As many of you who have studied public health and infectious disease well know, quarantine has been a staple of outbreak response essentially since before we even understood the underlying cause of disease. The practice of quarantine dates back at least as far as the 14th century, when ships arriving from ports with ongoing plague outbreaks were required to remain in the harbor for 40 days to prevent the importation of the deadly disease. The term “quarantine” originates from the phrase quaranta giorni, which is Italian for “40 days.”

Interestingly, multiple instances of infectious disease quarantines around the world, two involving ships, have populated the news over the past week or so, and we will take a quick look at the circumstances surrounding these events and briefly discuss the potential benefits and limitations of quarantine in the context of outbreak response.

Quarantine vs. Isolation

At their core, quarantine and isolation both aim to interrupt chains of infectious disease transmission, but they accomplish this in different ways. Quarantine refers to separating individuals at risk of becoming infectious from susceptible populations, whereas isolation involves individuals who are already known to be infectious. The differences between these two interventions are subtle, based on the characteristics of various stages of disease, and these terms are often, and incorrectly, used interchangeably. We will define and characterize the relevant stages of disease and use Ebola as an example to illustrate the differences between quarantine and isolation.

The first step in communicable disease progression involves an individual coming into contact with an infectious agent, at which point the individual is considered to be exposed. This can take various forms and range from direct physical contact with an infectious individual or their bodily fluids to physical contact with a fomite—an inanimate object contaminated with a pathogen, such as a doorknob touched by an infectious individual—to inhaling infectious aerosolized materials (eg, respiratory fluid, anthrax powder). At this point, susceptible individuals (ie, those without sufficient immunity to the pathogen) may become infected. The period of time between when an individual is infected and when s/he become infectious (ie, capable of transmitting the infection to others) is called the latent period. The period of time between when an individual is infected and the onset of symptoms is referred to as the incubation period. In some instances, the latent period and incubation period are essentially the same—ie, the individual is not infectious until symptoms present; however, other diseases (eg, measles) may be transmissible before the individual exhibits symptoms, so the latent period would be shorter than the incubation period. The latent period does not apply to non-communicable diseases, such as anthrax, because cases do not become infectious.

Individuals infected with Ebolavirus are known to only be infectious after the onset of symptoms. When an individual is diagnosed with Ebola virus disease (EVD), they are placed in isolation, ideally in dedicated treatment centers where they will not be in contact with other, non-EVD patients. Additionally, healthcare providers and other personnel will wear appropriate personal protective equipment (PPE) to prevent exposure to the virus. EVD patients are placed in isolation because they are known to be infectious, and they can be removed from isolation once they are determined to no longer be infectious (1). Conversely, individuals who are not currently exhibiting symptoms (and are, therefore, not infectious) but who have been exposed to Ebolavirus (eg, via a needle stick or other breach of PPE while caring for an EVD patient) or, in some instances, were potentially exposed to Ebolavirus may be placed in quarantine. For Ebola, the quarantine period is typically set at 21 days, based on the longest expected incubation/latent period. Quarantining these high-risk individuals ensures that they are not in contact with others if they do develop symptoms of EVD, which prevents further transmission.

Quarantines in the News

Two of the quarantine stories from the past week involve quarantined ships. The first involves a US Navy ship, the USS Fort McHenry, which was under quarantine at sea due to an outbreak of mumps among the ship’s crew. Ships provide ideal circumstances for communicable disease outbreaks—in particular, prolonged close contact with others—and outbreaks on ships, including the Navy, are not uncommon. From personal experience, norovirus and adenovirus are fairly common on naval deployments; however, outbreaks of vaccine-preventable diseases are more rare, as most military service members are fully vaccinated. The outbreak resulted in 28 cases out of the 703 personnel onboard. All Sailors onboard the Fort McHenry were provided with a vaccine booster as a precaution, and the identified cases were isolated in the ship’s medical facility until they recovered. The first case was reported on December 22, 2018, shortly after the ship got underway for deployment, and the ship’s last port visit was on January 10. US Navy ships typically pull into port about once per month while on deployment, but the Fort McHenry has been underway continuously for nearly 4 months due to the quarantine. The maximum expected incubation period for mumps is 25 days, and the last active case was on April 2. One report indicated that the US Navy typically uses 30 days as the duration of quarantine for outbreaks onboard ships. The outbreak cannot be considered resolved until it reaches 2 full incubation periods, but several reports indicate that the quarantine period has now been declared over.

The second shipboard outbreak involves measles onboard a cruise ship owned by the Church of Scientology. A crew member on the cruise ship was diagnosed with measles on April 29, shortly after getting underway en route to St. Lucia. Health officials coordinated with the Pan American Health Organization (PAHO) and determined the need to quarantine the ship upon arrival to prevent introduction of measles to the island. The ship’s doctor reportedly requested 100 doses of the MMR vaccine, which was provided by the St. Lucia Department of Health and Wellness. The ship returned to its home port in Curaçao, where it remained under quarantine while health officials conducted a thorough investigation to identify any additional infected individuals. Of the 318 people onboard, only 41 were able to provide proof of vaccination or immunity. The other 277 were required to provide blood samples, which were sent for testing in the Netherlands to determine which individuals were susceptible before allowing anyone to disembark the ship. One report indicates that the results are expected today. The quarantine reportedly impacted the ship’s schedule, forcing the cancellation of multiple port visits.

