Viruses constantly change, adapting to new environments. Understanding these changes is crucial for public health, as they influence how viruses interact with human populations. This article explains antigenic shift, a process that profoundly impacts viral evolution and global health.
Defining Antigenic Shift
Antigenic shift refers to an abrupt, major change in a virus’s surface antigens. This process creates a new viral subtype to which most of the population lacks prior immunity, posing a significant threat. It primarily occurs in viruses with segmented genomes, such as influenza A viruses.
Such changes alter viral surface proteins, specifically hemagglutinin (HA) and neuraminidase (NA) in influenza viruses. These proteins are recognized by the immune system, but a dramatic change means antibodies from previous infections no longer protect. This leaves previously infected individuals susceptible to reinfection.
How Antigenic Shift Happens
The primary biological mechanism behind antigenic shift is genetic reassortment, sometimes referred to as “viral sex.” This process occurs when two different strains of a virus co-infect the same host cell simultaneously. For instance, an animal virus, such as an avian or swine influenza strain, and a human influenza virus can both infect a single cell.
Within this co-infected cell, their genetic material, organized into distinct segments for influenza viruses, can mix and match. This exchange of gene segments forms a new virus with a novel combination of surface antigens from both original strains. Pigs are often considered “mixing vessels” for influenza viruses because they can be infected by avian, human, and swine influenza strains, facilitating this reassortment. The new virus, with altered HA and/or NA proteins, can then infect new hosts.
Antigenic Shift Versus Antigenic Drift
Antigenic shift and drift are distinct viral changes with different implications for immunity and public health. Antigenic drift involves a gradual accumulation of small genetic mutations in existing viral strains. These minor changes lead to slight alterations in surface proteins over time, necessitating annual vaccine updates for seasonal influenza.
In contrast, antigenic shift is an abrupt, major change creating an entirely new viral subtype. While drift allows viruses to slowly evade immunity, shift introduces strains to which the population lacks pre-existing immunity. This difference means drift typically leads to seasonal epidemics, while shift can cause widespread pandemics.
Why Antigenic Shift is Significant
Antigenic shift’s emergence of novel influenza strains poses a significant global health threat. These new viral subtypes differ substantially from previous strains, meaning the human population lacks sufficient pre-existing immunity. This allows the virus to spread rapidly, leading to pandemics.
Historically, antigenic shift has caused several major influenza pandemics. Examples include the 1957 Asian Flu (H2N2), the 1968 Hong Kong Flu (H3N2), and the 2009 H1N1 Swine Flu pandemic. While the 1918 Spanish Flu was once believed to be caused by antigenic shift, more recent research suggests it resulted from antigenic drift of an avian virus that adapted to humans. The sudden appearance of such novel strains challenges vaccine development and public health preparedness, requiring rapid deployment of new vaccines.