💉 Measles Outbreak in Undervaccinated Population Simulator

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.