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Institute of Medicine (US) Forum on Microbial Threats; Knobler S, Mahmoud A, Lemon S, et al., editors. Learning from SARS: Preparing for the Next Disease Outbreak: Workshop Summary. Washington (DC): National Academies Press (US); 2004.

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Learning from SARS: Preparing for the Next Disease Outbreak: Workshop Summary.

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, M.D., , M.D., M.H.Sc., , M.D., and , M.D.

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Quarantine is an ancient tool used to prevent the spread of disease. The Bible describes the sequestering of persons with leprosy, and the practice was used widely in 14th-century Europe to control the spread of bubonic and pneumonic plague. To prevent disease transmission, ships were required to stay in harbor for 40 days before disembarkation (thus the term quarantine, which derives from the Latin quadragina or the Italian quaranta, meaning 40).

Quarantine has been used for centuries, but because it was often implemented in a way that equated disease with crime, the practice has negative connotations.

Persons under quarantine were often detained without regard to their essential needs. Those who were exposed but not yet ill were not always separated from the ill, allowing disease to spread within the detained group. Populations targeted for quarantine, such as foreigners, were stigmatized. In some cases, the power of quarantine was abused; for example, at the end of the 19th century, the steerage passengers on arriving ships were frequently quarantined while the firstand second-class passengers were allowed to disembark without being examined for illness.

Despite its history, quarantine—when properly applied and practiced according to modern public health principles—can be a highly effective tool in preventing the spread of contagious disease. It may play an especially important role when vaccination or prophylactic treatment is not possible, as was the case with severe acute respiratory syndrome (SARS). Even when direct medical counter-measures are available (e.g., smallpox and pneumonic plague), reducing mobility in the at-risk population may enable the most rapid and efficient delivery of postexposure vaccination and chemoprophylaxis.

Isolation and Quarantine

Before discussing the role of quarantine as a component of community response and containment for SARS, it is necessary to distinguish, from a public health perspective, between the related practices of isolation and quarantine. Both are usually imposed by health officials on a voluntary basis; however, federal, state, and local officials have the authority to impose mandatory quarantine and isolation when necessary to protect the public’s health.

Isolation refers to the separation and restricted movement of ill persons who have a contagious disease in order to prevent its transmission to others. It typically occurs in a hospital setting, but can be done at home or in a special facility. Usually individuals are isolated, but the practice may be applied in larger groups.

Quarantine refers to the restriction of movement or separation of well persons who have been exposed to a contagious disease, before it is known whether they will become ill. Quarantine usually takes place in the home and may be applied at the individual level or to a group or community of exposed persons.

Contact surveillance, in the context of quarantine, is the process of monitoring persons who have been exposed to a contagious disease for signs and symptoms of that disease. Surveillance may be done passively, for example, by informing contacts to seek medical attention if signs or symptoms occur. Contact surveillance can also be performed actively, for example, by having health workers telephone contacts daily to inquire about signs and symptoms or even having health workers directly assess contacts for fever or other symptoms. All quarantined persons should be monitored for development of signs and symptoms of disease to ensure appropriate isolation, management, and/or treatment. For persons without a known contact but believed to be at increased risk for disease or exposure, enhanced surveillance and education can be used for risk assessment monitoring. During the SARS epidemic, this approach was used effectively with airline passengers arriving in the United States from areas of high transmission during the SARS epidemic.

Principles of Modern Quarantine

Quarantine as it is now practiced is a public health tool and a collective action for the common good. Today’s quarantines are more likely to involve a few people exposed to contagion in a small area, such as on an airplane or at a public gathering, and only rarely are applied to entire cities or communities. The main goal of modern quarantine is to reduce transmission by increasing the “social distance” between persons; that is, reducing the number of people with whom each person comes into contact (see Figure 1-2).

FIGURE 1-2. Effectiveness of vaccination and quarantine to contain a smallpox outbreak after the release of bioengineered, aerosolized smallpox in an airplane carrying 500 people.


Effectiveness of vaccination and quarantine to contain a smallpox outbreak after the release of bioengineered, aerosolized smallpox in an airplane carrying 500 people.

