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 that circulate among people. Novel influenza A viruses are usually influenza A viruses that circulate among animals. Some novel influenza 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 gain the ability to spread easily from person to person, which might cause the next influenza pandemic. Human infection with a novel influenza A virus is a nationally notifiable condition reportable to CDC. 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 (e.g. pneumonia).
  • Several subtypes of avian influenza A viruses are known to have infected people (H5, H6, H7, H9, H10 viruses).
  • Highly pathogenic Asian avian influenza A(H5N1) and low pathogenic Asian A(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.
  • See avian influenza for more information.

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 17 countries since 2003. 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 H7N9 viruses were reported by WHO on April 1, 2013external icon and sporadic human infections continue to be reported in China. In late 2016, some low pathogenicity Asian H7N9 viruses developed mutations that made them highly pathogenic in poultry. These HPAI H7N9 Asian viruses continue to be associated with human infections in China. Many Asian H7N9 virus infected patients have had severe respiratory illness. During the past five annual epidemics of Asian H7N9 virus infections in people, the mortality rate in hospitalized patients has averaged about 40 percent. Probable, limited, non-sustained human-to-human spread of Asian H7N9 viruses also has been reported in China. Since the implementation of a large-scale H5-H7 poultry vaccination program in September 2017, few human infections with Asian H7N9 viruses have been reported.
  • 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 virusexternal icon 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.
  • See Asian H7N9 virus for more information.

Avian influenza A H9 viruses

  • Sporadic human infections with some low pathogenicity H9N2 viruses also have been reported in China, Hong Kong, Bangladesh, and Egypt. Most H9N2 virus 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

  • Sporadic human infections with swine influenza A viruses circulating among pigs can occur. 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 influenza, 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.
  • See variant virus infections for more information.
  • See FluView Interactive for an interactive table of variant virus infections reported in the United States since 2011.

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.