Answer: Not as much as you might expect.
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Excerpted from a September 2019 White House report which looked specifically at flu and coronavirus vaccines ….
There is considerable variation from year to year in how much the flu vaccine reduces the risk of contracting the seasonal flu and flu-related illnesses.
Over the past 14 years, influenza vaccine effectiveness has ranged between 10% and 60%.
Much of the variability depends on which viral strains predominate in a given year and, more specifically, whether the vaccine matches the viral strain that is circulating in a given flu season.
Although a mismatch between the vaccine and the virus circulating during a flu season reduces efficacy, vaccines still provide some protection against flu illness and decrease the severity of the illness, due to immunologic similarity between the viruses.
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When it comes to pandemic viruses …
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The rarity of pandemics makes it hard to determine vaccine effectiveness during pandemics.
A monovalent A(H1N1) vaccine was developed for the 2009-2010 swine flu pandemic.
That vaccine was 62% effective in protecting people under age 65 years …
… and 43% effective for those age 65 and older (the age group at highest risk of medical complications and death).
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Bottom Line: Don’t expect any coronavirus vaccine to be 100% effective.
Based on historical experience, there are roughly even odds that a coronavirus vaccine will be effective.
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For the record: Assuming that the clinical trials don’t reveal any significantly disturbing side effects, I plan to roll the dice and take the coronavirus vaccine when it becomes available.
The Homa Files 
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