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Is Hong Kong’s 21-Day Quarantine evidence-based or justified?

Written by: Dr David Owens New call-to-action

Answer: No

The only possible justification for 21-day quarantine is that it reduces total passenger volume and takes pressure off limited quarantine facilities. If this is the goal it could be achieved in other ways, either by increasing quarantine supply or using travel quotas.

There is no scientific evidence to support 21-day quarantine. It is neither evidence-based nor proportionate and is almost certainly doing more harm than good.


Why Do We Quarantine?

There are really two different situations in which we quarantine people in Hong Kong. Whilst it is not strictly quarantine, there is a further situation in which we isolate people in hospital until they are no longer infectious.

  1. Border Quarantine
  2. Quarantine of Close Contacts
  3. Isolation of infected individuals

Border Quarantine

This is the classic use of quarantine originating in the 14th century. The word derived from the Venetian ‘quarantena’ describing the 40 days that ships would sit at anchor before entering Venice, effectively giving a period of time to ensure that any individuals entering the geographic location were not infectious. This form of border control has been used widely during the Covid pandemic.

Quarantine of Close Contacts

The second use of quarantine is to segregate the close contacts of infected individuals and to isolate them for a period of time, whilst waiting to see if they have been infected, or not. I have described this previously as building a wall around the embers which are coming off the fire. In Hong Kong, early in the epidemic, around 5% of close contacts subsequently developed the illness meaning that we accepted a temporary loss of liberty for 19 people in order to prevent one person from potentially infecting many others.

Isolation of infected individuals

Whilst this is not strictly quarantine, it shares some of the same ethical considerations. In Hong Kong, individuals who test positive for Covid are kept in hospital, often for a protracted period of time and regardless as to whether or not they have symptoms. The majority of these patients are not ill in a traditional sense. Their hospital admission is compulsory and subject to sanctions, not unlike quarantine.

So am I arguing against quarantine?

Answer: No

I am an advocate for public health and have consistently been supportive of the Hong Kong public health response. Border controls and test, trace and isolate have clearly been shown to be the most effective strategy in the early phases of the pandemic.

Ultimately all epidemics of infectious diseases require a balance between the cost of the disease (human and economic) and the social, economic and political costs of the public health interventions. Restriction of movement and quarantine are decisions which balance the rights of the individual against the rights of the majority. This process can be critiqued in philosophical and political terms. It is important to appreciate that control of infectious disease, by definition, has a significant political and economic component and the narratives around the management of infectious disease must be understood in this context.

I would argue that restriction of liberty on the grounds of public health should be both evidence-based and proportionate to risk. That is to say it should balance the risk/benefit to the population with the personal cost to the individual. The compulsory use of seat belts is a classic public health intervention which is demonstrably evidence-based and proportionate. The evidence for mask use is less clear cut, but the personal costs are small and the ‘precautionary principle’ would generally apply. Why not do simple things with minimal cost which may help? When it comes to locking people up and withdrawing rights of movement there must surely be a high threshold of both evidence and proportionality.

So what is the evidence?

When considering the issue of risk we need to look at two different factors.

  1. What is the incubation period?
  2. How long is an infected person infectious for?

Incubation Period

This refers to the period between exposure and the point at which an individual develops symptoms. In the context of Covid we know that many people never develop symptoms, especially those who have been vaccinated. We also know that Covid can be transmitted in the pre-symptomatic phase. However, in the context of quarantine we also need to consider the point at which a PCR test will become positive. The evidence is that the majority of people become either symptomatic or test positive by 5 days and the vast majority by 10 days[1]. There is good evidence that the incubation period for Delta is shorter than with previous variants[2].

How long is an infected person infectious for

Hong Kong is a world centre for virology and specifically coronavirus research. A paper published by Professor Peiris’s team at the University of Hong Kong showed that, although dead virus could be found on PCR testing for many weeks after infection, live virus was ‘rarely detectable beyond 8 days after onset of illness’[3]. Another paper by Public Health England found similar results[4]. Although long-term shedding has been described in some immunocompromised and very ill patients it is generally considered by the majority of experts in virology that 9 - 10 days is the outer limit in mild and moderate illness with the possibility of longer infectivity in severe illness or in individuals who are immunocompromised.

Surely it pays to be prudent. Longer quarantine must be safer?

Answer: No

All public health interventions, as with all medical interventions, involve a risk-benefit analysis. Doing a little bit more to be a little bit safer has the potential to produce unintended consequences and the possibility to increase harm. Continuing antibiotics for a little bit longer or in a slightly higher dose to ensure the infection has been eradicated increases the risks of drug resistance which increases the long-term risk to the population. Doing extra tests or scans leads to unnecessary biopsies and procedures, this has the potential to do more harm than good to the individual and increase costs to the health system. Excessive public health controls have the potential to do harm directly and indirectly. In the case of quarantine, directly in that the statistical probability of hotel acquired infections in 21-day quarantine, is greater than it would be for 14-day quarantine and almost certainly exceeds any potential benefit of the extra week. That is to say 21-day quarantine actually increases the risks to the population. Even more important is the negative impact on the overall public health strategy.

