Final answer:
Antigenic shift refers to a major genetic change in influenza viruses, resulting in new strains that can evade the human immune response due to a lack of existing immunity. It differs from antigenic drift, which involves minor changes over time. The 1918 Spanish Flu pandemic's severity was due in part to an antigenic shift that produced a virulent H1N1 strain.
Step-by-step explanation:
Understanding Antigenic Shift
The concept of antigenic shift refers to a major change in an influenza virus, leading to the creation of a novel strain against which the human population has little to no immunity. This significant genetic change occurs through the process of gene reassortment.
For instance, it arises when two different influenza viruses infect the same host and exchange genetic material, leading to a new hybrid virus with a different set of surface proteins. In contrast, antigenic drift involves minor changes due to point mutations in the virus' genes over time. The deadliness of the 1918 Spanish Flu pandemic is partially attributed to an antigenic shift where a recombination of avian and human influenza virus genes resulted in a highly virulent H1N1 strain.
Because antigenic shift results in a strain of the virus that is drastically different from previous ones, the existing immune defenses in the human population, either from previous infections or vaccinations, are often ineffective against it.
This can lead to rapid and widespread infection and potentially high mortality rates, as in the case of the 1918 Spanish Flu. The continuous monitoring and analysis of influenza strains by organizations like the WHO and CDC are crucial for early detection of such shifts, allowing for timely vaccine development and public health responses.