I've seen various sources (notably the government of Canada) saying it was "unlikely", but how unlikely?

I'm asking because I wonder if prepping against flu pandemics would be a sufficient reason to get the seasonal flu shot (putting its other benefits aside)?

ETA: I just read "There is a chance that seasonal flu shots would confer some protection against the avian flu virus H5N1, which has raised fears of a deadly global outbreak. Researchers vaccinated mice against a common form of the flu and found that although the animals got sick they were less likely to die when infected with the related H5N1." (https://www.scientificamerican.com/article/can-seasonal-flu-shots-help/)

I couldn't find anything about it on the CDC and WHO websites.

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1 Answer 1


Based on the most recent large-scale and high quality Cochrane review on more than 80,000 healthy adults from 52 clinical trials aged 16 to 65 years, including pregnant women, over a single influenza season in North America, South America, and Europe who received vaccination between 1969 and 2009 (1):

71 healthy adults need to be vaccinated to prevent one of them experiencing influenza, and 29 healthy adults need to be vaccinated to prevent one of them experiencing an influenza-like illness (2)

These statistics (i.e., 71 and 29) are called number needed to be vaccinated/treated (NNV or NNT) representing the clinical effectiveness of vaccines (NNV) or medications (NNT). http://www.thennt.com/ is a quite useful source to find out such measures of clinical effectiveness of diagnostic, preventive, or therapeutic medical interventions.

Considering the difference between pandemics and seasonal outbreaks: Basically, the current flu vaccines are based on previously recognized sub-types (e.g., in previous epidemics/pandemics). If the virus of an upcoming epidemic/pandemic has been covered in the vaccine, these likelihoods may be true. However, it should be noticed that it is a bit unlikely for a previously well-recognized and covered sub-type to form a pandemic (i.e. an influenza virus that spreads on a worldwide scale and infects a large proportion of the world population [Wikipedia]), unless by mutating to a new pandemic strain. Also, note that studying vaccines' effectiveness against pandemics (the exact protection rate in populations) by current research equipment, methods and budgets is not feasible.
Also, we should not forget that human kind has only experienced 5 flu pandemics.

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    AFAICT, this answers the question "How likely is the seasonal flu shot to protect you from the seasonal flu?" My questoin is "How likely is the seasonal flu shot to protect you from a flu pandemics?"
    – Mati Roy
    Mar 24, 2019 at 20:28
  • You are right. There are differences between them. I tried to clarify it a bit by editing the answer. Note that seasonal flu shots are to protect against seasonal outbreaks not pandemics. Pandemics are basically caused by mutant (i.e., new) sub-types of virus which the majority of us are not immunized against, these mutants can spread worldwide hence the odds are comprehensively stacked against us ;)
    – maaniB
    Mar 25, 2019 at 0:46
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    Thank you for the precision!
    – Mati Roy
    Mar 28, 2019 at 18:03

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