Infectious Disease Epidemiology

Infectious disease epidemiology provides the tools to detect, characterise, and control outbreaks. Core concepts include the epidemic curve, reproduction number, attack rate, and outbreak investigation methodology.

An epidemic curve (epi curve) plots the number of new cases over time. Its shape provides critical information about the source and mode of transmission. A point-source outbreak — where all cases are exposed to a single common source at roughly the same time — produces a sharply peaked, bell-shaped curve with cases clustered within one incubation period. Examples: a foodborne outbreak from a shared meal, or a deliberate release of a biological agent. A propagated (person-to-person) epidemic curve shows successive waves, with each wave separated by approximately one incubation period. Examples: influenza, COVID-19. A mixed pattern (initial point source followed by person-to-person spread) is also recognised.

The basic reproduction number (R₀) is the average number of secondary infections produced by one infectious case in a fully susceptible population. R₀ depends on the probability of transmission per contact, the number of contacts per unit time, and the infectious period. For SARS-CoV-2 (original Wuhan strain), R₀ was estimated at 2–3; for measles, R₀ is 12–18 (among the highest known). The effective reproduction number (Rₑ or Rt) accounts for partial immunity in the population. Herd immunity threshold = 1 − 1/R₀; for measles with R₀ = 15, herd immunity requires approximately 93% population immunity.

The attack rate (AR) in an outbreak is the proportion of exposed individuals who become ill: AR = cases / exposed persons × 100%. Secondary attack rate (SAR) measures spread within households or close contacts.

The steps of outbreak investigation are: (1) verify the diagnosis and confirm an outbreak exists; (2) establish a case definition (clinical, laboratory, epidemiological criteria); (3) find cases systematically (active case finding); (4) describe the data by person, place, and time; (5) generate hypotheses from descriptive data; (6) test hypotheses using analytical studies (cohort or case-control); (7) implement control measures; (8) communicate and report findings.

In New Zealand, notifiable disease reporting is governed by the Health Act 1956 (now Health (Notifiable Diseases, Conditions, and Risks) Regulations 2016). Clinicians and laboratories must notify the Medical Officer of Health (MOH) within specified timeframes (immediately for some, within 24 hours or 7 days for others). Notifiable diseases include meningococcal disease, tuberculosis, measles, typhoid, salmonellosis, and COVID-19 (added 2020). Contact tracing is coordinated through Public Health Units.

New Zealand has experienced several significant infectious disease epidemics: a meningococcal B epidemic from 1991 to 2007 (addressed with a tailor-made MeNZB vaccine campaign 2004–2007); ongoing endemic meningococcal C requiring routine immunisation; and the COVID-19 pandemic from 2020, during which NZ's initial elimination strategy (border closure, Alert Level system, aggressive contact tracing) resulted in very low case counts before widespread vaccination.