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In the United States we mostly use a quadrivalent flu vaccine which covers the four most common expected strains and we use Asia and Australia's flu season to help predict the predominant strains coming our direction.I seem to recall the flu vaccine is ever only 75% they take the variables of the 9 biggest strains from the year before and just hope the mutations don't creep above it.
Still a bloody good vaccine mind.
Anything over 54% is deemed a success..
When a vaccine is said to be 75% effective it doesn't mean that It stops It disease three out of four times or that one in four people will get infected that are vaccinated.
It's a comparison of the vaccine in the group against the unvaccinated group.
If I have a group of 50 in the unvaccinated group and 25 develop influenza, The vaccinates group of 50 should develop about 6 cases.
So even a 50% effective vaccine would be expected to take the same numbers and result in 12 cases instead of 25. These vaccine effects are further enhanced if a vaccine limits transmission or duration of transmission.
It's difficult to assess a single line in the sand on what is considered to be an effective vaccine. You have to consider the prevalence of the disease in the unvaccinated population. You then have to consider the number needed to treat to reduce one morbidity or mortality outcome you are measuring.