How is infectivity calculated?

How is infectivity calculated?

• A. (number infected/ number susceptible) x 100
• B. (number infected/ total population) x 100
• C. (number susceptible/ number infected) x 100
• D. (number deaths/ number infected) x 100

Answer: (A)

Infectivity measures how likely it is for exposure to lead to infection in those who are susceptible. It’s expressed as the proportion of susceptible individuals who actually become infected, so you multiply by 100 to get a percentage. That means the calculation uses the number infected divided by the number susceptible, then multiplied by 100. The idea is to focus on risk among those who could actually catch the infection, not including people who are already immune or who weren’t exposed. Using the total population would mix in people who aren’t at risk, which doesn’t reflect per-exposure risk. Taking the inverse (susceptible divided by infected) isn’t meaningful for infectivity, and using deaths divided by infections gives a case-fatality rate, not infectivity.

Infectivity measures how likely it is for exposure to lead to infection in those who are susceptible. It’s expressed as the proportion of susceptible individuals who actually become infected, so you multiply by 100 to get a percentage. That means the calculation uses the number infected divided by the number susceptible, then multiplied by 100. The idea is to focus on risk among those who could actually catch the infection, not including people who are already immune or who weren’t exposed.

Using the total population would mix in people who aren’t at risk, which doesn’t reflect per-exposure risk. Taking the inverse (susceptible divided by infected) isn’t meaningful for infectivity, and using deaths divided by infections gives a case-fatality rate, not infectivity.