You have heard that two different flu epidemics may threaten

You have heard that two different flu epidemics may threaten your town next year, but you only have the budget to purchase one vaccine.

Strain A has R0 of 1.5. A similar strain swept through the town several years ago and you estimate that 2,000 people are already immune. Within your budget, you can purchase 1,500 doses of vaccine. The vaccine is 100% effective.

Strain B has R0 of 1.7. No one in town has been exposed to any similar strain, and you expect no one starts with any immunity. Within your budget, you can purchase 4,000 doses of vaccine, but the vaccine is only 50% effective.

       Do you purchase the vaccine doses for Strain A or Strain B? What criteria do you use?

Solution

The spread of an infection depends on the number of susceptible individuals. A chain of disease spread can occur only when susceptible individuals are available and the chain is not broken. With more number of individuals who are already immune to strain A, the chain of spread can be expected to be low. Also, a large number of persons are immune to the strain.

Since there is no known immunity to strain B, it is wise to purchase vaccine for strain B rather than for strain A. As none of the population is immune, the disease spread will be rapid and more severe. The chain cannot be broken and can lead to an epidemic/pandemic. Though the vaccine is only 50% effective, vaccination can somehow prevent the prevalance of the flu.