After a measles case was confirmed on the campus of the University of California Los Angeles (UCLA), university officials rushed to determine the vaccination status of 500 individuals thought to be at risk for exposure to the infected student. Ultimately, the vaccination status could not be confirmed for 119 students and several university staff, and Los Angeles County Public Health issued a “legally binding” quarantine order for these individuals. UCLA began requiring measles vaccination starting in 2019, and most of the quarantine notices were reportedly a result of a lack of records as opposed to vaccine avoidance. Individuals who were able to provide proof of vaccination or provide blood specimens to confirm immunity were allowed to leave, but the rest of the affected individuals were confined to one of the residence halls for 7 days. A similar quarantine was implemented at California State University, Los Angeles (Cal State LA) the week prior, with 127 staff and 71 students ordered to “stay home and avoid contact with others” as a result of measles exposure. Outbreak Observatory was not able to determine if the quarantined individuals were offered the MMR vaccine. It likely takes 10-14 days after vaccination to confer immunity to measles, so even if the vaccine was available, newly vaccinated individuals may not necessarily be permitted to leave right away.

Finally, the death of two individuals due to bubonic plague in Mongolia resulted in the quarantine of all individuals in the local area. The two individuals, a Mongolian couple, hunted marmots and consumed raw meat from the animals, including the kidney, which is believed by some Mongolians to have health benefits. Despite these beliefs, plague is endemic in local marmot populations, and hunting them is illegal in Mongolia. It is unclear whether the couple were infected due to consuming the raw meat/organs or if they were bitten by fleas that had infested the animals. The deaths prompted a 6-day quarantine of the local population as well as tourists, due to concern that the couple had pneumonic plague, which is transmissible person to person via respiratory droplets. More than 100 people were determined to be in contact with the two victims and were treated with prophylactic antibiotics. When no more cases emerged and the deaths were determined to be due to bubonic plague, the quarantine was lifted.

Ideal Conditions for Quarantine

In contrast to a number of other recent, high-profile examples of quarantine, the situations described above actually seem to be fairly reasonable and effective uses of this intervention. Two of the major challenges associated with quarantine are the ability to effectively identify and confine the at-risk individuals and the ability to provide care and services for those individuals. In the case of the two ships, this is nearly an ideal set of circumstances for implementing quarantine. The affected individuals are easily confined to the ship (especially while at sea), and sufficient food, water, shelter, and services (eg, medical, religious) available onboard as well as relative creature comforts to mitigate the inconvenience. I know from experience that missing port visits while on a military deployment is disappointing and inconvenient, but the crew is basically able to continue operating like it is any other day. In the case of the UCLA measles outbreak, the university was able to quickly identify those potentially at risk and congregate them at a centralized location to remove them from the rest of the campus population. They took care to identify those with immunity and release them, and they provided shelter, food, entertainment, and other services for those who remained under quarantine. Additionally, the county health department issued formal quarantine orders, which ensured due process and provided an avenue for affected individuals to contest the quarantine, if they chose to do so.

The quarantine in Mongolia may be on the least solid ground among these examples, but it does not appear to be wholly inappropriate. The affected area seemed to be relatively small and isolated, which made the quarantine easier to enforce and therefore more effective. And being a small town, there was likely sufficient food, water, shelter, and services available for the affected individuals, at least for the short duration of the quarantine. It was not clear, however, who was responsible for providing these supplies and services to the affected population—ie, were the quarantined individuals responsible for paying for them, or were they provided by the government? The quarantine was initially implemented over concern about person-to-person transmission of pneumonic plague, but the quarantine may not have been necessary after the antibiotic prophylaxis was administered to exposed individuals. Additionally, it seems as though health officials identified specific exposed individuals, so a mass quarantine of the entire town may have been an aggressive approach. That being said, health officials lifted the quarantine once no additional cases were identified and the deaths were determined to be due to the less-transmissible bubonic plague (rather than pneumonic), mitigating the undue imposition on the affected individuals.


The practice of quarantine has long been used as a non-pharmaceutical intervention to stem the spread of infectious disease outbreaks, but it is not an appropriate response in all situations. There are certain circumstances in which quarantine can be effective—including an identified risk of transmission, the ability to identify exposed individuals, the ability to separate exposed individuals from the rest of the susceptible population, and the capacity to provide supplies and services for affected individuals—but it can be misunderstood and misapplied under the wrong circumstances. The examples described this week illustrate scenarios in which quarantine can be reasonably and effectively implemented as a public health tool and highlight characteristics that can make for a successful quarantine. This is an important lesson, particularly as vaccine-preventable diseases reemerge around the world and health and elected officials are faced with difficult decisions regarding the best methods to contain these outbreaks.

  1. NOTE: Ebolavirus has been found to persist in certain bodily fluids (eg, breast milk, semen) for prolonged periods of time after recovery. The discharge criteria provided in US CDC guidance include both clinical and laboratory components. EVD patients must be free from symptoms for at least three days as an indication that they are no longer actively shedding the virus. Additionally, EVD patients are determined to no longer be infectious based on negative RT-PCR tests (typically performed on blood samples). Despite these discharge criteria, certain precautions are still necessary after discharge to prevent transmission via other infectious bodily fluids (eg, sexual transmission).

Photo courtesy of CDC

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.