Influenza viruses cause annual epidemics and occasional pandemics by constantly changing into new versions to which humans have little immunity. A frequent pattern is the emergence of new pandemic strains from East Asia, particularly China. Specific regional factors help explain why this area is a recurring origin point for novel influenza viruses, which is important for global public health preparedness.
Historical Influenza Pandemics Linked to the Region
The 20th century saw multiple influenza pandemics with origins traced back to the region. In 1957, a new influenza A virus subtype, H2N2, emerged in Guizhou, China. Known as the “Asian Flu,” this virus resulted from an existing human flu virus exchanging genetic material with an avian flu virus. The first cases were reported in China in February 1957, spreading to Hong Kong by April and then rapidly across the globe, causing an estimated one to four million deaths.
Just over a decade later, the “Hong Kong Flu” (H3N2) emerged in July 1968. This virus evolved from the 1957 H2N2 strain but retained some of its genetic components, meaning people exposed to the earlier strain had partial immunity. This may explain why the 1968 pandemic was less deadly, though it was still highly contagious and spread worldwide, causing an estimated one to four million deaths.
Why the Region is a Hotspot for New Flu Viruses
The region is a hotspot for influenza emergence due to a combination of agricultural practices and population dynamics. The primary mechanism is “antigenic shift,” where different influenza viruses infect the same cell and swap genetic material. This reassortment can create a new virus subtype, often between viruses that infect birds and mammals.
Pigs act as biological “mixing vessels” because their respiratory tracts have receptors for both avian and human influenza viruses. This allows a pig to be co-infected with strains from both species. Inside the pig’s cells, the genetic material can mix, creating a novel virus that can then jump to humans.
This process is amplified by agricultural systems where large populations of poultry and pigs are raised near dense human populations. Live animal markets also bring various species together in crowded conditions, creating ideal circumstances for interspecies viral transmission. This constant interaction provides a persistent opportunity for viruses to jump from animals to people.
The region’s location on major migratory bird flyways is another factor. Wild aquatic birds are the natural reservoir for most influenza A viruses and carry them over vast distances. Migrating birds introduce new avian influenza strains to local poultry and pig populations, providing fresh genetic material for reassortment.
Modern Zoonotic Influenza Strains
Highly pathogenic avian influenza viruses, such as H5N1, demonstrate the ongoing risk. First identified as a cause of severe illness in humans in Hong Kong in 1997, H5N1 is primarily a disease of birds that causes devastating outbreaks in poultry. While it does not easily infect humans, the resulting illness is severe with a high mortality rate.
Sporadic human infections with H5N1 have occurred, almost always following direct contact with infected poultry. The virus has not acquired the ability to transmit easily and sustainably among people. Any genetic changes that allow for human-to-human spread could trigger a major pandemic, given the lack of immunity to H5N1.
Another avian influenza virus, H7N9, emerged as a concern in 2013 when it began causing human infections in China. Like H5N1, H7N9 is an avian virus that can cause severe respiratory illness in humans. A distinguishing feature of H7N9 is that it often causes little to no illness in poultry, allowing it to spread silently through flocks.
Human cases of H7N9 have been linked to exposure to infected poultry, particularly in live bird markets. Although it has shown limited instances of person-to-person spread, it has not achieved the sustained transmission needed to start a pandemic. Both H5N1 and H7N9 are contemporary examples of animal influenza viruses in the region posing a threat to human health.
Global Influenza Surveillance and Prevention
In response to the recurring threat of new influenza viruses, a coordinated international effort is in place to monitor their evolution. The World Health Organization (WHO) oversees the Global Influenza Surveillance and Response System (GISRS), a network of over 100 national influenza centers. This system acts as a global alert mechanism for tracking new and circulating flu strains in both human and animal populations.
Laboratories within the GISRS network, including centers in China, play a direct role in this effort. They collect and test thousands of virus samples from patients with respiratory illnesses and from animal populations. This information is then shared with WHO Collaborating Centers, which analyze the genetic properties of the viruses to determine which strains are spreading and if they pose a pandemic risk.
This continuous surveillance provides the data for the annual seasonal flu vaccine. Twice a year, the WHO convenes experts to review the global surveillance data and recommend which influenza strains should be included in the upcoming Northern and Southern Hemisphere vaccines. This process ensures the vaccine is updated to protect against the most likely circulating viruses.