Could Rt number be a key indicator of whether we need return to lockdown mode?

Could Rt number be a key indicator of whether we need return to lockdown mode?

Parts of Canada are seeing surges in COVID-19 cases, leading scientists to wonder whether those regions need to shut back down to avoid further outbreaks.

Daily cases have tripled in B.C. over the last couple weeks, while a recent spike in Alberta is also causing alarm. Ontario, meanwhile, saw its daily case count inch up over 200 on Tuesday, before scaling back down to 103 by Thursday.

“You can either look at it and say: is this the first wave with a really long tail, or is this truly a second wave?” said Dr. Lynora Saxinger, an infectious disease expert at the University of Alberta.

“I think no matter what you call it, when I look at the numbers, I’m concerned.”

While provinces primarily look at daily case counts to indicate when to close or re-open regions, some scientists believe another metric — the effective reproduction rate of the virus — can also offer insight.

The reproduction rate, or R0 is the number of people one infected person is expected to infect. Without any safety precautions in place, the novel coronavirus’s R0 is between 2 and 4. H1N1 has an R0 of 1.5, while measles, which is highly contagious, is thought to be around 18.

The effective reproduction number however — known as Rt — changes over time and is dependent on our own behaviour like physical distancing and wearing face coverings, says Ryan Imgrund, a biostatician who calculates daily Rt values for Ontario.

“If the value is two, that means one person is accountable for two secondary infections,” said Imgrund, who calculated Ontario’s Rt at 1.16 on Thursday. “We really want to get that value under one in order to lower our numbers.”

Saxinger described the effective reproduction number as a way of determining when community spread is ”getting out of a controlled phase,” and said Rt value in Alberta, which was around 1.46 on Thursday, was troubling.

“If it’s consistently above one, you can become quite concerned that you’re not controlling things adequately,” Saxinger said.

“Certainly in some segments of the population (in Alberta) the number is high enough that I think measures likely are going to be required to turn things around again.”

Rt values have been used in Europe to determine when it’s safe to re-open, and Imgrund says they can also determine when more restrictions are needed.

He said a prolonged Rt value above 1.3 is “really, really troubling,” and one month at that rate would mean daily cases “increasing tenfold.”

“It would mean we would need to change things,” he said. “It’s all about keeping it under one.”

Imgrund tweets out his Rt calculations for Ontario daily, breaking down the regions of the province in graphs to show where the values are high (shown in red), moderate (yellow) and relatively low (green).

Different scientists use different methods to calculate Rt, in what Imgrund calls a “complex” process, but Saxinger says most of those methods result in the same number range.

There are downsides to Rt, however.

Experts say the value on its own doesn’t tell us everything, and the number can swing wildly depending on how many cases are in a community to begin with.

That happened last month in Kingston following a nail salon outbreak where 27 people became infected. Imgrund calculated the Rt for the area at 8.24 on June 27. The number dropped to 4.23 the following day and 2.20 five days later.

“If there are very few cases (to begin with) you can double or triple your case number very quickly and that will lead to a very, very high number for the Rt value,” Saxinger said. “And the Rt (wouldn’t) necessarily reflect all that much.

“So it’s best to look at it with things like the actual current number of active cases and the total number of cases that you’re seeing in the community.”

Imgrund says people also need to understand the confidence interval, which he includes in his charts, when exploring Rt values. Confidence intervals measure the degree of certainty or uncertainty in the method of calculation.

Imgrund said calculating Rt for larger areas will give you a smaller confidence interval, which can translate to a more accurate value.

“But if the community is very small, there’ll be a lot more fluctuation,” he added.

A high Rt value with a confidence interval close to 1 could mean an area is experiencing a high degree of community spread, Imgrund said, and localized closures and rolling back reopenings could be a solution to temper that.

Stephen Hoption Cann, a clinical professor at UBC’s School of Population and Public Health, believes Rt can’t offer a clear picture of transmission on its own.

An Rt around 1 won’t have the same implications everywhere, he said, pointing to the situation in the COVID hot spot of Florida, which had a Rt of 0.99 as of Thursday despite its high number of daily cases, according to the website Rt.live.

“Thirty cases in B.C. with an Rt of 1 means they’ll transmit to another 30 people. But if you have 10,000 cases a day, that means they’re going to transmit to another 10,000 cases,” he said. “So it’s not as reassuring in that setting.

“You also have to look at new cases and how many appeared for every hundred people you test. And yeah, if it looks like it’s going to explode, more precautions may need to be put in place.”

B.C. announced restrictions to bars and restaurants Wednesday amid the spike in cases in the province.

Saxinger expects that to be common across the country as case counts continue to ebb and flow.

“I think a lot of people just focused on flattening the curve, and the message seemed to be that once you got to the end of that hump that it was OK,” she said. “But what we actually are seeing is that as soon as places start to relax restrictions, it just starts to come up again.

“And it’s going to do that until we have the vaccine in place…. I would completely expect waxing and waning public health measures will be necessary for at least a year, possibly longer.”

This report by The Canadian Press was first published July 24, 2020.

