As we enter another influenza season, one question continues to vex medical and public health professionals: How do you stop people from catching the flu? The best way to prevent the flu is by getting an influenza vaccine every year. However, in the event of a large-scale influenza outbreak of a new virus strain or a pandemic, when influenza vaccine may not be promptly available, we will see tremendous demands on the health care system and its workers. Thus, it’s critical to understand how influenza is transmitted from person to person so that we can determine the best ways to protect health care workers while still enabling them to do their jobs.
The typical incubation period for influenza is 1-4 days (average: 2 days). Adults shed influenza virus from the day before symptoms begin through 5-10 days after illness onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by 3-5 days after onset in an experimental human infection model. Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms. Severely immunocompromised persons can shed virus for weeks or months.
Experts think influenza may be spread to uninfected people in three ways: large-particle respiratory droplet transmission, airborne transmission, and contact (or fomite) transmission. Most experts think that influenza viruses are spread mainly by large-particle respiratory droplets produced when people infected with influenza cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled. 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 a short distance (usually less than 2 meters) through the air. Airborne transmission (via small particles suspended in the air for extended periods of time) also is thought to be possible, although data are limited. Less often, a person might also become infected with influenza by touching a surface or object that has influenza virus on it and then touching his or her own mouth or nose.
A better understanding of influenza transmission could help improve the infection control procedures and equipment used by health care workers. NIOSH has been conducting research for several years to learn more about the underlying science of influenza transmission, with a particular focus on airborne transmission and the protection of health care workers during a future pandemic. Here are some of the questions that are being addressed:
Is there a correlation between exposure to airborne influenza and illness among healthcare workers? A combination of three approaches is being used to better understand worker exposure to influenza and its consequences. First, aerosol sampling will be conducted in a health care clinic. Second, the amount of genetic material (RNA) from influenza virus on surgical masks, respirators and gloves worn by health care workers and on the surfaces of equipment and furniture in the clinic will be measured. Third, the health status and any occurrences of influenza among the workers in the study will be monitored. This project is part of a large multi-hospital study to compare the relative effectiveness of surgical masks and respirators in preventing influenza among health care workers.
Can better methods be developed to detect infectious airborne influenza virus? The current methods for determining the infectivity of influenza aren’t sensitive enough to work with the small amounts of virus in a typical aerosol sample. NIOSH researchers are developing new, more sensitive methods of measuring influenza virus viability. One technique is a hybrid system called a “viral replication assay” that combines PCR with a more traditional culture method for increased sensitivity. A second technique uses genetically modified cells that glow faintly when they are infected with the influenza virus. This luminescence can be detected using standard laboratory equipment.
How well do different types of personal protective equipment perform under different exposure scenarios? To explore this, NIOSH researchers constructed a simulated medical examination room containing a custom-built coughing machine that can cough an influenza-laden aerosol into the room much like a patient would, and a breathing machine that can simulate a health care worker treating the patient. The breathing machine can be outfitted with personal protective equipment (PPE), such as surgical masks, respirators, face shields, and powered air-purifying respirators (PAPRs). The simulated exam room is now being used to study how well different types of PPE and combinations of PPE protect from large spray droplets and small aerosol particles at shorter and longer distances.
Researchers at NIOSH have presented their work at scientific and public health conferences, and several articles have been published in peer-reviewed scientific journals. Information on NIOSH influenza research and influenza-related Health Hazard Evaluations can be found at http://www.cdc.gov/niosh/topics/flu/transmission.html and http://www.cdc.gov/niosh/topics/flu/hhe-projects.html. In addition, the CDC provides a comprehensive set of guidelines for preventing transmission of influenza in healthcare settings at http://www.cdc.gov/flu/professionals/infectioncontrol/healthcaresettings.htm.
William G. Lindsley, PhD
Dr. Lindsley is a Research Biomedical Engineer in the NIOSH Health Effects Laboratory Division.