Today’s evacuation of about 180 passengers from the cruise ship Diamond Princess to serve another period of quarantine back in Australia has raised questions about the best way to control spread of the coronavirus.

The passengers had already spent 14 days quarantined on board the ship, which had been docked in Japan, and now face another 14 days at the Howard Springs quarantine facility close to Darwin.

Di Stephens, Northern Territory’s acting chief health officer, told the ABC today:

These people need to go into quarantine because we are not entirely convinced that the quarantine procedures on that ship were 100% effective.

By contrast, Japan’s health ministry is allowing hundreds of people to leave the ship without being subject to further quarantine.

So what’s behind Australia’s announcement to impose a second quarantine period? And what were conditions like on board to prompt this decision?

What’s quarantine?

Quarantines have been put in place around the world as part of the global public health response to COVID-19 – the disease caused by a new coronavirus, now named SARS-CoV-2.

The idea is to limit the spread of the virus within and between countries.

Formal measures designed to limit contact between infected (or potentially infected) people are called “social distancing”. And they have been used to control communicable diseases for at least 2,500 years.

Read more: Remote village to metropolis: how globalisation spreads infectious diseases

Today, the term quarantine refers to the separation or restriction of movement of people who are not ill but are believed to have been exposed to an infectious disease.

This differs to isolation, which is the term used for the separation or restriction of movement of people who are ill, thereby minimising onward transmission.

How long should quarantine last?

Quarantine periods are determined by certain characteristics of the infectious agent, most notably the incubation period. This is the period between being exposed to it and symptoms appearing.

For COVID-19, the average incubation period is thought to be around six days, and can range from two to 11 days.

While a preliminary report has suggested a longer incubation period of up to 24 days, this is considered unlikely.

Read more: How contagious is the Wuhan coronavirus and can you spread it before symptoms start?

People who have been in close contact with someone confirmed to have COVID-19 are considered to have been potentially exposed to the virus. As a precaution, these people are placed in quarantine, essentially to “sit out” their potential incubation period.

The quarantine period of 14 days currently being used in Australia and elsewhere for COVID-19 takes into account the maximum known incubation period for this disease, plus a few extra days as a reasonable precaution.

In quarantine, people will either develop the disease and have symptoms or they will remain well. In theory, if a person remains well after their period of quarantine, they are deemed uninfected and restrictions are lifted.

Another factor that influences how long someone needs to be quarantined is the infectious period. That’s the period during which the infection can be transmitted from one person to another.

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If the infectious period starts before the symptoms (from asymptomatic or minimally symptomatic individuals), the virus can be transmitted silently. This can substantially complicate disease prevention and control.

When a new virus emerges – as with SARS-CoV-2 – the infectious period is largely unknown. While the proportion of asymptomatic or minimally symptomatic COVID-19 cases is not clear, it is increasingly apparent people can be infected without having any symptoms. However, further evidence is needed to see if these people can infect others.

When is it best to extend the quarantine period?

Crucial to quarantine is ensuring that best possible infection control practices are put in place to prevent ongoing transmission.

It is also essential to assess real-time data about newly diagnosed cases, which tells us how effective quarantine measures have been.

In some circumstances, it may be necessary to extend a person’s period of quarantine, as in the case of the Australian citizens on board the cruise ship Diamond Princess.

Read more: Cruise ships can be floating petri dishes of gastro bugs. 6 ways to stay healthy at sea this summer

So, what happened on board the Diamond Princess?

Data from the World Health Organisation (WHO) give us clues to what’s behind Australia’s decision to impose a second period of quarantine.

The graph below shows there may have been up to four possible waves of infections on board, including an initial undetected wave before quarantine measures were imposed.

These data demonstrate ongoing SARS-CoV-2 transmission between people on board during the quarantine period. It also indicates breaches in infection control on board may have contributed to ongoing waves of infections, which an expert highlights in the video below.

Evidence of ongoing transmission during the quarantine period supports the decision by several countries to evacuate their citizens from the Diamond Princess, including Australia, to “reset the clock” and to impose a further 14-day quarantine period.

This additional measure – while causing considerable and understandable frustration to those affected – is designed to limit transmission of COVID-19 within Australia.

The rights of individuals versus public good

Implementing public health measures, such as isolation and quarantine, requires decision-making that balances the rights of individuals and public good.

When appropriately designed and implemented, quarantine and isolation work. Even when quarantine is not absolutely adhered to, it can still be effective at reducing the likelihood of large-scale outbreaks.

With SARS (severe acute respiratory syndrome), these strategies were thought to have been an important part in controlling the epidemic, though they were resource and labour intensive.

Read more: Yes, there's merit in quarantining people on Christmas Island to prevent the spread of coronavirus

The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.