Q&A: Testing and tracing - Figuring out what the numbers mean

When it comes to testing and tracing, there’s no shortage of numbers.

Some 1.54 million tests, 258,230 contact tracing calls; calls one, two and three; average numbers of close contacts; means, medians and turnaround times. Hundreds of new staff and hundreds of millions of euro.

The contact tracing system throws off reams of data, which can be confusing and disorientating. Health systems researchers from UCD have said that 80 per cent of contacts should be quarantined within four days of a person developing symptoms. But how close are we to this goal?

What is an end-to-end turnaround time?
This is one of the key metrics. Somewhat confusingly, it has been used interchangeably to describe different things: the time taken from when a person is referred for a test to when they get the result of the test, but also from the time of referral to the completion of contact tracing.

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Really, the latter is a better measurement. Testing and tracing is designed not just to tell you who is sick, but to allow you to intercept their contacts and stop them circulating. Earlier this week, Dr Tony Holohan said about 10 per cent of close contacts get infected, so if 100 infected people have three close contacts each, that could mean about 30 people have contracted the disease and are circulating. The logic of test and trace is to track those people down before they pass it on.

This has to be done quickly; there’s no point in contacting someone after they’re no longer infectious, and if you do it too late, the damage may be done.

And how are we doing on this?
It is hard to get a single, universal figure for this. The HSE publishes times for separate parts of the process, but says they cannot simply be added together to get a picture of the entire turnaround time.

The overall figure can also be flattered by the parts of the process that move quickly. For instance, hospital tests are generally quicker than tests taken at community sampling centres like Croke Park. So, for example, it takes a median of 1.4 days for someone to get their result when Covid is not detected; but for community samples, that figure is 2.1 days. When you include contact tracing, the median figure for both is three days.

The community side of test and trace has proved more susceptible to slowing down. In the middle of October, as contact tracing came under more pressure, the figure was 4.1 days.

What don't these figures tell us?
The use of medians to describe the data, much like the inclusion of fast-track hospital testing streams, can obscure important information. So, for example, HSE statistics released to The Irish Times show that during the surge of case diagnoses which led to 2,000 people being asked to do their own contact tracing, more than half of positive cases (53.2 per cent) took more than four days to complete the whole process.

Over the last bank holiday weekend, this improved, but 45.7 per cent of positive cases are still taking more than 72 hours to complete contact tracing. We also don’t have a clear idea how long it takes between the onset of symptoms and referral for a test.

Dr Kevin Kelleher, assistant national director of the HSE, estimates it as between 12 and 24 hours. In June, Nphet said 47 per cent of people were taking longer than four days from the onset of symptoms to being swabbed.

So, we don’t know precisely how close we are to achieving the target outlined by the UCD academics. Even if we take the rosiest assumptions about how long it takes for someone to be referred for a test after developing symptoms, the data suggest that at the current high level of cases, the system would struggle to get 80 per cent of contacts to isolate within four days.