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I recently saw this video featuring Robert Scott Bell, where he basically says that the flu shot is not terribly effective.

Under the video is this description, in part:

Reports in 2013 state that flu shots were only 9% effective for the elderly.

9% sounds dismal. Is that actually the case? Do the reports show that in early 2013 the flu shot was only 9% effective in the elderly?

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Is that actually the case? : Maybe

Do the reports show that in early 2013 the flu shot was only 9% effective in the elderly? : No

It might be the case that the flu shot's effectiveness was only 9% in the elderly, but the analysis that the CDC does isn't able to support a statement as strong as "the flu shot's effectiveness in group X is Y%". They can only give ranges.

What is true is that the CDC's point estimate for the flu vaccine effectiveness against the 2013 flu A (H3N2) in people aged 65 or older is 9%, with a 95% confidence interval that spans from -84% to 55%. They suggest interpreting these results with caution.

In more general terms, the CDC says the following things:

CDC's vaccine effectiveness study measured lower vaccine effectiveness among people 65 and older against flu A this season than it did among other age groups.

[...]

One possible explanation for this is that some older people did not mount an effective immune response to the flu A (H3N2) component of this season’s vaccine; however, it’s not possible to say this for certain.

Against flu B, effectiveness in the 65 and older age group was found to be similar to that in other age groups: 67% with a 95% confidence interval that spans from 51% to 78%.

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    I'm sorry, but what? If the shot is given to 1000 people how many are effectively protected? If it's 9% then I would think 90 people are protected.
    – user11643
    Commented Apr 10, 2013 at 18:44
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    The CDC's measure of vaccine effectiveness represents the reduction in risk provided by the flu vaccine: cdc.gov/flu/about/qa/vaccineeffect.htm#present-data
    – user5582
    Commented Apr 10, 2013 at 18:51
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    How many people are protected is a more complicated question, and would require analyzing the hampered spread of a disease through a partially vaccinated population vs a completely unvaccinated population. For example, even if the vaccine was 100% effective, and given to 400 people in a 1000 person population, that would give some protection to the entire population because the virus can't easily be passed on through those 400 vaccinated people.
    – user5582
    Commented Apr 10, 2013 at 19:01
  • What does it mean for the confidence interval to include negative values? How does one interpret a statement like that?
    – Joshua Frank
    Commented Nov 15, 2018 at 14:00
  • @JoshuaFrank presumably negative effectiveness would indicate that it increased flu cases. In any case the confidence interval is so wide I wouldn't draw conclusions from this
    – Criticizing Israel not allowed
    Commented Apr 19, 2021 at 9:56

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