What happens to the unknown cases in the NSW Covid outbreak?
Part of the original plan for easing restrictions in New South Wales relied on reducing the number of cases infectious in the community to “as close to zero as possible”.
However, on average about 68% of cases in the past week have been “under investigation”, which means it was unknown how much time they had spent in the community while infectious and how big a risk they posed to others.
A similar amount had not yet been classified as “linked” or “unlinked” – that is, whether the case can be traced to a known cluster or other case. About 70% of local cases on average have been reported as under investigation in the daily briefings in the past week:
This number has climbed over the course of the pandemic as contact tracers’ jobs are made harder by the rising case numbers.
However, NSW Health does provide an update to both figures as they resolve cases. So what do these updates tell us?
There have been at least 6,295 cases for which the source of transmission was unknown at the daily briefing since 1 July. In the more detailed case data that NSW Health publishes online, they often retrospectively update the source of these cases when possible.
This means that of the 6,295 unknowns, one quarter, or 1,573, were subsequently linked to a known source, as of 19 August.
For the isolation status of cases, NSW Health publishes an update on these in a weekly epidemiological report. There is however a lag of about two weeks, with the most recent report published on 20 August covering cases up to 7 August.
By comparing this data to the figures from daily briefings, which we have been tracking since 26 June, we can see how many of the ‘under investigation’ cases have been resolved, and how.
This shows that of the 1,149 cases under investigation, the majority either ended up as low risk (isolating) or high risk (active in the community).
NSW Health also provides a measure of how well contact tracers and testing labs are meeting their goals in its weekly epidemiological report.
There’s a two-week lag between the report’s publication date and the data it includes, but the latest reports show a decline in the number of people with Covid who have been interviewed by public health staff within one day of notification to NSW Health, and a decline in the speed in which testing labs are notifying NSW Health of new positive cases: