Canada Covid Positivity Metric July 30

The map shows that in Canada 8929 deaths have been attributed to Covid19, meaning people who died having tested positive for SARS CV2 virus.  This number accumulated over a period of 182 days starting January 31. The daily death rate reached a peak of 177 on May 6, 2020, and is down to 7 as of yesterday.  More details on this below, but first the summary picture. (Note: 2019 is the latest demographic report)

Canada Pop Ann Deaths Daily Deaths Risk per
2019 37589262 330786 906 0.8800%
Covid 2020 37589262 8929 49 0.0238%

Over the epidemic months, the average Covid daily death rate amounted to 5% of the All Causes death rate. During this time a Canadian had an average risk of 1 in 5000 of dying with SARS CV2 versus a 1 in 114 chance of dying regardless of that infection. As shown in a previous post, the risk varied greatly with age, much lower for younger, healthier people.

The Key Covid Metric

With easing of lockdowns and increased testing in many places, epidemiologists are focusing on a key metric to inform public policies: Positivity. The positivity metric is the rate (%) of people who test positive out all people sampled. The significance is that (by definition) a presumed case is a person who tests positive once. If a second test comes back positive it is a confirmed case. The metric is not perfect for two reasons.

The first problem is false positives from the testing procedure itself or from errors in the data processing and reporting. For this we have to hope that quality assurance protocols are being followed and mistakes corrected along the way.

The larger issue appeared in Florida recently when officials discovered that numerous batches of samples were reported 100% positive and other batches 100% negative. While the latter result is expected sometimes, all people testing positive seems unlikely. Behind this is the reality that in many situations (eg hospital ICU) a single patient will be tested many times with many positive results in the course of monitoring that individual’s clearing of the virus. Obviously a batch of samples from that ICU might legitimately be 100% positive.

But it is also true that 10 or 20 positive tests from one patient should not be reported as 10 or 20 new cases. In some jurisdictions, officials say they go to the effort to link test results to the individuals tested, and can distinguish between number of cases and number of positives. In other places, cases and positives may be the same number. Thus confirmed cases could be only 1/2 of the total positives, or less.

How is Canada Doing?

Because of some irregularities in national data reporting, this update is based on Ontario and Quebec statistics combined.  Together the two provinces account for 72% of national testing, 85% of cases and 95% of deaths after testing positive for Covid19.  Like many places, the Canadian contagion is not a pandemic, but rather a few hot spots within a largely untouched geography,  This post is reporting the two central provinces as representing the Canadian epidemic.  Quebec data is here:  Ontario data is here:

The line shows the Positivity metric for Canada starting at nearly 10% for new cases April 22, 2020.  That is, for the 7 day period ending April 22, there were a daily average of 13225 tests and 1283 new cases reported. Since then the rate of new cases has dropped down, now holding steady at ~1% for the last month. Yesterday, the daily average number of tests was 35,747 with 274 new cases. So despite 2.7 times the testing, the positivity rate is not climbing.

Another view of the data (all Canadian provinces) is shown below.

Note that increased testing has led to a slight bump in new cases, but the positivity rate is little changed.  Meanwhile, the death rate has continued to decline from a high of 177 on May 6 down to 7 deaths presently.

Meanwhile the national messaging focuses on rising cumulative case totals, ignoring evidence the contagion is winding down.

Canada Mask2

My bad. That is the Health Minister of PEI solemnly announcing in July their first Covid case in two months.

Background at previous post Canada Succeeds on Key Covid Metric

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