Q2.20. You are devising a vaccination program for a strain of
influenza with an estimated Ro of 2.5. What proportion of a
susceptible population do you need to vaccinate to impart herd
immunity and prevent an epidemic?

Taking into account that the influenza strain has an estimated R₀ of 2.5, the proportion of the susceptible population that needs to be vaccinated is 60%, i.e. 6 out of 10 people.

Explanation:

The basic rate of reproduction or R₀ is an epidemiological indicator that guides the possibility of an infected disease becoming epidemic. R₀ means the number of people who can be infected by contact with a sick person or carrier.

When scheduling a vaccination program against a transmissible disease, the value of R₀ serves to determine what coverage should be ensured by immunization to avoid an epidemic. The calculation is made as follows:

Susceptible population to be vaccinated (SPV) = 1 - (1 / R₀)

Taking into account that R₀ = 2,5

SPV = 1 - (1 / 2,5) = 1 - 0,4 = 0,6

This means that, for a population of 1,000,000 inhabitants, in the presence of an influenza strain with an estimated R₀ of 2.5, vaccination coverage must include at least 600,000 people, i.e. 60% of that population , since 1.000.000 X 0,6 = 600.000.

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