We Are Still Not Doing Enough Testing: A Case Study of New York and Liverpool Schools

The Wall Street Journal is reporting that “New York City on Wednesday will close public schools and nonessential businesses in parts of Brooklyn and Queens that have registered a week-long spike in coronavirus cases”

Let’s look at New York and then compare to a UK city, Liverpool.

Cases are high in some New York boroughs. Up to 216 cases per 100,000 per week. But school closures are also being implemented in areas with 89 cases per 100,000 (source: New York Times)

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Source: New York Times
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Source: New York Times

Let’s compare with Liverpool. Here is the latest @PHE_uk report. Liverpool has cases of 238 cases per 100,000 in a week. Which is slightly higher than the highest rate ZIP code in NYC.

But remember, Liverpool’s figures are for the whole local authority.

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Source: PHE Week 40 Report


Let’s dig a little deeper into Liverpool. Here’s the map. We can see some areas with incidence in excess of 1200 cases per 100,000. That’s very high. And don’t forget this is detected cases. The number of cases will be much higher.

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Source: PHE Week 40

But how do we know that there hasn’t been enough testing? We look at positivity. Positivity is the number of people who test positive divided by the number of people tested. And this is what NYC uses to determine whether schools should be closed.

If an area of NYC has positivity greater than 3% – three in every 100 tests being positive – then schools close. What does positivity tell us? Whether enough tests are being performed.

“the World Health Organization recommended in May that the percent positive remain below 5% for at least two weeks before governments consider reopening.” (Johns Hopkins University)

So, given that positivity is set at a threshold of 3% for school closures in NYC and WHO suggest 5% before reopening, this begs the question – What is the positivity in Liverpool?

Just under 15%, according to the latest published data (PHE week 40 reporting). Which means that around 15% of all tests in Liverpool come back positive. That’s *very high*. And means not enough testing is being carried out. And this is a problem.

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Source: PHE Week 40

This is just an example of a city with large positivity. Extra testing capacity has been sent to Liverpool presumably due to students returning to universities there. This is not a Liverpool problem – it’s a national problem.

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