The number of people being tested for COVID-19 nosedived in late December in several provinces, raising concerns that cases are going undetected and that people may have put others at risk by attending holiday gatherings when they had the virus.
And while the lower test numbers have corresponded with falling case numbers in several provinces, including B.C. and Alberta, the number of cases in Ontario and Quebec has continued to rise.
Test numbers dropped significantly around Christmas in B.C., Alberta, Ontario and Quebec, with public-health officials saying some people may have chosen to put off getting a test until after the holidays.
In Ontario, testing hit a low of just over 34,000 tests on Dec. 29 and averaged just under 40,000 for that week – well below the average of about 60,000 tests per day a week earlier.
Testing also dropped off in Quebec, which reported fewer than 20,000 tests on Boxing Day, compared with an average of about 35,000 tests per day the week before.
In Alberta, on Dec. 26, the province identified 459 new cases after completing only 6,866 tests – the lowest daily testing number since July. The percentage of people who tested positive in the past week has fluctuated between 6.4 and 9.5 per cent.
Alberta’s Chief Medical Officer of Health, Deena Hinshaw, has pointed to the low testing rates over the holidays to explain in part why the province’s new infections dropped considerably in the days following Christmas.
“We know that over the holidays, fewer people will likely want to go to a testing centre or perhaps they may delay for a day or two, wait until the holidays are over,” Dr. Hinshaw said at a briefing this past week.
Lynora Saxinger, an infectious-diseases specialist at the University of Alberta, said the relatively high positivity rates suggest cases are being missed because tests are not being done.
“The community testing numbers are down from the holidays, so there will probably be a bit of a catch-up phenomenon with more testing and more results coming out,” she said.
“[The recent positivity rates] suggest that we are undertesting for the number of cases. We’re just getting the upper part of the iceberg,” she added.
Still, Dr. Saxinger said it’s difficult to know how many cases are being missed simply by looking at the positivity rates.
“The people who have the greatest urgency to test are the ones who are probably coming through [for tests]. But it’s still not reassuring because we would expect that, with a full complement of testing, there would have been additional cases identified, additional cases isolated, and additional cases who wouldn’t necessarily be transmitting,” she said.
In B.C., too, fewer people were tested in late December.
On Dec. 26 in B.C., the province reported 3,793 tests – compared with an average of 11,000 tests the previous week. Its average positivity rate for the week ended Dec. 30 was 8.8 per cent.
The positivity rate is one of several metrics used to track both the spread of COVID-19 and the adequacy of the testing system.
A high positivity rate can either mean that infection rates are high or that there are too few tests being conducted. A high rate suggests there could be a significant proportion of infections that are not being detected.
The World Health Organization recommends keeping the positivity rate below 5 per cent.
Across the province, the number of people going for tests had been down by as much as 50 per cent in recent weeks, Provincial Health Officer Bonnie Henry said at a Dec. 29 press briefing that covered the five preceding days in the province.
Some people may have put off a test because they didn’t want to miss out on holiday gatherings, she suggested.
“And partly it’s people don’t want to be tested and have to isolate before the holiday, which is worrisome because we know that people are getting together, some people, and even if it’s just your household, that you may bring this into your household and spread it to them,” Dr. Henry said.
Both the number of tests and the number of cases are expected to rise this month, as people exposed over the Christmas period begin to show symptoms after the incubation period, which is between five to seven days.
Sign up for the Coronavirus Update newsletter to read the day’s essential coronavirus news, features and explainers written by Globe reporters and editors.