Title: Default for Step 3.6 Calculating vaccine efficacy and herd immunity threshold - Quiz

Q: Assume that Ebola has a basic case reproduction number (R_0) of 2. What percentage of the population must be immune in order to reach the herd immunity threshold?
- 67% (Remember the formula for herd immunity threshold: Herd immunity threshold = (R_0-1)/R_0 You may find 3.5 How would vaccines help? useful.)
- 50% (The formula for herd immunity threshold = (R_0-1)/R_0. As such, in this case herd immunity threshold: (2-1)/2= 0.50 = 50%)
- 33% (Remember the formula for herd immunity threshold: Herd immunity threshold = (R0-1)/R0 You may find 3.5 How would vaccines help? useful.)
A: 50%

Q: Vaccines are not 100% effective. The percentage of the population that needs to be vaccinated to reach the herd immunity threshold (the vaccine coverage) depends on the vaccine efficacy.

Assume a vaccine for Ebola is available with an efficacy of 75%, and that the R0 for Ebola is 2 (as in the previous example).

What proportion of individuals must be vaccinated in order to reach the herd immunity threshold? (Hint: the HIT has already been calculated and is 50%.)
- 67% (Required vaccine coverage = herd immunity threshold / vaccine efficacy. Therefore, the proportion that would need to be vaccinated to get R=1 = 50% / 75% = 0.667 or 67%.)
- 50% (Remember that the required vaccine coverage = herd immunity threshold / vaccine efficacy. You may find 3.5 How would vaccines help? useful.)
- 33% (Remember that the required vaccine coverage = herd immunity threshold / vaccine efficacy. You may find 3.5 How would vaccines help? useful.)
A: 67%

Q: If the R_0 for Ebola was 1.3 rather than 2 (e.g. in a totally susceptible population where procedures for contact tracing and isolation were already in place), which of the following is true, compared to the scenario in Q2
- You would need to vaccinate more people to reach the herd immunity threshold. (Remember the herd immunity threshold is R0-1/R0. If the herd immunity threshold is lower you would need to vaccinate fewer people. You may find 3.5 How would vaccines help? useful.)
- For the same vaccine coverage a vaccine with lower efficacy could be used to reach the herd immunity threshold (Remember the herd immunity threshold is R_0-1/R_0, so is now (1.3-1)/1.3 = 23%. With a vaccine coverage of 67% (as in Q2) even a vaccine with quite low efficacy could be helpful.)
- This would make no difference to the proportion of the population that would need to be vaccinated to reach the herd immunity threshold (Remember the herd immunity threshold is R0-1/R0. If the herd immunity threshold is lower you would need to vaccinate fewer people. You may find 3.5 How would vaccines help? useful.)
A: For the same vaccine coverage a vaccine with lower efficacy could be used to reach the herd immunity threshold