Social Distancing

US Dept of HHS Pandemic Influenza Plan Supplement 4 Infection Control

Modes of transmission

Despite the prevalence of influenza year after year, most information on the modes of influenza transmission from person to person is indirect and largely obtained through observations during outbreaks in healthcare facilities and other settings (e.g., cruise ships, airplanes, schools, and colleges); the amount of direct scientific information is very limited. However, the epidemiologic pattern observed is generally consistent with spread through close contact (i.e., exposure to large respiratory droplets, direct contact, or near-range exposure to aerosols). While some observational and animal studies support airborne transmission through small particle aerosols, there is little evidence of airborne transmission over long distances or prolonged periods of time (as is seen with M. tuberculosis). The relative contributions and clinical importance of the different modes of influenza transmission are currently unknown.

Droplet transmission (www.cdc.gov/ncidod/hip/ISOLAT/std_prec_excerpt.htm)

Droplet transmission involves contact of the conjunctivae or the mucous membranes of the nose or mouth of a susceptible person with large-particle droplets containing microorganisms generated from a person who has a clinical disease or who is a carrier of the microorganism. Droplets are generated from the source person primarily during coughing, sneezing, or talking and during the performance of certain procedures such as suctioning and bronchoscopy. Transmission via large-particle droplets requires close contact between source and recipient persons, because droplets do not remain suspended in the air and generally travel only short distances (about 3 feet) through the air. Because droplets do not remain suspended in the air, special air handling and ventilation are not required to prevent droplet transmission.

Based on epidemiologic patterns of disease transmission, large droplet transmission has been considered a major route of influenza transmission. However, data directly demonstrating large droplet transmission of influenza in human outbreaks is indirect and limited.

Contact transmission (www.cdc.gov/ncidod/hip/ISOLAT/contact_prec_excerpt.htm)

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 personnel turn patients, bathe patients, or perform other patient-care activities that require physical contact. Direct-contact transmission also can 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.

Contact transmission of influenza may occur through either direct skin-to-skin contact or through indirect contact with virus in the environment. Transmission via contaminated hands and fomites has been suggested as a contributing factor in some studies. However, there is insufficient data to determine the proportion of influenza transmission that is attributable to direct or indirect contact.

Airborne transmission (www.cdc.gov/ncidod/hip/ISOLAT/airborne_prec_excerpt.htm)

Airborne transmission occurs by dissemination of either airborne droplet nuclei or small particles in the respirable size range containing the infectious agent. Microorganisms carried in this manner—such as M. tuberculosis— may be dispersed over long distances by air currents and may be inhaled by susceptible individuals who have not had face-to-face contact with (or been in the same room with) the infectious individual. Organisms transmitted in this manner must be capable of sustaining infectivity, despite desiccation and environmental variation that generally limit survival in the airborne state. Preventing the spread of agents that are transmitted by the airborne route requires the use of special air handling and ventilation systems (e.g., negative pressure rooms).

The relative contribution of airborne transmission to influenza outbreaks is uncertain. Evidence is limited and is principally derived from laboratory studies in animals and some observational studies of influenza outbreaks in humans, particularly on cruise ships and airplanes, where other mechanisms of transmission were also present. Additional information suggesting airborne transmission was reported in a Veterans Administration Hospital study that found lower rates of influenza in wards exposed to ultraviolet radiation (which inactivates influenza viruses) than in wards without UV radiation. Another study indicated that humidity can play a role in the infectivity of aerosolized influenza, although the influence of humidity on the formation of droplet nuclei was not evaluated.

Small-particle aerosols. 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. 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. While this study supports the possibility of droplet nuclei transmission of influenza, the proportion of infections acquired through droplet nuclei—as compared with large droplet or contact spread—is unknown.

It is likely that some aerosol-generating procedures (e.g., endotracheal intubation, suctioning, nebulizer treatment, bronchosocopy) could increase the potential for dissemination of droplet nuclei in the immediate vicinity of the patient. (Although transmission of SARS-CoV was reported in a Canadian hospital during an aerosol-generating procedure [intubation], it occurred in a situation involving environmental contamination with respiratory secretions.) Although this mode of transmission has not been evaluated for influenza, additional precautions for healthcare personnel who perform aerosol-generating procedures on influenza patients may be warranted.

Respiratory aerosols Infectious respiratory aerosols of varying sizes are generated when an individual coughs, sneezes or talks.[1] A cough can contain up to 100,000 particles and a sneeze can generate 20 times more particles than a cough.[2] The greater the force and pressure involved in aerosol generation, the smaller the expelled particles will be. The smallest particles evaporate quickly and the dried residues that remain (droplet nuclei), are so small that they can be carried on air currents a considerable distance from the source and remain suspended in the air for substantial lengths of time and infect people at some distance from the source.[1] Particle size determines where particles are deposited in the respiratory tract of the host. Where the particles are deposited can determine whether or not infection will occur; e.g., smaller particles may be deposited lower in the respiratory tract than larger particles.[3]

Infectious particles are generally measured in microns (one inch is equivalent to 25,400 microns. CDC infection control guidelines cite a particle size of 5 microns (µm) as a break point that distinguishes between diseases spread by “droplet transmission” (particles > 5 µm) and diseases spread by “airborne transmission” (particles < 5 µm).[4] Larger droplets are thought to typically travel no more than 3 feet while small particle aerosols have the ability to travel longer distances. Larger droplets are thought to be deposited mainly in the mucous membranes of the nose, eyes, and mouth; small particle aerosols are more likely to be deposited in the lower respiratory tract. … However, transmission of respiratory particles is quite complex. In reality, there is not a clear delineation between droplet and airborne transmission, and the distances that particles travel can vary (e.g., particles > 5 µm can travel more than three feet).[5] In addition, the length of time particles remain airborne varies and is determined by particle size, settling velocity, and airflow in the area. Also there is not a predictable size for droplet nuclei; final size depends on the nature of the fluid that contained the organism, the initial size of the aerosol, environmental conditions (e.g., temperature, relative humidity, airflow), the time spent airborne, and the size of the organism within a droplet. … Human influenza transmission Using current CDC infection control terminology, there is evidence that influenza is transmitted between humans via small particle aerosols (airborne transmission), larger droplets (droplet transmission), as well as by direct and indirect contact (contact transmission)[3, 4, 6, 7]. The relative importance of each route of transmission is unclear. …. In 1979, aircraft passengers, including a passenger who became acutely ill with influenza within 15 minutes of boarding the plane, were detained on a runway for 4.5 hours during which time the ventilation system was turned off for 2–3 hours. The ill passenger stayed on the plane the entire time and the other passengers and crew were free to come and go. Within 72 hours, 72% of the passengers and crew subsequently developed influenza-like-illness (91% with confirmed influenza). The risk of illness was dependent on the amount of time spent on board.[10] … Experimental evidence of airborne transmission of influenza in animals has been provided in several published studies. In one study, infected and uninfected mice were placed in a closed chamber in which the airflow could be manipulated. As the rate of airflow increased, the rate of influenza transmission decreased proportionately. In a setting of constant airflow, one group of uninfected mice were separated from infected mice by a screen while another group of uninfected mice were on the same side of the screen as the infected mice. The infection rates in both groups of initially uninfected mice were similar.[15] In another study, uninfected mice placed in an unventilated room with constantly agitated air and low relative humidity, became infected with influenza as late as 24 hours after virus was aerosolized into the room. As relative humidity levels were increased, the virus was infective for shorter periods of time. http://www.health.state.mn.us/divs/idepc/diseases/flu/avian/airborne.html