Final answer:
The person likely contracted a different strain of the flu not covered by the vaccine due to the high rate of mutation and antigenic drift in influenza viruses. The flu vaccine creates antibodies for specific strains, and different strains can emerge, against which the immune system is not prepared.
Step-by-step explanation:
When a person given a flu vaccine in November comes down with a severe case of influenza in January, the most likely conclusion is that they were infected with a different strain of the flu virus not covered by the vaccine. The flu vaccine is designed to elicit antibodies in the person's body that are specific to the strains of the flu virus determined to be most common for the coming flu season. Unfortunately, the flu virus can undergo antigenic drift, where small genetic mutations lead to changes in the surface proteins of the virus, rendering the immune system unable to recognize the new strains immediately.
Therefore, the reason you have to be immunized with a new influenza vaccine each year is largely due to the mutation and evolution of the flu virus, which outpaces the long-term effectiveness of a single vaccine. This is why each year's flu vaccine includes protection against what's predicted to be the most prevalent strains for that year, and why a person can still contract the flu even after vaccination if a different strain becomes widespread.Vaccination relies on the concept that exposure to noninfectious antigens, derived from the vaccines, generates a primary immune response without causing illness. The cross reactivity between the immune system and the virus may not occur if the strains are too different, which seems to be the case in the scenario described.The flu vaccine elicited antibodies in the person's body that were specific to a particular flu virus. Unfortunately, the flu virus that infected the person later in January was different enough for cross reactivity to occur between the virus antigens causing the infection.