Quarantine and Isolation

Quarantine and isolation are two different ways to limit the spread of certain infectious diseases by [Page 875]reducing contact between individuals at risk of spreading infectious disease and the rest of the population. The types of diseases for which quarantine and isolation are useful public health measures are those that involve direct transmission of infection by close contact (e.g., aerosol or droplet transmission). There must also be detectable symptoms that allow individuals who have been infected to be distinguished from those who have not. The aims of quarantine and isolation can vary and include stopping local spread of disease, global eradication of a disease, or simply slowing down the progress of an epidemic to gain time in which to vaccinate or administer drugs.

Fundamentally, the difference between quarantine and isolation depends on whether the individual has a confirmed infected/infectious status (depending on the specific disease, determining infection may be easier than determining infectiousness, which is the ability to transmit the disease). If an individual's infected/infectious status is confirmed, they are isolated, which means removed to an environment designed to prevent them from spreading the infection to other individuals. These environments can range from an individual's own home to a highly secure medical facility. Individuals can also receive treatment while in isolation, with the health workers taking precautions against compromising the isolation (e.g., physical barriers or vaccination). Isolated individuals remain in isolation until they are no longer considered to be at risk of spreading infection (established, e.g., by a serology test or a clinical assessment). Occasionally, isolation is enforced by law, as was done with tuberculosis (TB) in New York City during the 1990s. In this case, the aims included the prevention of rapid emergence of drugresistant forms of TB, caused in part by patients not completing their drug treatment. At around the same time (1986–1993), Cuban residents who were HIVpositive were isolated in sanitariums, though this controversial policy evolved so that patients had a choice of how and where to be treated.

Individuals may be quarantined if they are considered at risk of having been exposed to an infectious disease (from an infected individual or another source) but do not display symptoms of the disease. The ‘at risk’ assessment can be made by the individual who may have been exposed or by a third party (e.g., doctor, public health official), and it can be based on contact tracing (determining an individual's recent close contacts by interview or questionnaire), or on the individual's having been to a certain region where the infectious disease is endemic or epidemic. The quarantine conditions can be as strict as those for isolation, but they are often based more on clinical observation and can be as simple as self-reporting and staying at home. If an individual develops symptoms, then he or she meets the criteria for isolation. The length of time that an individual is quarantined for is related to the specific infectious disease; in fact, the origin of the word quarantine comes from the 40 days that people arriving by ship had to remain on their ships before coming to land in case they had been exposed to the plague but had not yet become symptomatic. Time required to be spent in quarantine should relate to the incubation period of a particular disease, that is, the time between infection and the onset of detectable symptoms. Mathematical modeling has shown how this quarantine period can best be set and modified based on updated information about the incubation period, which is especially useful for emerging infectious diseases where little epidemiological data is known.

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