alcohol harm reduction); measures addressing social determinants of health; surveillance; vaccination where possible; screening where appropriate.
- Primary prevention stops infection occurring, e.g. HPV vaccination, associated with over two-thirds reduction in cervical cancer and about one-third fewer precancer changes requiring treatment. Secondary prevention detects disease early through screening, e.g. organised cervical screening, associated with cervical cancer incidence in NZ declining steadily from about 12-13 to about 7 per 100,000 women between 1990 and 2002.
- A NZ study (Sykes et al. 2025) using national cervical screening and vaccination registers found vaccination was associated with a reduction of more than two-thirds in cervical cancer, and vaccinated people were about one-third less likely to have cervical precancer changes requiring treatment.
- HPV testing replaced cytology as the primary cervical screening method in NZ on 12 September 2023. As a suitable test, it is more sensitive (90-95%) than cytology for low-grade lesions (though less specific), gives better reassurance from a negative result, and provides 60-70% more protection against invasive cervical cancer than cytology. As high-quality evidence, Ronco et al. (Lancet 2014), following up four European RCTs, demonstrated HPV-based screening’s efficacy in preventing invasive cervical cancer.
- Defining & measuring the problem: cervical cancer incidence and HPV as its main cause. Describing causes & consequences: persistent infection with certain HPV types causing cervical cancer. Developing & evaluating interventions: HPV vaccination (primary prevention) and HPV-based screening (secondary prevention), evaluated against the NZ screening criteria (a suitable test, and high-quality RCT evidence of reduced mortality/morbidity). Disseminating effective policy & practice: national/international policy such as the WHO cervical cancer elimination strategy and NZ’s 2023 move to HPV primary screening.