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Connecticut Measles Outbreak: Epidemic Dynamics Lab

Connecticut Phenomenon · 2014 Measles Cluster · NGSS HS-LS2-6

In early 2014, a measles case was confirmed in a Connecticut school community linked to an unvaccinated student who had recently traveled internationally. Health officials scrambled to identify who had been exposed and whether vaccination rates in the school were high enough to stop the outbreak. How does a single case become an epidemic — and what determines whether it stays that way?

NGSS HS-LS2-6: Evaluate the claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may result in a new equilibrium or a new ecosystem state.

🔮 Before You Simulate — Make a Prediction!

If measles has a spread rate of about 3 (out of 5), what percentage of students would need to be vaccinated to prevent the outbreak from spreading? Write your prediction before you press Start.

Population Over Time

📊 Analysis Questions

  1. Set Vaccination to 0% and Spread Rate to 3. Start the simulation. How quickly does the disease reach peak infection? What percentage of the population was ultimately infected?
  2. Keeping Spread Rate at 3, increase Vaccination from 0% to 100% in steps of 10%. At what vaccination percentage does the epidemic stop spreading significantly? How does this compare to the displayed Herd Immunity Threshold?
  3. Change the Spread Rate to 5 (high, like measles). How does the required vaccination coverage change? Use the formula (Threshold = 1 – 1/R₀) to explain why highly contagious diseases need higher vaccination rates.
  4. Set Vaccination to 50% and run with 0% social distancing vs. 60% social distancing. What does combining interventions show you about ecosystem stability?
  5. In the real 2014 Connecticut measles cluster, some students who could be vaccinated had chosen not to be. How does your simulation demonstrate why vaccination is a community (ecosystem) decision, not just a personal one?

🦠 The Science Behind the Simulation

Measles is caused by the Morbillivirus and spreads through respiratory droplets. It is one of the most contagious pathogens known — its basic reproduction number (R₀) ranges from 12 to 18 in unvaccinated populations, meaning one infected person infects 12–18 others on average.

The SIR model at the heart of this simulation classifies every individual as Susceptible, Infectious, or Recovered. Recovered individuals are immune and cannot re-enter the susceptible pool, so the outbreak naturally burns out when it runs out of hosts.

Herd immunity emerges when enough individuals are immune that the average infected person transmits to fewer than 1 new person — the effective reproduction number Reff falls below 1 and the outbreak shrinks. The threshold is approximately 1 – 1/R₀. For measles (R₀ ≈ 15), this is about 93%.

NGSS Connection (HS-LS2-6): The "stable state" of a school community is no active disease. Introducing an infectious pathogen is a changing condition. High vaccination coverage represents the mechanism that restores stability — a new ecosystem equilibrium where the pathogen cannot persist.