CORONAVIRUS OP-ED: Why don’t we know how many people have been infected with Covid-19 in South Africa? By Max Price
. The severity of our response and the economic costs we are willing to bear would be quite different compared to the currently anticipated total of 800,000 deaths.
The fourth implication is on the achievement of threshold herd immunity. This is the proportion of the population that needs to become immune before the epidemic slows down and eventually fizzles out. In South Africa, with modest physical distancing and no lockdown, it is assumed that this will be about 60%, or 36 million. In the absence of a vaccine, this occurs only through infection.
A good quality test would aim to have a specificity of about 99%. This means that if you test 100 people who havehad the Covid-19 infection, and therefore do not have antibodies, the test will give the correct result 99 times, but may incorrectly classify one of the 100 as positive. There may be many reasons for this.
But clearly that would be wrong. Even if we had a brilliant test with a 99.9% specificity, we would still find six false positives in our sample of 6,000, and we would incorrectly conclude that instead of 10,000 cases, there were actually 60,000. There areNote that the error rate is much less significant as the epidemic takes off.
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