If quarantine is to be used, the basic needs of those infected and exposed must be met. The following key principles of modern quarantine ensure that it strikes the appropriate balance between individual liberties and the public good:

  • Quarantine is used when persons are exposed to a disease that is highly dangerous and contagious.
  • Exposed well persons are separated from those who are ill.
  • Care and essential services are provided to all people under quarantine.
  • The “due process” rights of those restricted to quarantine are protected.
  • Quarantine lasts no longer than is necessary to ensure that quarantined persons do not become ill. Its maximum duration would be one incubation period from the last known exposure, but it could be shortened if an effective vaccination or prophylactic treatment is available and can be delivered in a timely fashion.
  • Quarantine is used in conjunction with other interventions, including—
    • Disease surveillance and monitoring for symptoms in persons quarantined.
    • Rapid diagnosis and timely referral to care for those who become ill.
    • The provision of preventive interventions, including vaccination or prophylactic antibiotics.

Quarantine encompasses a range of strategies that can be used to detain, isolate, or conditionally release individuals or populations infected or exposed to contagious diseases, and should be tailored to particular circumstances. Quarantine activities can range from only passive or active symptom monitoring or short-term voluntary home curfew, all the way to cancellation of public gatherings, closing public transportation, or, under extreme circumstances, to a cordon sanitaire: a barrier erected around a geographic area, with strict enforcement prohibiting movement in or out. In a “snow day” or “sheltering in place” scenario, schools may be closed, work sites may be closed or access to them restricted, large public gatherings may be cancelled, and public transportation may be halted or restricted. People who become ill under these conditions would need specific instructions for seeking evaluation and care; they would only expose others in their households—or perhaps no one at all, if precautions are taken as soon as symptoms develop. The fact that most people understand the concept of sheltering at home during inclement weather, regarding home in these circumstances as the safest and most sensible place to be, increases the likelihood that similar conditions of quarantine will be accepted. “Snow day” measures can be implemented instantaneously, and most essential services can be met without inordinate additional resources, especially if the quarantine lasts only a few days.

Another important feature of quarantine is that it need not be absolute to be effective. Even a partial or “leaky” quarantine, such as occurs with voluntary compliance, can reduce the transmission of disease. Voluntary measures, which rely on the public’s cooperation, reduce or remove the need for legal enforcement and leverage the public’s instinct to remain safely sheltered. In contrast, compulsory confinement may precipitate the instinct to “escape.” If an effective vaccine is available, partial quarantine can be an effective supplement to vaccination, further reducing transmission of disease. For example, Figure 1-3 shows a model illustrating various outcomes of a hypothetical scenario of 500 people, all of whom are vaccinated against smallpox, exposed to an intentional aerosol release of that contagion on an airplane. In the model, all 500 people are offered postexposure smallpox vaccine; the model assumes that the vaccine is 95 percent effective. Even under these unlikely and theoretical circumstances, the addition of even partial (50 percent to 90 percent) quarantine to vaccination can have a profound effect on reducing the number of eventual cases in the community. This trend remains significant even at low rates of transmission (“reproductive rates”).

FIGURE 1-3. Impact of varying Ro and percent quarantined on total smallpox cases.


Impact of varying Ro and percent quarantined on total smallpox cases. Even with 100 percent vaccination against smallpox, quarantine effectively reduces the spread of disease in the community. This effect remains significant even at lower reproductive (more...)

In order to implement modern quarantine effectively, there must be a clear understanding of the roles of public health staff at federal, state, and local levels, and each group should know their legal authorities. Effective implementation also requires identifying appropriate partners, including transportation authorities and law enforcement officials, and engaging them in coordinated planning. Finally, quarantine can be most successful if the public has advance knowledge of the disease threat and understands the role of quarantine in containing an epidemic. People who are actually quarantined need to believe that their sacrifice is justified and that they will be supported during the period of quarantine.

Quarantine and the Response to SARS

Containment strategies employed during the recent SARS epidemic included case and contact management, infection control in hospitals and other facilities, community-wide temperature screening, mask use, isolation and quarantine, and the monitoring of travelers and response at national borders. Various combinations of these strategies were applied in different places, depending on factors such as the magnitude and scope of the local outbreak, the availability of resources to support containment, and the level of public cooperation and trust. In the United States, where only eight laboratory-confirmed cases of SARS and no community transmission occurred (see Figure 1-4), the principal strategies of containment were education of high-risk populations (e.g., international travelers and health-care workers); early detection of suspected and probable cases; and rapid implementation of isolation and other infection control tools. Additional measures such as quarantine were used in other countries where SARS presented a greater threat.