Melissa Couto Zuber, The Canadian Press

Coronavirus

Get local stories you won't find anywhere else right to your inbox.
Sign up here

Just Posted

2020 Ponoka business awards
Ponoka chamber 2020 Business Award winners

The Ponoka and District Chamber of Commerce 2020 Business Awards were held… Continue reading

Ryen Williams, 11, with a lost miniature horse at JJ Collett Oct. 23. Photo by Don Williams
UPDATE: Owner found

Father and son found miniature horse while out for a walk at JJ Collett

Alberta has 3,651 active cases of COVID-19. (File photo)
432 new COVID cases sets another record Friday

Central zone holds steady at 126 active cases

(Emily Jaycox/Ponoka News)
Ponoka FCSS’ Empty Bowls sells out

For the first time ever, Ponoka Family and Community Support Services’ (FCSS’s)… Continue reading

"We are looking seriously at the spread and determining what our next steps should be," says Alberta’s chief medical officer of health, Dr. Deena Hinshaw, as the daily number of COVID-19 cases continues to climb.
427 new COVID cases is highest in Alberta ever

Central zone has 126 active cases of COVID-19

B.C. Liberal leader Andrew Wilkinson, B.C. NDP leader John Horgan and B.C. Greens leader Sonia Furstenau. (Black Press Media)
VIDEO: One day until B.C. voters go to the polls in snap election defined by pandemic

NDP Leader John Horgan’s decision to call an election comes more than a year ahead of schedule and during a pandemic

Comedic actor Seth Rogen, right, and business partner Evan Goldberg pose in this undated handout photo. When actor Seth Rogen was growing up and smoking cannabis in Vancouver, he recalls there was a constant cloud of shame around the substance that still lingers. Rogen is determined to change that. (Maarten de Boer ohoto)
Seth Rogen talks about fighting cannabis stigma, why pot should be as accepted as beer

‘I smoke weed all day and every day and have for 20 years’

Leader of the Opposition Erin O’Toole rises during Question Period in the House of Commons Thursday October 22, 2020 in Ottawa. THE CANADIAN PRESS/Adrian Wyld
O’Toole tells Alberta UCP AGM Liberals were ‘late and confused’ on COVID response

He says Alberta Premier Jason Kenney has taken charge and not waited to make things happen

Alberta Premier Jason Kenney arrives for an announcement at a news conference in Calgary, Alta., Tuesday, Sept. 15, 2020. THE CANADIAN PRESS/Todd Korol
Inquiry into oil and gas foes to deliver report next year: Kenney

A lawsuit filed by environmental law firm Ecojustice argues the inquiry is politically motivated

The Canadian border is pictured at the Peace Arch Canada/USA border crossing in Surrey, B.C. Friday, March 20, 2020. More than 4.6 million people have arrived in Canada since the border closed last March and fewer than one-quarter of them were ordered to quarantine while the rest were deemed “essential” and exempted from quarantining. THE CANADIAN PRESS/Jonathan Hayward
Majority of international travellers since March deemed ‘essential’, avoid quarantine

As of Oct. 20, 3.5 million travellers had been deemed essential, and another 1.1 million were considered non-essential

This photo provided by Air Force Reserve shows a sky view of Hurricane Epsilon taken by Air Force Reserve hurricane hunter team over the Atlantic Ocean taken Wednesday, Oct. 21, 2020.   Epsilon’s maximum sustained winds have dropped slightly as it prepares to sideswipe Bermuda on a path over the Atlantic Ocean.  The National Hurricane Center says it should come close enough Thursday, Oct. 22, evening to merit a tropical storm warning for the island.  (Air Force Reserve via AP)
Hurricane Epsilon expected to remain offshore but will push waves at Atlantic Canada

Epsilon is not expected to have any real impact on land

A voter places her absentee ballot in the ballot box, Wednesday, Oct. 7, 2020, at Merrill Auditorium in Portland, Maine. THE CANADIAN PRESS/AP/Robert F. Bukaty
American voters living in Canada increasingly being counted in presidential race

The largest number of Canadian-based American voters cast their ballots in New York and California

A composite image of three photographs shows BC NDP Leader John Horgan, left, in Coquitlam, B.C., on Sept. 25, 2020; BC Green Party Leader Sonia Furstenau, centre, in Victoria on Sept. 24, 2020; and BC Liberal Party Leader Andrew Wilkinson Pitt Meadows, B.C., on Sept. 24, 2020. THE CANADIAN PRESS/Darryl Dyck, Chad Hipolito
British Columbia votes in snap election called during COVID-19 pandemic

NDP Leader John Horgan called the snap election one year before the fixed voting date

Nunavut's provincial flag flies on a flag pole in Ottawa, on Tuesday June 30, 2020. The annual report from Nunavut's representative for children and youth says "complacency and a lack of accountability" in the territory's public service means basic information about young people needing services isn’t tracked. THE CANADIAN PRESS/Adrian Wyld
Nunavut’s young people ‘should be expecting more’ from government services: advocate

‘The majority of information we requested is not tracked or was not provided by departments’

Most Read