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Novel Swine-Origin Influenza A H1N1 Virus and Air Travel
Abstract & Commentary
By Mary-Louise Scully, MD
Dr. Scully is Director, Travel and Tropical Medicine Center, Sansum Clinic, Santa Barbara, CA.
Dr. Scully reports no financial relationships relevant to this field of study.
Synopsis: CDC guidance for prevention of novel swine-origin influenza A (H1N1) during air travel focuses on basic measures of hygiene, cough etiquette, and identification and isolation of ill travelers, rather than imposing broad travel restrictions.
Source: CDC. Interim Guidance for Airlines Regarding Flight Crews Arriving from Domestic and International Areas Affected by Novel H1N1 Influenza. www.cdc.gov/h1n1flu/guidance/air-crew-dom-intl.htm Accessed 5/09/09. CDC. Interim Guidance to Assist Airline Flight Deck and Cabin Crew in Identifying Passengers Who May Have Novel H1N1 Flu. www.cdc.gov/h1n1flu/aircrew.htm. Accessed 5/09/09.
A novel swine-origin influenza A (H1N1) virus (S-OIV) has been identified as the cause of the recent outbreaks of febrile respiratory infection in Mexico with subsequent global spread to more than 28 countries. News of the outbreak quickly triggered the World Health Organization (WHO) and governmental agencies to initiate pandemic preparedness measures that had been developed for the possibility of pandemic avian influenza A (H5N1).
Novel Influenza A (H1N1) is transmitted from person to person in the same ways as seasonal influenza, with droplet exposure from coughing and sneezing of infected people as a primary mode of transmission. In addition, influenza also can be spread through contact with contaminated hands or surfaces and, less often, by airborne (droplet nuclei) routes.
The CDC interim guidelines address concerns about the potential for transmission of novel influenza A (H1N1) during air travel. First and foremost, the absolute importance of ill or potentially infected persons to avoid travel is stressed. Any ill person is advised to stay home from work or school and limit contact with other persons. Next, standard infection control and industrial hygiene practices are stressed, such as frequent hand washing and cough etiquette. Airline flight crews are advised to wear impermeable, disposable gloves on board the aircraft if they need to have direct contact with potentially contaminated surfaces such as airplane seats, tray tables, and lavatories used by ill passengers. If a crew member needs to assist an ill person, a facemask at a minimum, but ideally use of a respirator rated N-95 or higher, should be used.
Cabin and flight deck crew should be fully knowledgeable about the possible symptoms of influenza so as to be able to better identify potentially ill travelers. During a flight, if a person shows observable signs of novel influenza A (H1N1) illness, an attempt should be made to isolate the ill person (6 feet) from others, and the ill person should wear a facemask. If a face-mask cannot be tolerated, then tissues should be provided with a bag for proper disposal of contaminated items.
If a flight is bound for the United States and a person shows observable signs of an S-OIV illness, the captain is required by law to report the illness to CDC Quarantine Station in the jurisdiction of the airport where the plane is expected to land. The quarantine officials will then make arrangements for appropriate medical assistance, disease control and containment measures, passenger and crew notification and surveillance activities, as well as airline disinfection procedures. Of note, the flight deck crew should ensure that the aircraft air conditioning and ventilation system stay on until all passengers and crew have disembarked to maximize removal of virus particles from the cabin air.
It is estimated that more than 1 billion people undertake air travel each year. The confined cabin space, prolonged exposure time, and the process of ventilation and recirculation of cabin air raise concerns of increased transmission of respiratory pathogens. Yet it is very difficult to measure the actual rate of transmission of respiratory infections from air travel, in part secondary to the sheer numbers of travelers and the difficulty in proving the source, variable incubation periods of respiratory pathogens, pursuing follow up, and even differentiating cabin exposure from contact within the airline terminal before the flight.1
Despite these challenges, several studies have evaluated in-flight transmission of influenza. One well-known example is the 1979 Alaskan passenger jet that suffered engine failure during take-off, which resulted in a 3-hour ground delay. All 54 passengers remained on board during the delay. The index patient developed influenza-like symptoms while on board, and within 72 hours, 40% of the crew and 72% of the passengers became ill with influenza.2 It was noted that during the 3-hour delay the cabin ventilation was turned off. Mostly as a result of this outbreak, it is now recommended that airplanes with more than 30-minute ground delays keep the aircraft ventilation system in operation.
In a more recent report, a person ill with influenza-like symptoms flew on a 75-seat plane for a 3.5-hour flight. Despite adequate air circulation and filtration, 20 other passengers developed similar illnesses during the next 3-4 days after the flight. All but two of the 20 cases were seated in close proximity to the index patient, supporting the notion that transmission occurred through droplet exposure as the person was coughing and sneezing throughout the flight.3
Most commercial passenger airplanes have complex systems to pressurize the air in the cabin, control ventilation and filtering of cabin air, as well as adjust the temperature and humidity of the air for the comfort of the passengers. Typically, outside air is compressed, heated, conditioned, and then mixed with an equal (50:50) amount of filtered recirculated air. The recirculation system takes air from the cabin and passes it through high-efficiency particulate air (HEPA)-type filters. These filters remove particles and microorganisms sized as small as 0.3 µ. This should be efficient for removal of most bacteria, which are approximately 1.0 µ in size. Viruses, although often much smaller in size (0.01-0.10 µ), are thought to form clumps or ride on dust particles that are large enough to become trapped in the HEPA filter as well.4
In the ventilation systems of most modern aircraft, air circulation is laminar, i.e. side to side, where air enters the cabin at the overhead system of a seat row and leaves the cabin at the floor level of the same row. This should result in very little air exchange front to back within the cabin, but rather mostly along the seat row. The importance of physical proximity to an index case was demonstrated during the SARS outbreak in which people seated in the three rows in front of the index patient had a higher relative risk of developing illness.5 Droplet exposure through coughing and sneezing of the ill passenger would be consistent with this proximity. The CDC interim guidance for in-flight measures that recommend physical isolation and masking of ill passengers puts this concept into action.
The use of U.S. quarantine centers addressed in the CDC interim guidance has an interesting history. The first U.S. quarantine station and hospital were built in 1799 at the port of Philadelphia in response to the 1793 yellow fever outbreak. In 1878, the National Quarantine Act moved the quarantine powers from the states to the federal government. In 1967, the CDC (then known as the National Communicable Disease Center) took over federal quarantine functions. During the 1970s, the number of quarantine stations was reduced from 55 to 8, but after the September 11, 2001, attack on the World Trade Center and later with the 2003 SARS outbreak, the CDC increased the number of quarantine centers from 8 to 20. The quarantinable diseases include cholera, diphtheria, infectious tuberculosis, plague, smallpox, yellow fever, viral hemorrhagic fevers, and SARS (added in April 2003). New types of influenza with pandemic potential were added in 2005.
Some experts feel that the recent novel influenza A (H1N1) outbreak serves as a good testing ground for the pandemic preparedness policies adopted for avian influenza A (H5N1) and the new strengthened International Health Regulations. In an address to world health administrators on May 9, 2009, WHO Director-General Dr. Margaret Chan stated, "The world is better prepared for an influenza pandemic than at any time in history." One hopes this is indeed the case, and that we are having the "dress rehearsal" for a performance we hope never takes place.