People play volleyball on a beach in Vancouver, while the number of new cases has tripled in British Columbia during the past two weeks.
July 25, 2020 10: 45 am
COVID-19: the growth of the index, Rt concerned experts in Canada
The canadian Press
Some regions in Canada are experiencing an increase in cases of COVID-19, leading scientists to wonder if these places should be confined again to avoid further outbreaks.
The number of new cases has tripled in British Columbia during the past two weeks. A recent spike has troubled the authorities in Alberta. In Quebec, the number of active cases has surpassed Thursday the threshold of 2000 for the first time since June 26.
“We can look at these data and wonder whether it is the long end of the first wave or the beginning of a second wave,” observes Dr. Lynora Saxinger, an infectious disease specialist at the University of Alberta. No matter as it is called, when I look at the numbers, I am concerned.”
The provinces looking primarily at the number of cases daily to determine whether they should close a region, but some scientists believe that another measure – the reproductive rate of effective virus – may also offer insight.
Known under the name of Rt, the value of this index is difficult to evaluate, because it may change with time and depends on social behaviors, such as distancing or physical port of a covers-face, ” says Ryan Imgrund, a biostatistician, which calculates the daily values of Rt for Ontario.
“If the value is two, this means that a person is responsible for two secondary infections, says it. We really want this value falls below 1, in order to reduce the number of cases.”
The Rt has high Thursday to 1.16 in Ontario.
Dr. Saxinger said that the reproductive number effective enables you to determine if a spread begins to no longer be controlled. According to it, the data in Alberta, where the Rt has reached 1,46 Thursday, are a cause for concern.
“If this value is consistently greater than one, we should worry about this, because it means that we do not control the spread of adequately, she argues. It is certain that, in some segments of the population, 1/8 in Alberta3/8 that the Rt is sufficiently high to ensure that measures are probably necessary to reverse the situation.”
The index Rt has been used in Europe to determine if it is safe to open a sector. Mr. Imgrund is believed that it also allows you to determine if you should impose restrictions.
He points out that a index Rt higher at 1.3 for a long time is “really, really disturbing”. At such a rate, the number of new cases could increase in a month.
“This means that we should then change things. The goal is to keep the index under 1.”
Different scientists use different methods to calculate the Rt. This process is “complex,” acknowledges Mr. Imgrund. Dr. Saxinger said, however, that the scientists arrive at similar values.
“I expect fully that public health measures, sometimes more restrictive, sometimes more flexible, are still necessary for at least a year, maybe more. ”
Dr. Lynora Saxinger
The Rt has its disadvantages.
According to experts, the index itself does not tell us everything. Its value can vary enormously according to the number of cases present in a community at the start.
So to Kingston, where an outbreak has erupted in a nail salon last month, affecting 27 people. Mr. Imgrund has calculated a Rt of 8.24 for the 27th of June in this area. The figure has fallen 4.23 the next day and 2.20 five days later.
“If there are very few cases (to start with), you can double or triple very quickly the value of the index. This will lead to a very, very high for the Rt, which does not give an accurate picture of the situation, warns Dr. Saxinger. Therefore, it is best to examine it with other elements such as the current number of active cases and the total number of cases that have been registered in a community.”
Stephen Hoption Cann, a professor in the School of Population and Public Health of the University of British Columbia, believes that the Rt may not provide a clear picture of the transmission.
A Rt of around 1 will not have the same implications everywhere, ” he stressed. He mentions that the one in Florida – where there has been a sharp upsurge in the number of cases, was that of 0.99, Thursday.
“If we had thirty cases in British Columbia with a Rt of 1, this means that they will transmit the virus to 30 people. However, if there are 10 000 per day, the transmission will affect 10,000 other people. This is not as reassuring, depending on the contexts,” says Mr. Cann.
Several provinces, including Quebec, have announced restrictions on bars because of the increase in the number of cases, especially in young adults.
Dr. Saxinger reports that the number of cases continues to fluctuate.
“A lot of people are focused only on the flattening of the yield curve. The message suggested that when we have crossed that hump, everything would be fine. That we see fit, it is as soon as one begins to loosen the restrictions, everything starts over.”
According to her, it will be as well as a vaccine will not be available.
“I expect fully that public health measures, sometimes more restrictive, sometimes more flexible, are still necessary for at least a year, maybe more.”