FIGURE 1-4. Reported cases of SARS, United States, May 19, 2003.


Reported cases of SARS, United States, May 19, 2003.

Case and Contact Management

In the United States, most people with suspected or probable SARS were isolated at home; hospital isolation was reserved for those who required such care or had no suitable home environment. (e.g., homeless, out-of-town visitors). Isolation was continued while symptoms persisted and for 10 days thereafter. In some other countries, most persons with suspected or probable SARS were isolated in the hospital. For contact management, the U.S. Centers for Disease Control and Prevention (CDC) recommended quarantine only for health-care workers who had a high-risk exposure to a SARS patient. In several states, however, local health officials “furloughed” health-care workers who were exposed to high-risk probable cases. In general, CDC recommended only passive surveillance. Persons who were exposed to suspected or probable SARS, as well as travelers returning from areas with SARS transmission, were asked to monitor their health for 10 days and seek medical attention immediately if fever or respiratory symptoms developed. Active surveillance was reserved for probably and lab-confirmed cases and their high risk close contacts; this was usually conducted by members of the local or state health departments.

In some countries other than the United States (e.g., China, Taiwan [ROC],8 Singapore, and Canada), home quarantine was used for most close contacts of people with suspected or probable SARS. Designated quarantine facilities were used in some situations for homeless persons, travelers, and people who did not wish to be quarantined at home. In some situations, as a result of staffing shortages and relatively high exposure rates in hospitals, exposed health-care workers and ambulance personnel were placed on “work quarantine,” which entailed working during their regular shifts, using comprehensive infection control precautions and personal protective equipment, and staying either at home or in a building near the hospital when off duty. Most persons in home quarantine were asked to monitor their temperature regularly, once or twice a day; health workers called them twice a day to get a report on temperature and symptoms. Other health-care workers had their temperature checked twice a day or more at work. In Singapore, video cameras linked to telephones were occasionally used to monitor patients.

Authorities used a variety of methods to enforce quarantine during the SARS epidemic. In select places, quarantine orders were given to all persons placed in quarantine, while in the majority, only those who demonstrated noncompliance were given orders. Under some orders, noncompliant individuals were isolated in guarded rooms; others were confined at home wearing security ankle bracelets; yet others received fines or even jail sentences. However, these instances of compulsory enforced quarantine orders were clearly the exception rather than the norm during the SARS epidemic.

Community Containment

In the United States, community containment strategies consisted mainly of coordinating the SARS response activities through emergency operations centers and providing information and education to the public, health workers, and others. This strategy included publishing guidelines and fact sheets on websites, holding press conferences, making presentations to a variety of audiences, and meeting with groups and communities who were experiencing stigmatization.

On some occasions, such as occurred in mainland China, Hong Kong (SAR), Taiwan (ROC), and Singapore, large-scale quarantine was imposed on travelers arriving from other SARS areas, work and school contacts of suspected cases, and, in a few instances, entire apartment complexes where high attack rates of SARS were occurring.

In addition to imposing large-scale quarantine in some cases, many areas with high transmission (e.g., Hong Kong, Singapore, Taiwan, Toronto, and mainland China) used strategies such as mandated fever screening before entry to schools, work, and other public buildings; requiring masks in certain settings; and implementing populationwide temperature monitoring and disinfection campaigns. Community mobilization programs were also developed to educate the public about SARS and what to do to prevent and control it; for example, a populationwide body temperature monitoring campaign and a SARS hotline to promote early detection of fever as a warning sign for SARS. Taiwan and mainland China also undertook a series of community disinfection campaigns in which streets, buildings, and vehicles were sprayed with bleach and bleach was distributed free throughout the community.

