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Pandemic Influenza – 1 Contact Hour
What Is Pandemic Influenza?
An influenza pandemic is an influenza epidemic that spreads on a worldwide scale, infecting a large proportion of the human population. In contrast to regular seasonal epidemics of influenza, these pandemics occur irregularly, with the 1918 Spanish flu the most serious pandemic in recent history. Three influenza pandemics occurred in the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus in humans. Often, these new strains result from the spread of an existing flu virus to humans from other animal species. History teaches that the influenza virus mutates to cause a worldwide epidemic about twice a century, on average. But scientists have yet to figure out what causes these viral mutations, when they will occur and what makes certain viruses more lethal than others.
Seasonal Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae, which affects birds and mammals. The name influenza comes from the Italian, meaning “influence”. The most common
symptoms of the disease are chills, fever, pharyngitis, muscle pains, severe headache, coughing, weakness and general discomfort. In more serious cases, influenza causes pneumonia, which can be fatal, particularly for the young and the elderly. In children the immune responses are still developing, while the elderly immune function tends to be weaker due to the aging process. Influenza, although often confused with the common cold, is a much more severe disease and is caused by a different type of virus.
Most people who get the flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. In an average year, influenza kills an estimated 250,000 to 500,000 people worldwide, with about 36,000 deaths in the United States and many more hospitalizations. Flu-related complications can occur at any age; however, the elderly and those with chronic health problems are much more likely to develop serious complications. By contrast, pandemic flus often strike young, healthy people the hardest.
Modes of Transmission
Influenza can be highly contagious, particularly among persons without pre-existing antibodies against influenza, such as young children during the inter-pandemic phase and anyone during a pandemic. Roughly 50% of all infections are, however, asymptomatic. Asymptomatic infection is especially common in children. The influenza virus is transmitted in most cases by droplets through the coughing and sneezing of infected persons, but it can be transmitted by direct contact as well.
Direct Contact
Direct-contact transmission involves skin-to-skin contact and physical transfer of microorganisms to a susceptible host from an infected or colonized person, such as occurs when healthcare personnel turn patients, bathe patients, or perform 
other patient-care activities that require physical contact. Direct-contact transmission can also occur between two patients (e.g., by hand contact), with one serving as the source of infectious microorganisms and the other as a susceptible host. Indirect-contact transmission involves contact of a susceptible host with a contaminated intermediate object, usually inanimate, in the patient’s environment (such as needles or countertops).
Droplet
Droplet transmission occurs when contagious droplets produced by the infected host are propelled a short distance through coughing or sneezing and can come into contact with another person’s conjunctiva, mouth or nasal mucosa. Influenza can be transmitted by large droplets, which generally travel 3 to 6 feet. Since these droplets are generally greater than 10 micrometers and do not stay suspended in the air, this mode of transmission is not affected by special air handling or control of room pressures. Large droplets appear to be the primary route of nosocomial transmission.
Airborne (Droplet Nuclei)
Droplet nuclei (airborne) transmission entails the production of infectious droplet nuclei, generally 5 micrometers or less in diameter. In contrast with larger droplets, these droplets can remain suspended in the air and be disseminated by air currents in a room or through a facility, to be inhaled by a susceptible host. Small droplet nuclei and aerosols can remain suspended in the air for prolonged periods and travel significant distances. Small particles appear to be more infectious, with both the degree of infectivity and the severity of illness directly related to particle size. Aerosols smaller than 10 microns have been shown to cause more severe disease and require a smaller inoculum than large intranasal droplets. Preventing the spread of droplet nuclei requires the use of special air handling and ventilation procedures.
There is no evidence that influenza transmission can occur across long distances (e.g., through ventilation systems) or through prolonged residence in air, as seen with airborne diseases such as tuberculosis. Organisms transmitted in this manner must be capable of sustaining infectivity, despite desiccation and environmental variation that generally limit survival in the airborne state. However, transmission may occur at shorter distances through inhalation of small-particle aerosols (droplet nuclei), particularly in shared air spaces with poor air circulation. An experimental study involving human volunteers found that illness could be induced with substantially lower virus titers when influenza virus was administered as a small droplet aerosol rather than as nasal droplets, suggesting that infection is most efficiently induced when virus is deposited in the lower rather than the upper respiratory tract.
Evidence supporting the relative contribution of each route of transmission for influenza is limited; however, droplet transmission is thought to be the predominant form of spread in a setting with an appropriate number of air exchanges and standard ventilation. In the absence of appropriate ventilation and air exchange, airborne transmission may play a greater role, such as in a crowded space where air exchange is limited.
