Final answer:
The flu vaccine aims to produce specific antibodies against strains of the flu virus, but if a person still gets ill, it may indicate that the infecting virus was different yet similar enough for some antibody cross reactivity. Vaccine effectiveness can be compromised by rapid mutations in flu viruses, highlighting the difficulty in predicting prevalent strains.
Step-by-step explanation:
When a person is given a flu vaccine, the goal is to stimulate the body to produce antibodies that are specific to the strains of the flu virus included in the vaccine. It is possible for a mismatch to occur if the actual flu virus that infects someone is different enough from the strain in the vaccine, leading to limited effectiveness of the vaccine.
This mismatch can still result in cross reactivity, where the antibodies raised by the vaccine react to some extent with the new flu strain, but the reaction may not be strong enough to prevent illness. Therefore, if a person vaccinated in November becomes ill with the flu in January, it suggests that the virus they were infected with was different from the vaccine strain, albeit similar enough to produce some cross-reactive antibodies.
The process of updating flu vaccines is ongoing and relies on predicting which flu strains will be most prevalent in the upcoming season. Accurate prediction is challenging, as flu viruses mutate rapidly, making it a constant race to match the vaccine as closely as possible to the circulating strains.