Several important lessons can be gained from the experience of countries where large-scale quarantine measures were imposed in response to SARS. First, when the public was given clear messages about the need for quarantine, it was well accepted—far better, in fact, than many public health officials would have anticipated. Indeed, voluntary quarantine was effective in the overwhelming majority of cases. Yet, despite the widespread acceptance of and cooperation with quarantine, it represented a great sacrifice for many people through consequences such as loss of income, concerns for the health of their families, feelings of isolation, and stigma. Finally, large-scale quarantine was found to be complicated and resource intensive to implement, creating enormous logistic, economic, ethical, and psychological challenges for public health authorities. Recent data evaluating the efficacy of quarantine in Taiwan and Beijing, China, during the SARS epidemic suggest that efficiency could be improved by focusing quarantine activities on persons with known or suspected contact with SARS cases. In order to prepare for future epidemics, enhanced systems and personnel will need to be established to deliver essential services to persons in quarantine, to monitor their health and refer them to necessary medical care, and to offer mental health and other support services.

Border and Travel Response

Several strategies for border and travel response were used in the United States, including issuing travel advisories and alerts, distributing health alert notices at ports of entry, and meeting planes and responding to reports of ill passengers. Additional strategies used in other countries (e.g., Canada, China, and Singapore) included predeparture screening of temperature, SARS symptoms, and recent exposures to SARS patients; postarrival disembarkation screening with questions about travel and exposure to SARS, maintaining “stop lists” of people with suspected SARS and their contacts at airports to prevent such individuals from traveling, isolation of ill travelers with suspected or probable SARS, and quarantine of healthy travelers returning from other areas with high SARS transmission.

In the United States, CDC issued a series of travel alerts and advisories related to SARS (see Table 1-7). A travel alert describes a health situation in a particular area and gives recommendations about appropriate precautions, while a travel advisory goes a step further and recommends postponement of nonessential travel to an affected area. During the SARS epidemic, CDC staff met nearly 12,000 flights and distributed more than 2.7 million health alert notices to passengers arriving directly and indirectly from affected areas. The notices instructed travelers to monitor their health for fever and respiratory symptoms for 10 days and immediately seek medical attention (with advance notice to the health-care facility) if the symptoms occurred. Health alert notices (HANs) were also distributed at 13 U.S.–Canada land crossings, as well as in the predeparture area at the Toronto airport. If an ill passenger was reported on a flight arriving in the United States, it was met by members of the CDC quarantine staff. They evaluated the affected passenger for possible SARS, provided referral to a health-care provider, collected locating information from other passengers, and coordinated with federal, state, and local public health authorities.

TABLE 1-7. Travel Alerts and Advisories for SARS, March–July 2003.


Travel Alerts and Advisories for SARS, March–July 2003.

Preparedness Planning

Preparations for a resurgence of SARS (or indeed an outbreak of any contagious disease) should be made at all levels of government. Plans must encompass general logistics and planning for case and contact management, including quarantine. A framework for the community containment of SARS (see Figure 1-5) lists several criteria for establishing movement restrictions and a range of options for containment that could be applied in response. In deciding whether and how to restrict movement during an epidemic, community officials would consider factors such as:

FIGURE 1-5. Range of available responses to SARS at the national, state, and community levels.


Range of available responses to SARS at the national, state, and community levels.

  • the number of suspected, probable, and confirmed cases;
  • whether cases have well-defined exposure risks;
  • how many potential new exposures each case has been in contact with;
  • what type of transmission is predominant (e.g., airborne, droplet, fomite);
  • how many generations of transmission have occurred; and
  • the morbidity and case-fatality rate of the epidemic.

Decision makers would also need to consider the baseline amount of movement in the community, the impact of curtailing movement on critical infrastructure, the resources available to support containment, and the public’s reaction to the epidemic.

Planning for Community Containment

In some circumstances, containment of SARS or other microbial threats at the community level could be accomplished without restricting movement, with the focus instead on educating the public through such means as press releases and travel alerts and advisories (as was done in the United States in 2003). In other situations, targeted restrictions, including quarantine of close contacts and restriction of some group gatherings, would be appropriate. A more restrictive option would include general voluntary movement restrictions, including measures such as fever screening at entrances of public places, “snow-day” or “shelter- in-place” quarantines, closing public places, canceling public gatherings, and restricting mass transit. Rarely, in the most extreme circumstances, compulsory movement restrictions, including the closing of airports and borders, would be warranted.

