Editor’s note:This is the first of two articles on reducing transmission of respiratory viruses. For the latest news on coronavirus disease 2019, visit https://www.aappublications.org/news/2020/01/28/coronavirus.
Successful transmission of an infectious agent in a health care setting requires a source, a susceptible host and a mode of transmission. Sources of infection include people who are infected asymptomatically, are in the incubation period of an infection or may be colonized with a pathogenic organism.
Disease expression in a susceptible host is a complex interaction between the host and the infectious agent. It depends on the dose of the organism, the route of exposure, the immune status of the host and the virulence factors of the organism. Standard precautions as well as transmission-based precautions are recommended for management of patients with a viral respiratory tract infection.
The ease of transmission of an infectious agent varies by type of organism and may involve more than one route. The most common mode of transmission is by contact, either direct contact (such as herpes simplex virus from a patient with herpes whitlow to the unprotected hands of a health care worker) or indirect contact (such as shared toys or a contaminated stethoscope or otoscope).
Which statement is false?
A. Airborne transmission refers to transmission over long distances, often beyond the patient’s room environment.
B. Aerosol-generating medical procedures alter the risk of transmission of an infectious agent and may necessitate personal protective equipment.
C. Group A streptococcus is spread by fomites and household pets.
D. Some evidence indicates that norovirus and rotavirus may be transmitted by respiratory droplets.
E. The Centers for Disease Control and Prevention recommends wearing cloth face coverings in public settings to help slow the spread of coronavirus disease 2019 where other social distancing measures are difficult to maintain (e.g., grocery stores and pharmacies) especially in areas of significant community-based transmission.
Answer: C is false.
Droplet transmission refers to respiratory droplets carrying an infectious agent that travel from the respiratory tract of an infectious person to the mucosal surface of a susceptible person. Respiratory droplets are generated by sneezing, coughing, talking or during certain aerosol-generating medical procedures such as respiratory tract suctioning, intubation, sputum induction, pulmonary function testing, cardiorespiratory resuscitation and autopsy procedures. Any procedure that irritates the airways such as tracheal intubation may elicit a forceful cough.
For respiratory viruses, the important portals of entry for respiratory droplets are the nasal mucosa or ocular conjunctiva and less frequently the oral mucosa.
Because of their relatively large size, most respiratory droplets travel less than 3 feet from the patient, although some respiratory droplets travel at least 6 feet before gravity pulls the particle to a surface. The distance a droplet travels depends on a number of factors, including the velocity of the cough or sneeze, the density of droplets and environmental factors such as temperature and humidity. Traditionally, particles spread by droplet transmission are greater than 5 microns in diameter.
Examples of viruses spread in this manner are respiratory syncytial virus, rhinoviruses and adenoviruses as well as SARS-CoV-2. Because the droplets tend to fall out of the air quickly, measures to control airflow generally are not indicated. This is considered transmission by direct contact. Although more studies are needed to clarify this issue, a face mask generally should be donned when within 6 feet of an infectious patient.
Airborne transmission refers to small particles of respiratory secretions that are less than 5 microns in diameter, remain suspended in the air for longer periods of time than droplet particles (e.g., measles virus can remain suspended for up to two hours) and contain an infectious agent or fungal spores. These respirable particles are less influenced by gravity and can be inhaled directly into the lower respiratory tract without contacting the mucous membranes of the upper respiratory tract. Because they remain suspended, air currents may carry the particles a long distance where they may be inhaled by susceptible individuals who have not had face-to-face contact with the infectious person. Transmission of this sort is considered to be indirect contact.
Some aerosols consist of both small and large particles. Evidence suggests certain respiratory viruses such as influenza may be transmitted between people by droplet as well as small particle aerosol.
Minimizing airborne transmission requires special air handling and ventilation systems. Infectious agents spread by airborne transmission (small particle) include Mycobacterium tuberculosis, measles virus, varicella-zoster virus and smallpox virus. The spores of Aspergillus fumigatus and some other opportunistic fungi have a diameter of about 3 microns and can be spread by an airborne route. Pneumocystis jirovecii appears to be spread person to person via airborne transmission.
When Middle East respiratory syndrome coronavirus was circulating, airborne precautions were recommended because of the severity of the disease and the unclear method of transmission. Ventilation of a patient’s room who is on airborne precautions should include six to 12 air exchanges per hour with either direct exhaust to the outside or through a high-efficiency particulate air filter.
Norovirus outbreaks usually result from fecal contamination of food or water, but transmission from person to person is facilitated by contaminated fomites or infectious particles, particularly during the process of vomiting. Aerosolized infectious particles generated during vomiting may be inhaled or swallowed by a susceptible contact.
Repeat education of health care workers is essential so that the scientific basis for precautions is understood and so that precautions can be modified based on changing requirements or changes in available resources. One study described the likelihood of health care workers developing severe acute respiratory syndrome was associated strongly with less than two hours of infection-control training on proper hand hygiene and use of personal protective equipment.
Dr. Kilpatrick, an AAP resident member, is a PGY-3 pediatric resident at Floating Hospital for Children and will be starting neonatology fellowship at Duke University Medical Center in July. Dr. Meissner is professor of pediatrics at Floating Hospital for Children, Tufts Medical Center. He also is an ex officio member of the AAP Committee on Infectious Diseases and associate editor of the AAP Visual Red Book.