There are many things we would like to know about the novel coronavirus but don’t.
How many cases are out there? Are we testing enough? How could we tell if we were testing enough?
Epidemiologists say looking at the ratio of positive tests to deaths gives us about as good an idea as we’re likely to get.
In general, the higher the number of tests in relation to each death, the better the surveillance is, and the better the chance that it’s finding people who are asymptomatic. The lower the number, the better the chance that only a few of the most obvious cases are being found.
Positive tests per death are a measure of how thorough testing is in a place in relation to the size of the problem.
“Having more tests per death suggests that you are finding more of the less severe infections,” says Ashleigh Tuite, an epidemiologist at the University of Toronto. “It means that you probably have a better view of what is going on in terms of the amount of disease that’s in your population.
“The deaths are the tip of the iceberg.”
By that standard, some provinces are doing much better than others:
“If your percentage of tests per death is small — for example, Ontario is much smaller than Nova Scotia — that suggests that we are finding the more severe cases and not finding the less severe cases, which is consistent with what we know in terms of testing right now,” she says.
Internationally, Canada is doing better than some of the harder-hit countries in Europe, but not as well as countries that have tested on a much more ambitious scale, like South Korea or Iceland.
“If you look at a country comparison, the fact that Iceland is so much higher than the other countries, that suggests that they’re finding a lot more cases, probably doing better surveillance, and the other countries that are high there are countries that, looking at them from our perspective, seem to be doing a good job: South Korea and Australia,” Tuite says.
“In South Korea, we know that they are doing a lot of testing. Australia seems not to be having a lot of cases in general, but this suggests that they are finding the cases that they do have.”
Iceland, which has tested for the coronavirus far more extensively than any other country, may be showing us something close to the COVID-19‘s real death rate, something in the range of 0.5 per cent, Tuite says. The death rate for the coronavirus is hard to calculate without knowing the total number of people who have the virus, and many studies have shown that large numbers of people who test positive have no symptoms.
“How prevalent is this, how dangerous is this? It’s maddening, and this is the epidemiologist’s lament,” says Colin Furness of the University of Toronto. “You never know. You don’t know the denominator. You never know how many cases there are.
“Epidemiology is all about measurement and counting. That’s all it is. It’s massively frustrating that we can’t answer these really simple questions.”
Another number that’s often brought up is the “test positivity rate”: the percentage of tests that come back positive. The higher the percentage of negative tests, the better your surveillance is, more or less.
“When positivity is really high, it indicates that you’re missing a lot of infections,” Tuite says. “Really high positivity is a sign of a health system that’s in a bad place, or under stress.”
Test positivity numbers from some other Western countries are alarming. France, Spain, the Netherlands and the U.S. are all between 18 and 23 per cent. The U.K.’s is over 30 per cent.
New York City has an alarming test positivity rate of 55 per cent and some zip codes in the city have rates as high as 75 per cent.
“We would certainly like to see countries testing at the level of 10 negative tests to one positive, as a general benchmark of a system that’s doing enough testing to pick up all cases,” said World Health Organization official Maria Van Kerkhove on March 30.
Experts Global News talked to cautioned against comparing different jurisdictions’ positivity rates, because approaches to testing can be so different in different places.
Tuite doesn’t see Quebec’s 11 per cent level, Canada’s highest, as alarming.
“You want it to be lower in general, because it suggests that you’re screening a bit more broadly, but it’s not 50 per cent, it’s not 70 per cent,” she says.
“I don’t know that there’s necessarily a target number that you want. In general, lower is better. Eleven per cent doesn’t set off red flags, or saying that they’re doing something bad or missing something.”
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