Biology: Epidemiology: R0, Outbreak Graphs, and Interventions
Analyzing disease spread, outbreak graphs, and public health strategies
Biology: Epidemiology: R0, Outbreak Graphs, and Interventions
Analyzing disease spread, outbreak graphs, and public health strategies
Biology - Grade 9-12
- 1
Define R0 in your own words. Explain what it means if a disease has an R0 of 3 in a fully susceptible population.
Focus on the average number of secondary infections caused by one case.
R0, called the basic reproduction number, is the average number of new infections caused by one infected person in a fully susceptible population. If a disease has an R0 of 3, each infected person infects about 3 other people on average when no one has immunity and no interventions are in place. - 2
A disease has an R0 of 0.8. Predict whether the outbreak is likely to grow or shrink over time, and explain why.
The outbreak is likely to shrink over time because each infected person infects fewer than 1 new person on average. When R0 is below 1, transmission is not enough to sustain a growing outbreak. - 3
In a town, one infected traveler starts an outbreak. The disease has an R0 of 2. If conditions stay the same, estimate the number of new infections in the next three rounds of transmission after the traveler. Show the pattern.
Multiply the number of infected people in each round by 2.
The first round would have about 2 new infections, the second round about 4 new infections, and the third round about 8 new infections. The pattern is 1 infected traveler, then 2, then 4, then 8 because each case infects about 2 others on average. - 4
A virus has an R0 of 4. What fraction of the population would need immunity to reach the herd immunity threshold? Use the formula: herd immunity threshold = 1 - 1/R0.
Substitute 4 for R0 in the formula.
The herd immunity threshold is 1 - 1/4, which equals 3/4 or 0.75. About 75% of the population would need immunity to reduce sustained spread. - 5
Look at an outbreak graph where the number of new cases rises slowly at first, then rapidly increases, reaches a peak, and then declines. Describe what this graph suggests about the course of the outbreak.
The graph suggests that the outbreak began with limited spread, then entered a rapid growth phase as infections increased. The peak shows the highest number of new cases, and the decline suggests that transmission decreased, possibly because of immunity, behavior changes, interventions, or fewer susceptible people. - 6
An epidemic curve shows two separate peaks, with the second peak occurring about one incubation period after a large public event. What is a reasonable explanation for the second peak?
Think about the delay between exposure and symptoms.
A reasonable explanation is that the public event caused many new exposures, and those infected people developed symptoms after about one incubation period. This produced a second wave of cases on the epidemic curve. - 7
A county reports these new cases over 6 weeks: Week 1: 10, Week 2: 18, Week 3: 35, Week 4: 70, Week 5: 68, Week 6: 40. Identify the week when new cases peaked and describe the overall trend.
New cases peaked in Week 4 with 70 cases. The overall trend shows rapid growth from Week 1 to Week 4, followed by a slight decrease in Week 5 and a larger decrease in Week 6. - 8
Explain the difference between incidence and prevalence using an infectious disease example.
Incidence focuses on new cases, while prevalence focuses on total existing cases.
Incidence is the number of new cases that occur in a population during a specific time period. Prevalence is the total number of current cases at a specific time or during a period. For example, new flu infections this week are incidence, while everyone currently sick with flu today is prevalence. - 9
A school introduces a mask policy and improves ventilation. Before the policy, the effective reproduction number was 1.6. After the policy, it drops to 0.9. Explain what this change means for the outbreak.
This change means that transmission has decreased enough that the outbreak is expected to shrink. Before the policy, each infected person infected more than 1 other person on average, so cases grew. After the policy, each infected person infects fewer than 1 person on average, so cases should decline. - 10
Compare R0 and Rt. Why is Rt often more useful during an ongoing outbreak?
The letter t refers to a particular time.
R0 describes transmission in a fully susceptible population with no interventions. Rt, also called the effective reproduction number at time t, describes transmission at a specific time during an outbreak. Rt is often more useful because it changes as immunity, behavior, vaccination, and interventions change. - 11
A contact tracing team finds that one infected person attended a crowded indoor party and infected 12 people, while most infected people infected no one. Explain how superspreading can affect outbreak control.
Superspreading can cause sudden jumps in cases even if average transmission seems moderate. Outbreak control may need to focus on preventing high-risk events, improving ventilation, limiting crowding, and quickly tracing contacts from large exposure settings. - 12
A graph compares two outbreak curves. Curve A is tall and narrow, while Curve B is shorter and wider. Both curves include about the same total number of cases. Explain which curve would be easier for hospitals to manage and why.
Think about hospital capacity and the number of patients at one time.
Curve B would be easier for hospitals to manage because cases are spread out over a longer time, so fewer people need care at the same time. Curve A could overwhelm hospitals because many cases occur during a short period. - 13
A vaccine reduces the chance that a vaccinated person becomes infected and also reduces their chance of transmitting the pathogen if infected. Explain two ways this can lower Rt.
The vaccine can lower Rt by reducing the number of susceptible people who can become infected. It can also lower Rt by reducing transmission from breakthrough infections, so each infected person infects fewer people on average. - 14
In a city of 100,000 people, 20,000 people are currently immune to a disease. The disease has an R0 of 2.5. Use the herd immunity threshold formula to decide whether the city has enough immunity to prevent sustained spread.
First calculate the percent needed, then compare it with 20,000 out of 100,000.
The herd immunity threshold is 1 - 1/2.5, which equals 1 - 0.4, or 0.6. The city would need about 60% immunity, which is 60,000 people. Since only 20,000 people are immune, the city does not have enough immunity to prevent sustained spread. - 15
A public health department must choose between starting isolation of confirmed cases, contact tracing, vaccination clinics, and public communication. Explain how using several interventions together can reduce an outbreak more effectively than using only one.
Using several interventions together can reduce transmission at different points in the chain of infection. Isolation reduces spread from known cases, contact tracing finds exposed people, vaccination reduces susceptibility and transmission, and public communication helps people change behavior. Together, these actions can lower Rt more than one action alone.