How a 45% decrease in testing and lack of data over the holidays have obscured COVID-19 transmission rates

Over the last two weeks in British Columbia, a number of factors have made it difficult for people tracking the COVID-19 pandemic to make firm determinations about whether transmission of the virus is declining.

"I think we'll just have to wait for another week or so to see," said Jens Von Bergmann, a Vancouver data scientist.

This is largely due to a decline in testing of around 45 per cent — from a daily average of around 12,000 in late November and early December to around 6,500 a day since Dec. 20. 

It means that while the average number of daily cases has declined by 13 per cent since Dec. 20, the positivity rate has risen to its highest average in the course of the pandemic. At the same time, hospitalizations have risen slightly.

"It's really hard to put a lot of certainty into anything," said Von Bergmann.

Here's what we know about the decrease in testing, and when we'll be able to have a better sense of where B.C. stands.

Why the testing decrease?

According to Daniel Coombs, a UBC mathematician who has worked with the province on its pandemic modelling, the decline in testing can be attributed to three main factors.

The first is a sharp decrease over the holidays in testing by private organizations — which usually account for 20 to 30 per cent of overall tests — much of which is due to film productions going on pause.

The second factor is some people choosing not to get tested when they otherwise would due to personal holiday considerations — a point emphasized by Chief Health Officer Dr. Bonnie Henry last week.

But the third factor has less to do with the holidays, and more to do with the trend line B.C. was on before they began.

"Since a month ago, there's been a decline in the number of cases before testing went down," said Coombs. 

"Not massive, but it has been going downward. And less cases means less contact tracing required, less testing of asymptomatic people."

Coombs said the one exception is in Northern Health, where cases rapidly increased before the holiday period, and the positivity rate has been above 17 per cent in recent days. 

More information Friday

Both Coombs and von Bergmann said it was difficult to know what percentage of people who typically would have gotten tested over the holidays would have been positive. 

"We could build better models that also account for holidays … but I don't think anyone has done this, "said Bergmann. 

In addition, since Dec. 18, the B.C. Centre for Disease Control has not issued its weekly situation report, which provides additional data, including the public testing positivity rate.

Its next report will be released on Friday, two weeks after Christmas. That information will come at the same time positive tests based on transmissions happening at Christmas gatherings will be fully known.

Those who track the numbers are hopeful that additional information will provide additional clarity. 

"If you called me in two days, I would feel more comfortable," said Coombs.

"But right now … it's just a little too early to say."