Positive Case Rates

Case number in San Diego County are on the rise, but so are testing numbers.

Testing is useful in tracking the spread of Covid-19 in San Diego County, but case numbers alone give us an incomplete picture.

How do we know if Covid-19 is spreading faster, or if we are just detecting more cases?

It’s easy to understand that more testing will find more cases. At the start of the outbreaks in San Diego, with tests in short supply, testing was limited to people with certain risk factors, including travel to areas with widespread outbreaks or exposure to someone who had tested positive. As you would expect, a relatively high percentage of those early tests came back positive.

As tests became more widely available, the county tested more people. And more people with Covid-19 were identified. But as testing rates went up, the percentage of positive cases dropped.

Evidence suggests that many people who acquire Covid-19 have no symptoms, or symptoms so mild, they do not realize they have Covid-19. So maybe we are just catching those folks now that we are testing more people.

So if case numbers go up when testing numbers go up, can we really know if cases are rising more quickly?

YES! We can.

This is why the county keeps track of how many tests come back positive. The percentage of positive tests is a key indicator in how fast the virus is spreading.

There are 3 charts below. The top one shows the number of Covid-19 tests reported each day. The middle chart shows the number of new cases reported each day. Note that each chart axis has a different scale, relative to the data it is displaying. This makes it easier to see the trends for each set of data. The two lines on the bottom chart use the same scale.

See here for the latest charts: Positive Case Rate

On average, testing has been steadily rising since mid April. But new cases were generally hovering between 100-200 cases until mid June.

On the bottom chart, the blue line plots the percentage of daily reported tests that were positive. The orange line plots the county’s calculated 14 day rolling average of positive cases. Since the percent positive jumps up and down day to day, the rolling average helps to see the general trend balanced out from the results in a 2 week period.

As testing levels rose, the percent of tests that came back positive came down. And for a period in May and early June, the percent of positive cases hovered around 2-3%. The fact that the percent positive was relatively stable despite more positive cases implies that rising cases were a result of more testing.

However, since June, although testing rates are still higher, the percent that come back positive is rising. The daily reported percent positive has not been below 3% since June 22. At first, it could have just been a variation in testing. Which is why the orange line is important. The rolling average of the percent of tests that come back positive is trending up, even when taking 2 weeks of testing results into account.

Case numbers rise when testing levels rise. But when the percentage of tests that are positive is rising, we know that the virus is spreading faster than before.