Rationale for MDH Infection Control Recommendations for Avian and Pandemic Influenza Patients
MDH recommends that healthcare workers use full barrier precautions,
including respirators, when working with known or suspect avian or pandemic
influenza patients. The rationale for these recommendations follows.
In making these recommendations, MDH acknowledges that supplies of PPE necessary to implement full barrier precautions, particularly respirators, may be limited during a pandemic. MDH will provide guidance on prioritization and possible reuse of PPE when supplies are limited.
On this page:
Human influenza transmission
Evidence for airborne transmission of influenza
Influenza transmission in healthcare facilities
Infectious respiratory aerosols of varying sizes are generated when an individual coughs, sneezes or talks. A cough can contain up to 100,000 particles and a sneeze can generate 20 times more particles than a cough. 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. 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.
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). 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.
Communicable diseases are classified by their presumed route of transmission (e.g., droplet, contact, or airborne) and infection control recommendations are based on this classification. Current CDC guidelines recommend that healthcare workers wear a surgical mask when working within 3 feet of patients with an infection spread via the droplet route, and a respirator when in the same room as a patient with an infection spread via the airborne route.
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). 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.
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.
The explosive spread of influenza after introduction into a community has long suggested the possibility of airborne transmission. There are several reports in the literature describing observational evidence of airborne transmission of influenza in humans. Tuberculosis patients housed in a building with ceiling ultraviolet radiation (which is known inactivate influenza virus and to reduce airborne disease transmission) during the 1957-58 pandemic were less likely to become infected with influenza than tuberculosis patients housed in a building without ultraviolet radiation. 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.
Other published reports have provided experimental evidence of airborne transmission of influenza in humans. The infectious dose of influenza is 10-100 fold lower when small particle aerosols are delivered to the lower respiratory tract (mimicking airborne transmission), rather than when delivered as intranasal drops (mimicking droplet transmission). In addition, influenza virus administered intranasally typically does not cause cough or lower respiratory tract symptoms, whereas early onset of cough and protracted cough are associated with natural influenza infection. Two recent articles have demonstrated that H5N1 preferentially binds to cells of the lower respiratory tract rather than the upper respiratory tract. This finding may explain the relatively rare amount of person-to-person transmission seen to date.[13,14]
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. 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. The possibility of re-aerosolization of influenza virus is supported by increased infectivity of the air after the floor of the room was vigorously swept. Another study found that a highly transmissible influenza strain could be recovered easily from the air surrounding infected mice during the period when they were most infectious, but there was no recoverable virus in the air surrounding mice infected with a less transmissible influenza strain during the same period. Finally, efficient transmission of influenza from infected to uninfected ferrets was demonstrated whether or not the ferrets were separated by a long straight air duct or by air ducts in the shape of an 's' or a 'u.'
Unfortunately, the data on influenza transmission in health care facilities are very limited and it is not possible to determine all the modes of potential transmission. Available data are often cited as indicating that droplet transmission is the primary mode of transmission in healthcare facilities; however, other modes cannot be ruled out. During the 1957-58 influenza pandemic, an acutely ill patient was admitted to a four-person hospital room with no precautions. Subsequently, roommates, health care workers, and other ward patients became ill. The epidemic curve suggested a point source outbreak with additional droplet or contact spread, rather than a single source outbreak, which would be more likely to be associated with airborne trans-mission. More recent influenza experiences at two U.S. hospitals have also been reported. In one hospital, transmission of influenza was rarely noted; most rooms were private, but had positive pressure. In the other hospital, transmission of influenza in pediatric patients was most often observed among patients in the same room, particularly those in adjacent cribs. Patients in other rooms in the same ward were less likely to become infected, even though room doors were open and influenza patients were not housed in negative pressure rooms. It should be noted in interpreting the latter study, that pediatric patients do not typically have a forceful cough and are known to be less likely to transmit airborne diseases such as tuberculosis. Clearly, additional data would be helpful in determining the relative importance of the different modes of transmission of influenza in various settings including healthcare facilities.
It is important to understand how respiratory protection equipment works. Respirators are designed to protect the wearer from respiratory aerosols expelled by others. Surgical masks are designed to protect the sterile field from respiratory aerosols expelled by the wearer and are not designed to offer respiratory protection to the wearer. Although there are no data on the efficacy of respirators vs. surgical or procedure masks in preventing transmission of influenza to health care workers, there are data demonstrating the poor filtration and fit capacity of single or even multiple surgical masks worn at one time.[21,22,23,24] In addition, surgical and procedure masks are not evaluated for fit and cannot be properly fitted to the face or tested for fit and do not prevent leakage around the edge of the mask when the user inhales. It is important to note that there are no minimum standards for surgical or procedure mask filter efficiency, there are a wide variety of filter efficiencies among available masks, and most masks do not effectively filter small particles from the air.
- Influenza may be transmitted by small particle aerosols and surgical masks do not offer adequate protection against the inhalation of these particles.
- To minimize exposure of healthcare workers to avian and pandemic influenza virus, MDH recommends that healthcare workers use full barrier precautions, including respirators (if available*), when working with known or suspect avian or pandemic influenza patients.
- Providing appropriate protection
to healthcare workers during a pandemic is critical because:
- vaccine for the pandemic influenza strain is unlikely to be available in the initial stages of a pandemic;
- antiviral supplies are likely to be limited; and
- pandemic influenza may cause disproportionate morbidity and mortality in younger, healthier people, such as healthcare workers, as it did in the 1918 pandemic
*If respirators are unavailable, use a tight fitting surgical mask
2. Evans D. Epidemiology and etiology of occupational infectious disease. In:Couturier A, ed. Occupational and environmental infectious diseases: epidemiology, prevention and clinical management. Beverly Farms, MA: OEM Press; 2000:37-132.
17. Schulman J. Experimental transmission of influenza virus infection in mice. IV. Relationship of transmissibility of different strains of virus and recovery of airborne virus in the environment of infector mice. J Exp Med. 1967;125(3):479-488.
19. Blumenfeld HL, Kilbourne ED, Louria DB, Rogers, DE. Studies on influenza in the pandemic of 1957-1958. I. An epidemiologic, clinical and serologic investigation of an intrahospital epidemic, with a note on vaccination efficacy. J Clin Invest. 1959;38(1 Pt 1-2):199-212.
21. Kaye K, Weber D, Rutala W. Nosocomial Infections Associated with Respiratory Therapy. In: Mayhall C, ed. Hospital Epidemiology and Infection Control. 3 ed. Philadelphia: Lippincott Williams & Wilkins; 2004:1207-1222.
Adapted from March/April 2006 DCN (PDF: 187KB/16 pages), Minnesota Department of Health Infection Control Recommendations for Avian and Pandemic Influenza, March/April 2005, pages 12-14.