In the case of Covid the following are the public health priorities for Hong Kong, in order:

  1. Increasing vaccine uptake in the old and vulnerable.
  2. Increasing vaccine uptake in the old and vulnerable.
  3. Increasing vaccine uptake in the old and vulnerable.

The published scientific evidence on Covid supports an optimal quarantine period of 10 - 14 days. The shorter incubation of Delta is tipping the evidence towards 10 days. Singapore recently reduced quarantine to 10 days in response to this evidence.

What does the data in Hong Kong show us?

In the 3 months June - August 2021 Hong Kong reported 163,233 passenger arrivals. There were 188 cases of imported Covid (0.12% of total arrivals) of which 101 (54%) tested positive on arrival and 185 (98.4%) tested positive at or before day 7. Only 2 tested positive on day 19 and none on day 26.

One response to this data is that two cases justifies the process and is a reward for prudence. However, one of the two cases was discharged from the hospital within three days which indicates that they were excreting dead virus and therefore not infectious. The other may or may not have been a true infection but if so it is statistically most likely to have been quarantine acquired. We know of at least two quarantine acquired infections and one infection via an airport worker that have been picked up in the community.


There is no evidence which supports 21-day hotel quarantine for vaccinated PCR negative arrivals and the current policy is almost certainly increasing risks in both the short term, via quarantine infections and the long term by diverting priorities from the single most important public health intervention, which should be increasing vaccination rates in the old and vulnerable.

There is no evidence to support the quarantining of contacts of individuals who contact Covid overseas within the artificial, none evidence-based, 21-day window especially when there is no current community transmission in Hong Kong.

There is no evidence to support routinely keeping asymptomatic infected individuals in hospital for 21 days, recently suggested as one of the criteria for opening the border. There is actually no evidence for keeping asymptomatic people, especially children with positive stool samples, in hospital for protracted periods regardless.

In all of the situations above, in addition to the separation of infected children and parents, neither the scientific evidence nor the principle of proportionality, justify the compulsory loss of liberty or agency. This runs the risk of harm to the individuals involved and damage to the reputation of the Hong Kong Public Health system.

Hong Kong has some of the world’s leading virologists and Public Health experts. Some of the most respected research in the field of Covid has originated in Hong Kong. Current public policy is ignoring this evidence.

This issue is discussed further in a recent podcast with Professor Ben Cowling, Division Head of Epidemiology and Biostatistics in the School of Public Health at Hong Kong University.



1. Guan WJ; Ni ZY; Hu Y; Liang WH; Ou CQ; He JX; Liu L; Shan H; Lei CL; Hui DSC; Du B; Li LJ; Zeng G; Yuen KY; Chen RC; Tang CL; Wang T; Chen PY; Xiang J; Li SY; Wang JL; Liang ZJ; Peng YX; Wei L; Liu Y; Hu YH; Peng P; Wang JM; Liu JY; Chen Z; Li G; Zheng ZJ; Qiu SQ; Luo J; Ye CJ; Zhu SY; Zhon. (n.d.). Clinical Characteristics of Coronavirus Disease 2019 in China. The New England Journal of Medicine. Retrieved October 7, 2021, from

2. Kang, M., Xin, H., Yuan, J., Ali, S. T., Liang, Z., Zhang, J., Hu, T., Lau, E. H. Y., Zhang, Y., Zhang, M., Cowling, B. J., Li, Y., & Wu, P. (2021, January 1). Transmission Dynamics and epidemiological characteristics of Delta variant infections in China. medRxiv. Retrieved October 7, 2021, from

3. Perera RAPM; Tso E; Tsang OTY; Tsang DNC; Fung K; Leung YWY; Chin AWH; Chu DKW; Cheng SMS; Poon LLM; Chuang VWM; Peiris M; (n.d.). SARS-CoV-2 Virus Culture and Subgenomic RNA for Respiratory Specimens from Patients with Mild Coronavirus Disease. Emerging infectious diseases. Retrieved October 7, 2021, from

4. Singanayagam A; Patel M; Charlett A; Lopez Bernal J; Saliba V; Ellis J; Ladhani S; Zambon M; Gopal R; (n.d.). Duration of infectiousness and correlation with RT-PCR cycle threshold values in cases of COVID-19, England, January to May 2020. National Library of Medicine. Retrieved October 7, 2021, from

Topics: COVID-19

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