Vaccine
Vaccines are essential to prevent diseases such as the flu. Therefore, the flu vaccine should be administered annually to the population.
It is necessary to remember that the virus that causes the flu undergoes many mutations, therefore, it is not efficient to apply the same vaccine every year.
The selection of which flu strain should be used for the manufacture of vaccines has some characteristics, such as:
• Which strain of the flu virus is causing the most illness during the season;
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• Which is spreading the most before the season even starts;
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• How effective is the previous vaccine for the strains that are most circulating.
Therefore, we can conclude that scientists choose the strain that will be used in the next year according to many criteria. So option "D" is correct.