Q1 Assume that Ebola has a basic case reproduction number (R0) of 2.

Assume that Ebola has a basic case reproduction number (R₀) of 2. What percentage of the population must be immune in order to reach the herd immunity threshold?

• 67%

• 50%

• 33%

Q2 Vaccines are not 100% effective.

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%

• 50%

• 33%

Q3 If the R0 for Ebola was 1.3 rather than 2 (e.g. in a totally susceptible population where procedures

If the R₀ 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.

• For the same vaccine coverage a vaccine with lower efficacy could be used to reach the herd immunity threshold

• This would make no difference to the proportion of the population that would need to be vaccinated to reach the herd immunity threshold