Our glorious leader seems to have inherited from her predecessor the knack of coming up with a response to a question which is not only unsatisfactory, but almost completely irrelevant and deeply offensive.

Consider her response last week when asked to respond to the refusal of some medical experts to take the “universal but voluntary” Covid-19 test, which the government is offering in September.

Carrie Lam. Photo: GovHK.

Note that none of them actually said that people should not take the test. They said, for a variety of reasons, that they were not themselves going to volunteer. The reasons were technical and medical. No political overtones intruded.

But for Ms Lam these days, if the political overtones are not inserted by someone else, then they will be inferred by her. Ms Lam said she could not figure out why some experts were not keen on the test. They were politically motivated, she said.

This followed: “The so-called experts, doctors or members of the public kept finding excuses to stop citizens from participating in the test… There is only one intention behind this: political calculation. They are smearing the central [Beijing] government and it’s an effort to sever Hong Kong’s relations with the central government.”

Photo: May James/HKFP.

“So-called experts” is an ill-chosen phrase in this context. The people concerned are all doctors and many of them are indisputably experts in relevant areas. None of them is, as far as we know, part of any political movement and this is not, actually, a political matter.

In fact if anyone in this controversy is politically motivated it appears to be Ms Lam herself. The idea that a mass testing programme cannot be organised in Hong Kong without the central government’s help is not very convincing. Is the purpose of bringing Beijing into it to improve the service, or to make a political point: that our motherland loves us even if it has vandalised our constitution and perverted our police force?

Let us see if we can “figure out” what the problems might be in rolling out a mass testing programme for a condition which is, in all likelihood, still quite rare. They are not political; they are statistical.

We start with the assumption that the test is 99 percent accurate in detecting the disease in people who have it. This is an ambitious target but maybe the mainland genius team can manage it.

Government personnel collect sample for coronavirus testing. Photo: GovHK.

To get a high detection rate, though, you have to accept that there will be some “false positives“ — people who fail the test but who do not, actually, have the disease. The only way to reduce the number of false positives is to make the test less sensitive and reduce the detection rate. So let’s not go there. Say the false positive rate is three percent, which is a really good figure for a mass testing programme.

The other interesting variable is how many people in the population we are testing actually have the disease. So far, Hong Kong has seen less than 5,000 cases in a population of seven million, which suggests a possible infection rate of 0.02 per cent. But let us suppose there are a lot of undetected cases out there and put the infection rate at one percent.

Now we test 1,000 people and consider the outcome. In that group, assuming it is typical, there will be ten people with the disease, and the test will detect nine or ten of  them. The undetected person, if there is one, will go home reassured and breathe the virus all over his friends and family members.

Photo: Kaiser/HKFP.

There will also be the unavoidable 30 false positives: people who are told they have the disease but actually do not. This is a distressing moment for them, and a dangerous moment for us. It is easy to suppose that someone who tests positive in a 99 percent accurate test has a 99 percent chance of having the virus.

But this is an elementary error. We have 39 or 40 people who tested positive, only nine or ten of whom have the virus, and we still do not know which nine, or ten, are the genuine cases. The chance of you having the virus if you are one of this unfortunate group are actually only one in four.

But there will be a temptation to put them all on a bus together and send them off to a quarantine centre.  And if you have not been exposed to the virus when you get on the bus you may well have been exposed to it by the time you get off.

This is the sort of calculation which makes doctors wary of testing whole populations for diseases which very few people have. A positive test is distressing and disturbing. It may lead to mental health problems, disruption to work, or recriminations in the family.  It is also quite likely to lead to unnecessary medical procedures.

Photo: GovHK.

The rarer the disease, the worse the problem. If the figure for Covid-19 cases so far is typical, and the actual incidence is 0.02 percent, then our 1,000 tests will produce only one or two positives. And the 30 false positives will still be there to muddy the waters.

Ms Lam’s hysterical resort to political explanations for criticism is disappointing, if only because we have all become used to the daily performances of Dr Chuang Shuk-wan, the head of the Communicable Diseases Branch of the Centre for Health Protection.

Dr Chuang deploys a soft, melodious voice and a reassuring bedside manner which makes even the worst news sound like a manageable ailment. Her response to a similar question about mass testing was that it would be useful to find the “silent carriers” who were still among us, because some of the cases being detected were not traceable to known clusters.

Clearly, there is an interesting technical issue here on which well-intentioned experts could disagree with each other: Is it better to try to test everyone, or does it make more sense to concentrate on the most likely groups? Ms Lam needs to dial down the political paranoia and listen more carefully to people who know what they are talking about. We do not need a Hong Kong Ms Trump.

HKFP is an impartial platform & does not necessarily share the views of opinion writers or advertisers. HKFP presents a diversity of views & regularly invites figures across the political spectrum to write for us.

Tim Hamlett

Tim Hamlett came to Hong Kong in 1980 to work for the Hong Kong Standard and has contributed to, or worked for, most of Hong Kong's English-language media outlets, notably as the editor of the Standard's award-winning investigative team, as a columnist in the SCMP and as a presenter of RTHK's Mediawatch. In 1988 he became a full-time journalism teacher. Since officially retiring nine years ago, he has concentrated on music, dance, blogging and a very time-consuming dog.