Variant Influenza Viruses: Background and CDC Risk Assessment and Reporting

Background On Variant Influenza Viruses

Influenza viruses that normally circulate in pigs (i.e., swine influenza 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 exposed to pigs at an agricultural fair, people who raise pigs, or workers in the swine industry). This is thought to happen mainly when an infected pig coughs or sneezes and droplets with influenza virus in them spread through the air. If these droplets land in your nose or mouth, or are inhaled, you can be infected. There also is some evidence that you might get infected by touching something that has virus on it and then touching your own mouth or nose. A third way to possibly get infected is to inhale particles containing influenza virus. Scientists aren’t certain which of these ways of spread is the most common. Swine influenza has not been shown to be transmissible to people through eating properly handled and prepared pork (pig meat) or other products derived from pigs.

There have been documented cases of multiple people becoming sick after exposure to one or more infected 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 variant virus infection should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in humans and, if infected animals are identified, to limit further exposure of humans and other healthy animals to infected animals

Illness associated with variant influenza virus infection has been mostly mild with symptoms similar to those of seasonal influenza. Like seasonal influenza, however, serious illness, resulting in hospitalization and death is possible. In 2012, for example, of 309 human infections with H3N2v, 16 people were hospitalized and one of these people died. Most of the people who were hospitalized and the person who died had one or more health or age factors that put them at higher risk of serious flu-related complications. People at higher risk of serious complications from seasonal influenza and H3N2v include children younger than 5 years old, people with certain chronic conditions like asthma, diabetes, heart disease, weakened immune systems, pregnant people, and people 65 years and older. CDC has issued guidance for people attending fairs where swine might be present, including additional precautions for people who are at higher risk of serious flu complications.

CDC Assessment

Sporadic infections and even localized outbreaks among people with variant influenza viruses may occur. All influenza viruses have the capacity to change, and it’s possible that variant viruses may change such that they gain the ability to infect people easily and spread easily from person-to-person. The Centers for Disease Control and Prevention (CDC) continues to monitor closely for variant influenza virus infections and will report cases weekly in FluView and in the Novel Influenza A Virus Infections ( section of FluView Interactive.

Reporting Variant Influenza Viruses

Domestically, CDC reports these cases in its weekly national influenza surveillance report, FluView. CDC also is required to report the first cases of human infection per year of a specific variant virus subtype to the World Health Organization (WHO) as part of the International Health Regulations (IHR). (For example, the first human cases of H1N1v, H1N2v, or H3N2v that occur in a given year are reported WHO as part of the 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 confirmed cases of human infection with a “novel” (non-human) influenza virus.

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