Vaccination Question

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Forgive me if this is the wrong forum but I'm having a rough time deciphering some information.

Obviously, there is a lot of talk about vaccinations in the media due to COVID.

I'm hoping you all can answer a question for me.....

When the CDC says something like "this flu vaccine is 70% effective" what exactly does that mean?

Does it mean that 70% of people who take the vaccine become immune and 30% do not?

Does it mean that 100% of the people develop 70% immunity so that if they are infected with influenza, they only come down with a mild case?


The flu has a very large number of different strains (Types of Influenza Viruses). It is difficult to predict with 100% precision which strains will be the most common during a given year. Often they chose the 4 (hence calling it a Quadrivalent vaccine) strains that most likely to spread quickly during that year. This changes each year and is the reason why it is given every year as opposed to only a few times over your lifetime like most other vaccines.

Some people will still, unfortunately, become infected by a strain that is not covered by the vaccine and will fall ill with the flu. So the 70% statistic would be attributed to people who were protected from getting one of the four strains due to the vaccine. The 30% get sick from a flu strain not covered by the Quadrivalent vaccine.


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May 24, 2017
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The most relevant sections are these:

How does CDC present data on flu vaccine effectiveness?
CDC typically presents vaccine effectiveness (VE) as a single point estimate: for example, 60%. This point estimate represents the reduction in risk provided by the flu vaccine. CDC vaccine effectiveness studies measure two outcomes: laboratory confirmed flu illness that results in a doctor’s visit or laboratory-confirmed flu that results in hospitalization. For these outcomes, a VE point estimate of 60% means that on average the flu vaccine reduces a person’s risk of a flu outcome by 60%.

In addition to the VE point estimate, CDC also provides a “confidence interval” (CI) for this point estimate, for example, 60% (95% CI: 50%-70%). The confidence interval provides a lower boundary for the VE estimate (e.g., 50%) as well as an upper boundary (e.g., 70%). One way to interpret a 95% confidence interval is that if CDC were to repeat this study 100 times and calculate 100 confidence intervals, 95 times out of 100, the confidence interval would contain the true VE value. A simpler interpretation is that there is a 95% chance that the true VE lies within the confidence interval – therefore, there is still the possibility that five times out of 100 (a 5% chance) the true VE value could fall outside of the 50%-70% confidence interval.

What do recent vaccine effectiveness studies show?
Recent studies show flu vaccine can reduce the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well matched to the flu vaccine viruses.

This implies that the interpretation above is essentially the way the CDC presents it and the factors that influence effectiveness (based on strains). The percent of nonresponders also likely plays a factor, but it's harder to disentangle those (because some percent of nonresponders may get sick while others don't). Typically effectiveness doesn't take this into account and instead just looks at outcomes of everyone who was vaccinated.
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