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Viruses of Special Concern

A novel influenza A virus is one that has caused human infection, but is different from current seasonal human influenza A viruses spreading among people. Novel influenza A viruses can be viruses that originate in animals that gain the ability to infect humans or human viruses that change significantly so as to be different from current human seasonal influenza A viruses. Some novel flu A viruses are believed to pose a greater pandemic threat than others and are more concerning to public health officials because they have caused serious human illness and death and also have been able to spread in a limited manner from person-to-person. Novel influenza A viruses are of extra concern because of the potential impact they could have on public health if they gained the ability to spread from person-to-person easily, triggering a pandemic. The text below summarizes the novel influenza A viruses that are currently most concerning to public health officials.

Avian Influenza A Viruses

  • Avian influenza A viruses do not normally infect humans, but sporadic human infections have occurred.
  • Illness in humans caused by avian influenza A virus infections has ranged from mild to severe.
  • Three subtypes of avian influenza A viruses are known to infect people (H5, H7 and H9 viruses).
  • Highly pathogenic Asian avian influenza A H5N1 and low pathogenic Asian H7N9 viruses account for the majority of human infections with avian influenza A viruses.
  • Human infections with avian influenza A viruses have most often occurred after exposure to infected poultry or their secretions or excretions, such as through direct or close contact, including visiting a live poultry market.
  • More information about avian influenza is available.

Avian influenza A H5 viruses

  • Among H5 avian influenza A viruses, Asian highly pathogenic avian influenza (HPAI) A H5N1 viruses have caused the most human infections. Asian H5N1 viruses are currently circulating among poultry in Asia and the Middle East and human infections with these viruses have been reported in 16 countries. Human infections are often associated with severe pneumonia and mortality greater than 50%. Probable, limited, non-sustained human-to-human spread of Asian H5N1 viruses has been reported in several countries. (HPAI H5 viruses detected in birds and poultry in the United States are different and have not caused human infections.)
  • The World Health Organization (WHO) tracks the number of reported, confirmed, human infections with HPAI H5N1 viruses.
  • On January 8, 2014, the first case of human infection with Asian HPAI H5N1 virus in the Americas was reported in Canada in a traveler returning from China. (No human infections with Asian H5N1 virus have ever been reported in the United States.)
  • Sporadic human infections with HPAI H5N6 viruses also have been reported in China, resulting in severe illness and high mortality.

Avian influenza A H7 viruses

  • Among H7 viruses, low pathogenic Asian lineage avian influenza A H7N9 viruses have caused the most reported human infections; with most of them occurring in China. The first human infections with these viruses were reported by WHO on April 1, 2013 and sporadic human infections continue to be reported in China. In late 2016, some Asian low pathogenicity Asian H7N9 viruses developed mutations that made them highly pathogenic in poultry. These Asian HPAI H7N9 viruses continue to be associated with human infections in China. Many Asian H7N9 virus infected patients have had severe respiratory illness. During the first four annual epidemics of Asian H7N9 virus infections in people, the mortality rate was about 40 percent. Probable, limited, unsustained human-to-human spread of Asian H7N9 viruses also has been reported in China.
  • In January 2015, the Government of Canada and the Ministry of Health in British Columbia reported the first two cases of human infection in North America with Asian H7N9 virus in a husband and wife who had traveled to China. Cases of Asian H7N9 virus infection associated with travel to China also have been reported in Hong Kong, Taiwan and Malaysia.
  • More information about Asian H7N9 virus is available.

Avian influenza A H9 viruses

  • Sporadic human infections with some H9N2 viruses also have been reported in China, Hong Kong, Bangladesh, and Egypt. Most H9N2 infections in people have occurred in children after poultry exposures. H9N2 virus infection of humans generally causes mild upper respiratory tract illness.

Swine Influenza/Variant Influenza A viruses

  • Swine flu viruses do not normally infect humans, but sporadic human infections with influenza A viruses that normally circulate among swine have occurred. When this happens, these viruses are called “variant viruses.” They also can be denoted by adding the letter “v” to the end of the influenza A virus subtype designation.
  • Illness associated with variant virus infection has been mostly mild with symptoms similar to those of seasonal flu. Like seasonal flu, however, serious illness, resulting in hospitalization and death, is possible. In general, variant viruses have been associated with less severe illness and much lower mortality than human infection with avian influenza A viruses.
  • Most commonly, human infections with variant viruses occur in people with exposure to infected pigs (e.g., children who have direct or close contact with pigs at a fair or workers in the swine industry), but limited, non-sustained spread from person to person of some variant viruses has been detected.
  • Human infections with H1N1v, H3N2v and H1N2v viruses have been detected in the United States.
  • More information about variant virus infections in people is available.
  • A table of the variant virus infections reported in the United States since 2005 is available.

Risk Assessment

CDC and non-CDC influenza experts have developed a tool that assesses the potential pandemic risk posed by novel influenza A viruses. Influenza Risk Assessment Tool  results are summarized in a table.

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