Variant (Swine Origin) Influenza Viruses in Humans

Background On Variant Influenza Viruses

Swine flu viruses do not normally infect humans. However, sporadic human infections with influenza viruses that normally circulate in swine and not people 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 virus subtype designation. Human infections with H1N1v, H3N2v and H1N2v viruses have been detected in the United States.

Most commonly, human infections with variant viruses occur in people with exposure to infected pigs (e.g. children near pigs at a fair or workers in the swine industry). There have been documented cases of multiple persons becoming sick after exposure to one or more sick pigs and also cases of limited spread of variant influenza viruses from person-to-person. The vast majority of human infections with variant influenza viruses do not result in person-to-person spread. However, each case of human infection with a swine influenza virus should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in humans and to limit further exposure of humans to infected animals if infected animals are identified.

Reporting Variant Influenza Viruses

Domestically, CDC reports these cases in its weekly national influenza surveillance report, FluView. CDC also is required to report all cases of human infection with novel influenza viruses (which would include variant viruses) to the World Health Organization (WHO) as part of the International Health Regulations (IHR). The IHR is an international legal instrument entered into force in 2007 with the goal of helping the international community prevent and respond to public health risks with potential global impact. The IHR requires countries to report certain disease outbreaks and public health events, including any confirmed case of human infection with a "novel" (non-human) influenza virus.‬‬

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For information about 2009 H1N1 influenza (initially referred to as “swine flu” when it was first detected), visit the archived CDC 2009 H1N1 Flu website.

The links below offer information about human infections with variant influenza viruses.