Continuing with the saga of our Hypthetical Small Town, USA and its seasonal “flu” assessment.
In a normal year, the 100 citizens of Hypothetical Small Town, USA:
- 50 people remained clinically well
- 46 people had a Non-influenza-Influenza-Like Illness (NI-ILI)
- 4 people had an ILI caused by the Influenza Virus (the “true flu”)
Recapping the mayor’s problem:
Shall we vaccinate all 100 people to try to prevent these 4 cases of ILI which are caused by the Influenza Virus (the “true flu”)?
How effective is the vaccination for influenza? The CDC advises that the seasonal influenza vaccination averages about 40% effectiveness.
This means that 6 out of 10 people vaccinated are NOT protected. (I will return to this finding in a later discussion as this low effectiveness prevents the herd immunity phenomenon.)
If our mayor decides to vaccinate all 100 people of Hypothetical Small Town, USA, we expect that they can reduce the 4 “true flu” cases to 2.
Expressed another way:
In Hypothetical Typical Small Town, USA, they must vaccinate 50 people to prevent 1 case of Influenza.
This way of phrasing the statistic lets us compare a couple of studies.
The Cochrane Collaboration meta-analysis of the effectiveness of the Influenza Vaccine in healthy aduts found 71 people would need to be vaccinated to avoid one influenza case.
In healthy children, the influenza vaccination was more effective with 7 children needing to be vaccinated to prevent one influenza case.
In the elderly, 30 people would need to be vaccinated to prevent one influenza case.