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14 Nov 2021  (726 Views) 
Covid-19 crisis

Flaw in computing the degree of protection provided by vaccine
This article claims that the vaccine prevents 90% of covid deaths, based on a large scale study in Scotland. 

There could be a flaw in the calculation. 

Let me explain.

Suppose there are 1 million people in a population of which 800,000 (80%) are vaccinated (vax) and 200,000 (20%) are not vaccinated (unvax). 

Suppose we have  400 deaths from the vax and 1,000 death from the unvax. 

We first compute the death rate from the unvax as 1000/200,000 = .005.

If we apply this rate to the vax, we should get .005X800,000 = 4,000 deaths. 

The actual deaths is 400, so we say that the vaccine reduces the deaths by (4,000-400)/4,000 = 90%.

Here is the flaw in the computation.

We are assuming that the profile of the unvax is the same as the vax. 

This is a fallicious assumption. 

It is likely that a high proportion of the unvax are people with co-mobilities who are likely to avoid the vaccine because they fear the potential serious side effects. 

Even if there was no covid infection, the death rate from the second group (i.e. unvax) would still be higher, due to their profile. 

I do not know if the difference in the health profiles between the vax and unvax groups have been adjusted for in the above study. There was no mention that it had been adjusted. 

Based on the figures in Singapore, which showed a similar difference between the death rates for the vax and unvax groups. there was no adjustment for the difference in the age or health profile.

I believe that the vaccine does provide a good degree of protection against covid, but not the the extent of 90%. The actual figure could be much lower.

My view is that the vaccine does help to a fairly significant extent, but we should not over rely on it. 

Tan Kin Lian

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