Question 1

What does R₀ mean and why is it important?

Accepted Answer

R₀ (the basic reproduction number) is the expected number of new infections caused by a single infectious person in a population where everyone is susceptible. An R₀ greater than 1 means each person infects more than one other on average, so the epidemic grows. An R₀ below 1 means the disease will die out naturally. R₀ determines the scale of outbreak, required vaccine coverage, and effectiveness of interventions.

Question 2

What is the difference between the SEIR and simpler SIR models?

Accepted Answer

The SIR model has three compartments (Susceptible, Infected, Recovered) and assumes people become immediately infectious upon exposure. SEIR adds the Exposed (E) compartment to represent the incubation period—the delay between infection and becoming contagious. This delay is crucial for diseases like COVID-19, influenza, or measles where the incubation period significantly affects the shape and timing of epidemic curves.

Question 3

How does vaccination change the epidemic dynamics?

Accepted Answer

Vaccination moves individuals directly from Susceptible to Recovered (or Immune), effectively reducing the starting size of the susceptible pool. As more people are vaccinated, the effective reproduction number Rₑ = R₀ × (S/N) decreases. Once Rₑ falls below 1—the herd immunity threshold—the epidemic cannot sustain itself, protecting even unvaccinated individuals through indirect immunity.

Question 4

Why does the epidemic curve have a single peak and then decline?

Accepted Answer

As infections accumulate the susceptible pool shrinks and infected people increasingly encounter immune individuals. Eventually Re drops below 1 and new infections occur more slowly than recoveries, causing the infected compartment to peak and decline.

Question 5

What are the limitations of SEIR models?

Accepted Answer

SEIR models assume a well-mixed homogeneous population. Real populations have age structure, spatial clustering, and behavioral heterogeneity, which more advanced network and age-stratified models capture.

Question 6

What does the Incubation 1/sigma slider control, and why does it delay the epidemic without changing its final size?

Accepted Answer

1/sigma (1-14 days) is the average time Exposed before becoming Infectious. A longer incubation stretches the timeline and delays the peak but does not change R0 or the final infected fraction, which is governed by R0 alone.

Question 7

How does the Infectious 1/gamma slider affect the shape of the epidemic curve?

Accepted Answer

1/gamma (3-21 days) sets how long a person stays contagious. Since R0 = beta/gamma, changing 1/gamma while holding R0 fixed adjusts beta to compensate, but a longer infectious period on its own means more opportunities to spread disease.

Question 8

What does the Vax rate slider do, and why does a small percentage per day matter so much?

Accepted Answer

Vax rate (0-2%/day) moves that fraction of Susceptible directly into Recovered daily. This steadily shrinks S/N, compounding over weeks and pushing Re = R0 x (S/N) below 1 sooner, flattening or preventing the peak.

Question 9

Why does setting R0 below 1 prevent an outbreak from happening at all?

Accepted Answer

With R0 below 1 each infectious person infects fewer than one other on average even with a fully susceptible population, so transmission shrinks each generation instead of growing, and Exposed/Infectious decay back toward zero.

Question 10

Why do higher R0 values produce a taller but narrower epidemic peak?

Accepted Answer

Higher R0 means faster spread per generation, but also burns through the susceptible pool faster, so Re crosses below 1 sooner in absolute time, producing a sharper, higher peak that resolves faster than a low-R0 epidemic.

🏥 SEIR Epidemic Model — Exposed & Intervention Dynamics

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