January 25, 2010
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<The call for rationality>
This is what Thomas Tsang Ho-Fai said today :
“衛生防護中心表示，打流感針後宮內夭折個案為2宗，而懷孕不足24周打針後自然流產就有4宗。根據國際經驗，注射疫苗與胎死腹中沒有關係。根據本地及外地經驗，注射疫苗後出現夭折個案一般在千之次三至千分之五，而本港現時為千分之二，如有關數字升至千分之六及千分之七，才達警戒線，需加強警惕。
他強調，每5名孕婦中，就有1人自然流產，而本港每年有7萬宗懷孕個案，所以每日有數十宗自然流產。”
It is very difficult to explain to the difference between causality and correlation :
— Causality has a “if A then B” relationship. A causes B
— Correlation has a “A-and-B-just-coexist-coincidentally” relationship (technically, there could be no relationship whatsoever)Put in this context, the public should have perceived the 4 cases of natural miscarriage (自然流產; this English translation might not be correct) is caused by the injection of vaccination.
But (in very crude and explicit terms) what Tsang attempted to point out is that, regardless of the injection, some pregnant women were going have natural miscarriage. The injection has nothing to with the miscarriage, and the events are mere coincidence or correlation.
I think a better way to present the objective data (not necessarily a success in convincing the adamant general public) is :
First, calculated the number of daily natural miscarriage and as a share of total pregnant women over a long horizon. This should give you 38 (70000*20%/365) and 20%.Then you calculate the number of daily natural miscarriage for injected women and as a share of total pregnant and injected women. You should get 4 and a certain percentage (according to Tsang, 0.2%)By the way, since 20% is the average miscarriage rate, won’t be the 0.2% a too low number? Out of a total 100 pregnant women, 20 will have natural miscarriage regardless of the injection; but now we are told that the average miscarriage rate amongst injected pregnant women is only 0.2% (a reduction of 19.8% pt).
Reportedly, CHP is going to release more statistics later today. Let’s wait and see.
By the way, Thomas Tsang has very very good public image : professional, humble, trustworthy. He is one of the very few public officials that I have a good impression for many years.
Comments (4)
Tsang’s gentleness and dimples contribute quite a lot to his public image~
He was my boss – a great guy to work with. What you mentioned is actually the science of epidemiology and I guess they are working on that in the back offices (using SPSS
) .
Seems like 自然流產 = miscarriage = spontaneous abortion.
The cases Dr Tsang talked about are 宮內夭折, i.e. stillbirths, which are different from miscarriage.
According to some CHP figures (Box 2, p.101) released last month, the background rates of spontaneous abortion and stillbirth between 2003-2008 are respectively 5% and 0.3%, so:
1) that “每5名孕婦中，就有1人自然流產” is perhaps a mistake of Dr Tsang;
2) the current prevalence of stillbirth among those vaccinated pregnant women by no means alarming.
my excolleagues like your post
just in case you have time – introduce a new tool to you. this would expand your ability to analyze health incidents / policies. it’s called the ODDS RATIO – it is often used to approximate the relative risk (the ratio that you suggested) of getting a disease when the groups are very different sizes, or the relative risk itself cannot be calculated – they approach each other when probabilities are small.
odds ratio is the measure that is more often used in public health. this also allows groups with and without injections to be compared.
http://www.childrens-mercy.org/stats/journal/oddsratio.asp
this link will provide you with more info.