Advance planning is necessary to enable officials to assess risk, make decisions, and implement necessary measures as effectively as possible in the event of a disease outbreak. Jurisdictions should establish an emergency operations center structure and a legal preparedness plan, and forge connections among essential partners such as law enforcement officials, first responders, health-care facilities, educators, the media, and the legal community. Provisions must be made to monitor and assess factors such as those above to determine response level for both implementing and scaling back interventions and movement restrictions. Educational message strategies should be developed to disseminate information to government decision makers, health-care providers and first responders, and the public; it will be especially important to address the possibility that some people may experience stigmatization as a result of containment. A draft of the CDC SARS Preparedness Plan entitled, “Public Health Guidance for Community-Level Preparedness and Response to Severe Acute Respiratory Syndrome (SARS) is posted at http://www.cdc.gov/ncidod/sars/updatedguidance.htm. Appendices D and E specifically address Community Containment and Border Strategies, respectively. A SARS preparedness checklist (available at http://www.astho.org) also provides guidance for public health officials in developing such plans.

To plan for case and contact management, jurisdictions should secure necessary protocols for clinical evaluation and monitoring, contact tracing and monitoring, and reporting of disease. Standards, tools, and supplies must be established for home and nonhospital isolation facilities. A telecommunications plan should be developed to provide for case and contact monitoring and fever triage, as well as to provide information to decision makers, health-care workers, and the public. Provisions must be made to ensure that all isolated and quarantined individuals receive food, medicine, and mental health and other supporting services, including transportation to medical facilities. Jurisdictions should also identify and develop assessment procedures for appropriate nonhospital residential facilities. These sites could be used for quarantining contacts or persons for whom “home isolation” is indicated, but who do not have an appropriate “home” environment.

To prepare for the implementation of community containment measures, jurisdictions must establish legal authorities and procedures to implement all levels of movement restrictions. Essential personnel for the implementation of quarantine and other movement restrictions will include law enforcement officials, first responders and other deployable government services workers, and key personnel from the transportation, business, and education sectors. Training programs and deployment drills should be developed for these partners, as well as for public health personnel.

Preparing to Respond and Secure National Borders

Similar criteria to those used to determine community-level containment policy must be considered when determining appropriate responses to SARS at national borders (see Figure 1-6). In addition to considering circumstances in their area, officials contemplating movement across national borders must also monitor events in adjacent areas and, given the frequency of global travel, throughout the world. A limited border response could resemble that mounted by the United States in 2003 (i.e., issuing travel advisories and alerts; meeting flights from SARS areas to triage arriving ill passengers; and monitoring contacts for symptoms of illness). More intensive arrival screening could include questionnaires on symptoms and exposure to SARS, temperature screening, or even requiring health certification or registration with the local health department. In some circumstances, predeparture screening also would be appropriate. A further step would be to quarantine arriving passengers from affected areas, and under the most extreme circumstances, restriction of inbound and outbound travel may be necessary.

FIGURE 1-6. Range of available responses to SARS at borders.


Range of available responses to SARS at borders.


Modern quarantine represents a wide range of scalable interventions to separate or restrict movement (e.g. detain, isolate, or conditionally release) of individuals or populations infected by or exposed to highly dangerous contagions. These strategies can be an important part of the public health toolbox for suppressing transmission and stopping epidemics such as SARS. However, the ethical implementation of modern quarantine can be resource and labor intensive. Quarantine is most effective when it is tailored to specific circumstances and used in conjunction with other containment measures; people affected by quarantine must be ensured appropriate support services. The effectiveness of quarantine is further improved by comprehensive preparedness planning. Effective communication and public trust are quintessential components; consequently, the public must receive clear messages about the role and importance of quarantine as a means of containing certain infectious disease in advance of, as well as during, the epidemic.

If a future epidemic affects the United States as SARS did other countries in 2003, it may be necessary to recommend quarantine, among other containment measures, in this country. Thus, it is essential that planning for the effective implementation of quarantine and other containment measures be undertaken at every level of government, and well in advance of the need. Strategic and operative plans should be exercised at all levels to expose and rectify gaps and pitfalls in nonurgent settings to ensure our readiness in an emergency.


The authors thank Alison Mack, Katherine Oberholtzer, Alexandra Levitt, and Ava Navin for technical assistance in the preparation and review of the manuscript.



ROC stands for Republic of China.

Copyright © 2004, National Academy of Sciences.
Bookshelf ID: NBK92450


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