The Difference between Antigenic Drifts and Antigenic Shift

Antigenic drift and antigenic shift are two distinct processes that describe the changes in viral antigens, particularly those found on the surface of influenza viruses. These processes play a crucial role in the ability of the virus to evade the immune system and cause seasonal epidemics or pandemics. Here’s a detailed explanation of each process and their differences:

1. Antigenic Drift: Antigenic drift refers to the gradual accumulation of small genetic changes or mutations in the genes that encode viral surface proteins, particularly the hemagglutinin (HA) and neuraminidase (NA) proteins of influenza viruses. These proteins are responsible for the recognition of the virus by the immune system and the attachment to host cells.

The mutations that occur during antigenic drift are typically point mutations, which are changes in individual nucleotides in the viral genome. As a result of these genetic changes, the antigenic properties of the virus subtly change over time. This means that the antibodies produced by the immune system in response to a previous infection or vaccination may no longer recognize the new variant of the virus as effectively.

The accumulation of antigenic drift mutations is a continuous process that happens over multiple replication cycles of the virus. As a consequence, influenza viruses undergo gradual genetic changes, leading to the emergence of new strains or variants. These new strains may cause seasonal outbreaks of influenza as the population’s immunity to previous strains wanes over time.

2. Antigenic Shift: Antigenic shift is a more dramatic and rapid process compared to antigenic drift. It involves the reassortment or exchange of genetic material between different strains of influenza viruses, leading to the emergence of a novel virus with antigenic properties distinct from both parent viruses.

Antigenic shift typically occurs when two different influenza viruses that commonly infect different species, such as birds and humans or pigs, co-infect the same host. In this scenario, the genetic material of both viruses can mix and reassort during the replication process, resulting in a new strain that possesses a combination of genetic segments from both parental viruses.

The consequences of antigenic shift can be significant because the new virus may contain surface proteins that have not been previously encountered by the human immune system. This lack of pre-existing immunity in the population can lead to a widespread outbreak or pandemic if the novel strain is highly transmissible.

Antigenic shift events have historically been associated with major influenza pandemics. For example, the H1N1 influenza pandemic of 2009 was caused by an influenza virus resulting from antigenic shift between a swine influenza virus and human seasonal influenza strains.

In summary, the key differences between antigenic drift and antigenic shift are:

• Antigenic drift is a gradual process involving point mutations that accumulate over time, while antigenic shift is a rapid process resulting from the reassortment of genetic material between different influenza viruses.
• Antigenic drift leads to minor changes in viral surface proteins, making the virus less recognizable to previously generated antibodies. Antigenic shift, on the other hand, can result in the emergence of a novel virus with a completely different set of surface proteins, potentially causing severe epidemics or pandemics.
• Antigenic drift occurs commonly and contributes to seasonal influenza outbreaks, while antigenic shift events are relatively rare but have the potential for significant global impact.
• Antigenic drift can be addressed through regular updates of influenza vaccines to match the circulating strains. Antigenic shift, however, poses a greater challenge as it involves the emergence of completely new strains for which there is little to no pre-existing immunity in the population.