An Index Case Enters an Undervaccinated Community
One traveler brings measles into a low-coverage neighborhood.
- 12–18: Measles R0 (most contagious human virus)
- ~95%: Herd immunity threshold (coverage needed to block spread)
- ~2 hrs: Airborne persistence (virus lingers in room air)
- 4 days: Pre-rash contagious window (spreads before symptoms show)
Why measles ignites so fast
Each case infects twelve to eighteen unprotected contacts.
One case can seed a whole outbreak within days.
Undervaccinated pockets break herd immunity
Coverage below ninety-five percent leaves susceptible clusters exposed.
The first ninety-six hours
Contagious before rash, the index case travels unnoticed.
Early Spread Through Susceptible Contacts
Daily new cases climb sharply as chains of transmission multiply.
- 10–12 days: Incubation period (exposure to first symptoms)
- ~90%: Secondary attack rate (in susceptible household contacts)
- ~4–5 days: Doubling time (in low-coverage clusters)
- shared room: Airborne spread radius (no direct contact needed)
Exponential, not linear, growth
Each generation of cases multiplies rather than adds.
A handful of cases becomes dozens within two weeks.
Susceptible clustering accelerates spread
Unvaccinated pockets act as fuel for the chain.
Detection lags behind transmission
Cases are confirmed days after exposure already occurred.
Peak Transmission in the Outbreak Curve
The epidemic curve crests as susceptible supply and contacts intersect.
- 2–3 weeks: Typical outbreak peak (after index case)
- dozens/day: Cases at peak (small town) (in undervaccinated settings)
- ~1 in 5: Hospitalization rate (reported measles cases)
- ~1 in 20: Complication rate (develop pneumonia)
What drives the curve to crest
Growth slows only once susceptible contacts run short.
Peak height depends heavily on starting vaccination coverage.
Healthcare strain at peak
Clinics face surging visits for fever and rash.
The intervention window
Public health teams race to trace contacts now.
Contact Isolation Breaks the Chain
Quarantining exposed contacts removes fuel from the transmission chain.
- 21 days: Isolation window needed (from last exposure)
- ~20 contacts: Contact tracing effort (per confirmed case)
- well below 1: Effective R with isolation (at high compliance)
- ~1–2 weeks: Time to flatten curve (after isolation starts)
How isolation cuts transmission
Removing spreaders from circulation stops onward chains.
High compliance can flatten a curve within days.
Compliance is the deciding variable
Partial isolation still leaves gaps for spread.
Racing the exponential
Isolation must outpace the chain it is chasing.
Outbreak Decline and Containment
Daily cases fall as susceptibles deplete and isolation takes full effect.
- 6–10 weeks: Typical outbreak duration (from index to zero cases)
- majority: Cases prevented by isolation (at high compliance)
- catch-up drives: Post-outbreak vaccination push (to rebuild coverage)
- re-examined: Elimination status risk (after sustained transmission)
Two forces bend the curve down
Fewer susceptibles remain, and isolation removes spreaders.
Combined effects end outbreaks faster than either alone.
Declaring the outbreak over
Health authorities wait two incubation periods without cases.
Preventing the next outbreak
Closing the vaccination gap is the lasting fix.