High Coverage Herd Immunity
High coverage blocks measles from sustaining transmission.
- 12–18: Measles R0 (most contagious human virus)
- ≥95%: Herd immunity threshold (vaccination coverage needed)
- 97%: MMR two-dose efficacy (protection against infection)
- Near zero: Outbreak risk at 97% (chains fail to sustain)
Why measles needs 95%
Each measles case infects twelve to eighteen others.
Community protection
Vaccinated majority blocks chains before they start.
Two-dose MMR schedule
Two doses give about ninety-seven percent protection.
The 95% Threshold Line
95% coverage is the fragile herd immunity line.
- 95%: Threshold coverage (borderline herd immunity)
- ~0.75: Effective R at 95% (below epidemic threshold)
- 5%: Susceptible fraction (population still vulnerable)
- Small: Cluster risk (localized case clusters possible)
The threshold line
95% is the minimum for lasting herd immunity.
Still fragile
Small unvaccinated pockets still allow local clusters.
Why so high
Very high R0 demands a very high threshold.
Coverage Drops to 90%
Below 95%, measles regains its transmission advantage.
- 90%: Coverage (below the herd threshold)
- ~1.5: Effective R (sustained transmission begins)
- 10%: Susceptible pool (of the population)
- Weeks: Outbreak duration (chains persist locally)
Threshold broken
Below 95%, sustained transmission becomes possible again.
Effective R rises
Effective R climbs above one at ninety percent.
Local outbreaks
Unvaccinated clusters sustain multi-week case chains.
Coverage Drops to 80%
80% coverage lets outbreaks spread rapidly citywide.
- 80%: Coverage (well below threshold)
- ~3.0: Effective R (rapid case growth)
- 20%: Susceptible pool (of the population)
- 3×: Cases per generation (growth each transmission cycle)
Rapid growth
Each transmission generation roughly triples case count.
Community spread
Outbreaks move fast through schools and households.
Strained response
Contact tracing struggles to keep pace here.
Low Coverage Outbreak
Low coverage returns measles to pre-vaccine spread levels.
- 65%: Coverage (critically low)
- ~5.3: Effective R (explosive spread)
- 35%+: Susceptible pool (of the population)
- Majority: Final attack rate (of susceptible residents infected)
Widespread outbreak
Most susceptible individuals eventually become infected.
Pre-vaccine echo
Spread resembles measles before vaccination existed.
High stakes
Complications rise sharply among unvaccinated children.