The flu pandemics of 1957, 1968, 2009, and likely 1918 all involved the introduction of either HA or NA protein variants from either birds, swine, or both. Which of the following is a reasonable explanation as to why such an introduction could be associated with a pandemic?

• A. Antigenic shift introduced novel antigens for which humans had no immune defenses.
• B. These pandemics represent zoonoses, which are inherently more dangerous than other infections.
• C. Avian and swine variants of influenza more closely resemble human influenza type C, meaning they are significantly more virulent.
• D. Introduction of a novel NA protein would allow the flu virus to enter new types of cells, significantly increasing its virulence.

Answer: (A)

Verified step by step guidance

1. Understand that influenza viruses have two key surface proteins: hemagglutinin (HA) and neuraminidase (NA), which are important for virus entry and release from host cells.
2. Recognize that antigenic shift is a process where influenza viruses acquire new HA or NA proteins through reassortment, often involving animal (avian or swine) influenza strains mixing with human strains.
3. Realize that when a novel HA or NA protein is introduced into the human population, the immune system may not recognize these new antigens because humans have not been previously exposed to them.
4. This lack of pre-existing immunity allows the virus to spread rapidly and widely, potentially causing a pandemic due to the population's vulnerability.
5. Therefore, the introduction of new HA or NA variants from animals into humans via antigenic shift explains why such events are associated with pandemics.