Virus came to Quebec from Europe and U.S., study finds
Genome tracking of samples shows strains from popular destinations
The coronavirus pandemic that has infected more than 70,000 people in Quebec was brought into the province by as few as 247 travellers, mostly via Europe and the Americas.
The first case is believed to have entered Quebec City as early as Jan. 30 from someone travelling from the United Kingdom. But it was spring break travellers who initiated the major outbreak, new research suggests.
“Most of the early introductions of the virus into Quebec did not give rise to sustained transmission, but a barrage of introductions just after spring break eventually gave rise to the tens of thousands of cases we have seen since,” Dr. Jesse Shapiro, associate professor with Mcgill University's department of microbiology and immunology, said in a statement.
Shapiro was among a team of researchers from the Mcgill Genome Centre and the Institut national de santé publique du Québec (INSPQ) who announced initial results this week from their study into the genetic sequencing of the SARS-COV-2 genome, the virus responsible for the outbreak of COVID-19. Peer review of the study is still pending.
The findings indicate one-third of infections (32.7 per cent) came from Europe, with France having the highest number at 12.1 per cent. Another 31 per cent originated from the Caribbean and Latin America, and nearly a quarter (24 per cent) came from the United States. Very few came from Asia (1.2 per cent) and none came from China. The results correlate with popular spring break travel destinations for Quebecers.
To trace the pathway of the disease, a team of researchers extracted the genetic code of the virus from 734 swab samples of patients who tested positive in Quebec between February and April 1. Then, explained Dr. Carmen Lia Murall, research associate at the Université de Montréal and lead author of the study, scientists “compared the genetic code, which is like a string of letters across the virus, to look for similarities and differences.”
Viruses accumulate errors as they make copies of their genes during replication. Scientists use methods of evolutionary biology to basically rebuild a family tree of the virus, telling them which ones are more similar or distinct from each other, Murall said.
The sequences from quebec were compared with nearly 22,000 Canadian samples and a global repository of sequences from around the world, allowing them to pinpoint from where in the world different strains of the virus originated. Researchers also looked at the travel history of the patients swabbed.
“We could see in some cases you only need a few people to be transmitting once they get back, and not even realizing it because they are asymptomatic or pre-symptomatic,” Murall said. “That's why epidemics grow exponentially when they are not being controlled.”
Although initial indications show 247 travellers brought the virus in, Murall said the figure is probably larger because they were working with a small sample size.
In Quebec, the evidence collected showed most transmission of the virus came at the end of March break, backing up the theory that the province's earlier spring break might have increased its number of cases compared with the rest of Canada.
“This confirms the notion that COVID had spread much more widely than earlier believed and that by the early spring of 2020 it was much more than an endemic disease in China,” researchers wrote. “It is surprising to see how widely the virus had been dispersed at an early stage.”
Contrary to the first outbreak that was fuelled by travellers, Quebec's budding second wave appears to be fed by community transmissions, Murall said.
Scientists are continuing their work on the project, sometimes looking at local outbreaks at the request of health authorities to get a better understanding of how the virus is spreading. The findings allow policy-makers to better understand which containment measures are working and which aren't.
Genetic sequencing “makes the invisible visible,” Murall said. “You can actually look at it and see how it's connected and how it's changing.”
A barrage of introductions just after spring break eventually gave rise to the tens of thousands of cases we have seen since.