Is the 21-day Ebola quarantine enough to protect SE Asia?
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This week, Singapore announced that people from the three West African countries caught in the Ebola outbreak must apply for entry visas if they wish to visit. This is part of Singapore's screening process at air, land and sea checkpoints.
But supposing a person infected with Ebola makes it to our shores anyway. Say she is symptom-free when she lands. The screening won’t pick her out, and she will go anywhere she pleases, coming into contact with many people. What then?
Hopefully, local doctors and nurses maintain a high level of alertness, so that she is quickly diagnosed when she presents herself for treatment after symptoms do develop, and is then isolated meticulously for treatment.
What happens next is that all those who came into contact with her would have to be tracked and quarantined at home for 21 days, the incubation period before symptoms appear.
The authorities assert that Ebola patients are not infectious until they have symptoms. This may be as soon as two days after exposure, typically eight to 10 days but certainly within 21 days of being infected, say authorities.
However, is it true that the incubation period is just 21 days. If it were actually longer, would ending the quarantine at 21 days potentially imperil more of us?
The 21-day figure was statistically determined but has not been verified with lab tests of infected patients before they developed symptoms.
In one study of Ebola outbreaks between 1976 and 2000, the mean incubation period was found to range from 3.35 days to 12.7 days.
In the current outbreak, the World Health Organisation estimates the mean incubation period to be 11.4 days, with the upper limit – at the 95 per cent confidence level – being 21 days.
But what is the 95 per cent confidence level that leads to the magic number of 21 days?
What it means is simply that statistics show that 95 per cent of cases develop symptoms within 21 days.
However, in a new study published in the respected journal PLOS Currents Outbreaks, Drexel University experts in the United States modelled all available data of Ebola outbreaks, and the 21-day period fell a little short.
The study recorded a 0.2 per cent to 12 per cent chance of symptoms appearing after 21 days.
At a higher confidence level of 99 per cent, the incubation period could well be as long as 31 days.
So why was the 95 per cent confidence level – and thus 21 days – chosen?
The reason was that it struck a balance between the social and economic benefits of quarantine to society as a whole versus the social and economic costs of quarantine to individuals concerned.
But even if we accept 21 days, is it actually true that the patient cannot infect others if he is symptom-free? Has it been factually established that the patient does not shed Ebola virus in her body fluids, including perspiration, while she has no symptoms? The answer is not known.
There is no published research in which scientists measured the Ebola virus load in infected persons before they developed symptoms.
The data that we do have is that of blood levels of the virus load in Ebola patients on the very first day they have symptoms, when experts say they can infect others.
At this point, the virus levels are very high, suggesting there would have also been significant virus loads near the end of the symptom-free period.
If so, virus shedding may well occur in the last few days of incubation.
If this is correct, then the official position that patients are not infectious during the incubation period is an approximation to the truth to calm the public.
At its October 5 press briefing about the first person – a nurse – to catch Ebola on US soil, the Centres for Disease Control and Prevention asserted that there was “zero chance” of catching Ebola from an asymptomatic patient.
This assertion may have calmed the public but it was based only on anecdote, not scientific fact. It would have been more circumspect to say there was “very low risk”.
The Drexel study questions the science of stopping at 21 days, now accepted as public policy.
Of course, extending that official quarantine period would impose an extremely onerous burden on suspected Ebola patients and those who come into contact with them.
Quarantines minimise the risk of harm to everyone whereas those quarantined shoulder nearly all the economic, social and mental costs.
But extraordinary circumstances call for extraordinary sacrifices. So if quarantines under Ebola vigilance need to be longer than 21 days for us to be more than very sure, then maybe that is what we will need to do.
Let’s just hope we never have a case on these shores and never have to quarantine any contacts.
