9477 / 2027

Lesson 4 of 5 / Infection, vaccination and disease spread

Vaccination and interrupted transmission

How can one person's immunity reduce another person's exposure?

I can explain vaccination, smallpox eradication and benefits/risks.

  • I can explain vaccination, transmission interruption and smallpox eradication.Syllabus

    Syllabus 9477, Extension A(e). Explain vaccination, transmission interruption and smallpox eradication.

  • I can discuss vaccination benefits and risks.Syllabus

    Syllabus 9477, Extension A(f). Discuss vaccination benefits and risks.

About 7 min

Make a guess. You are not marked.

A vaccinated person still catches the disease. Does that prove the vaccine does not work?

  1. The pathogen must now be resistant.
  2. Compare risk in both groups.
  3. A working vaccine prevents each case.
Show the answer

Compare risk in both groups.

A vaccine can greatly lower risk without preventing every case. Compare vaccinated and unvaccinated outcomes.

The key idea

Vaccination primes adaptive protection; sufficient effective population immunity can interrupt chains of transmission.

Connect cause and consequence

Can the same protection count create different transmission paths?

Contacts are fixed in this modelISSSSSSSSSSSP protected; I initial source

Predict the reach before tracing the paths. With no protection, the contact network remains connected.

S = susceptible but not shown as reached; R = reachable in this model, not a recovered person. Protection is assumed perfect and every available path is traced; actual transmission is probabilistic, vaccines vary, and contacts are more complex. This explains the importance of mixing patterns, not a vaccine-coverage threshold.

Explanation

Vaccination presents antigens, or instructions leading to antigen production, in a way intended to stimulate protective adaptive responses without causing the target disease. Memory cells and other immune responses can reduce later infection, disease severity or onward transmission, depending on the vaccine and pathogen.

When enough people have effective protection against infection or transmission, an infectious person encounters fewer susceptible hosts and transmission chains can fail. This indirect protection depends on coverage, effectiveness, waning, mixing patterns and pathogen biology; it is not a guarantee that every vaccinated person is completely protected.

Smallpox eradication illustrates coordinated vaccination combined with surveillance and targeted containment of cases and contacts. Human-only maintenance of the infection and identifiable disease helped the campaign. Eradication means sustained worldwide interruption of natural transmission, not merely a temporary fall in one country's case count.

Benefits include reduced illness, deaths and disruption, and protection of vulnerable people through reduced transmission. Risks and limitations include adverse reactions, rare serious events, incomplete protection and practical access barriers. Evaluate their frequency and severity against disease risks using reliable evidence; this lesson explains population biology rather than choosing an individual medical treatment.

Step by step
  1. 1

    Explain individual protection

    Connect antigen exposure to adaptive memory.

  2. 2

    Explain the population effect

    Fewer successful transmissions can end chains.

  3. 3

    Evaluate proportionately

    Use evidence about both disease and adverse outcomes.

Worked example

Work through the evidence

Why might a highly vaccinated population still have some cases?

One way to explain it

Protection may be incomplete or wane, exposure may cluster among susceptible people, and a vaccine may reduce disease more strongly than infection. High coverage does not imply zero risk in every person.

Why this answer works
  • Population reduction and individual certainty differ.
  • Transmission assumptions must match the specific vaccine-pathogen system.

Watch out for this

A student says: "Any case after vaccination proves vaccination has no effect." What is wrong with this?

Show the answer

Effectiveness is assessed by comparative risk and outcomes, not a requirement that every case be prevented.

In the real world

Smallpox: a disease removed by vaccination

A global vaccination campaign led by WHO, with surveillance and containment of cases, ended smallpox. An independent commission certified its eradication in December 1979, and WHO declared it eradicated in 1980. It is the only human infectious disease eradicated so far.

Source: World Health Organization, Smallpox (health topic page)

Measles vaccination worldwide

WHO and the US CDC estimate that measles vaccination prevented about 60.3 million deaths worldwide from 2000 to 2023. Yet only 83 per cent of children received a first dose in 2023, and measles cases rose by about 20 per cent from 2022 to 2023, showing why high coverage matters.

Source: WHO and US CDC (2024), Progress toward measles elimination, worldwide, 2000-2023, MMWR 73(45)

Check your understanding

Which best explains indirect protection?

  1. Reduced transmission gives susceptible people fewer infectious encounters.
  2. Milder symptoms alone protect neighbours even if transmission is unchanged.
  3. Protection requires every person in the population to be individually immune.
Show the answer

Reduced transmission gives susceptible people fewer infectious encounters.

The chain is interrupted through population-level effects.

Quick recall

Card 1 of 3. Answer in your head, then check.

How can one person's immunity reduce another person's exposure?

You can return to this lesson any time.