why some cities get ravaged by covid-19 while others don't

by stuart thomsoncanada’s covid-19 outbreak has so far been mostly confined to its two largest cities, with two-thirds of confirmed cases in toronto and montreal.the question that consumes policymakers is whether that trend will continue or if the virus will someday sweep into the rest of the country with the same ferocity. the answer will sketch the outline of canadian life in the months ahead.new research suggests, though, that certain cities are far more susceptible to the virus than others. the findings could be crucial for public health officials planning for a possible resurgence of the virus.“i think we can safely say that certain drivers, such as a dense way of living, as well as age structure, in particular, the share of the population living in nursing homes, will strongly continue to predict worse outcomes,” said klaus desmet, an economist at southern methodist university, texas, in an interview with the national post.for months, experts have been tracking the capricious path of the new coronavirus. it left an appalling trail of destruction in italy, killing nearly 35,000 people, and then hit germany with a glancing blow and a quarter of the deaths that italy experienced. new york city suffered one of the worst outbreaks in the world while los angeles suffered a death toll one-sixth the size.in canada, greater toronto and montreal account for more than 66,000 of the roughly 100,000 cases, while the rest of canada accounts for the rest. desmet says it’s likely that canada’s largest two cities fell victim to the same characteristics as new york city.“what we find is that large metropolis are vulnerable mainly for two reasons. first, they are places with a dense way of life. second, in large urban areas it is very hard to create a disease-free bubble because there is always inflow and outflow,” said desmet.by isolating each variable, desmet and romain wacziarg, an economist at the university of california, los angeles, have found that a city’s density is a huge factor, along with the age of the population and its distance to an international airport. most importantly, the researchers believe that these characteristics are likely to define the shape of the outbreak in the future.the evidence so far suggests the severity of the disease will not equal out across cities, but remain uneven and with some cities grappling with persistent outbreaks while others remain relatively calm.if this trend continues, it means officials can concentrate resources in areas with the highest susceptibility without fearing that other areas will be left vulnerable to their own outbreak. it also means certain places can have less stringent lockdowns and earlier reopenings without the risk of overwhelming the health care systems.

density and public transportation

the most obvious guess about what drives the severity of an outbreak is density. anyone skimming the list of cities particularly affected by covid-19 will see a list of north america’s most dense cities.“this should come as no surprise: as with any other infectious disease, contact between susceptible and infected individuals is a key determinant of the spread of the disease,” reads a study from desmet and wacziarg on the differences in covid-19 outbreaks across the united states.the study considers several features of density, including the share of individuals living in multi-unit housing structures and the number of people per household. another factor measures the amount of density a random person in the city or county will encounter in a square kilometre.the researchers found multi-unit housing and the size of households are both positively associated with cases and deaths, and they expect this to be a feature of any future outbreak.the research also found that places with more people using public transit have much worse covid-19 outbreaks, particularly in terms of deaths.“any epidemiological model of infectious disease will tell you that one of the most important drivers of the spread of the disease is the infection rate. obviously that infection rate depends on how many other people an infected individual bumps into every day (and) that’ll be much larger if i use the metro than if i live on an isolated farm on the countryside,” said desmet.

airports

prime minister justin trudeau announced in march that international flights would only be arriving in vancouver, calgary, toronto and montreal, which provided laboratory conditions for measuring the effects of international travel on the pandemic.in the united states, the researchers also found a clear correlation between a city’s distance to an international airport and an early outbreak of covid-19.“there is a randomness in locations that got the virus early. one important exception is the variable capturing distance to international airports with connections to the top-5 covid incidence countries as of march 15,” the study reports.in toronto, the higher infection rates clustered around the airport. peel region, which is home to pearson international airport, has a rate of 328 cases per 100,000 people. that’s one of the highest rates in the province, second only to the city of toronto itself, which has 394 cases per 100,000 people.this could also be what drove some small differences in case numbers in alberta. in calgary, where the airport remained open to international flights, the infection rates surged ahead of edmonton, where the airport is smaller, farther away from the city and closed to international travel.it’s almost certain that cities close to international airports get infected earlier than others, but desmet said it’s less clear whether they will be worse off in the long run. the evidence does seem to be pointing in that direction, he said.

age of population

interestingly, the researchers found that if a county has more people over the age of 75, it is likely to have fewer cases. this is likely because younger people are more likely to take part in activities that spread the virus, like large social gatherings and events.“places with a large share of retired individuals may feature fewer places (bars, stadiums) where the disease spreads rapidly,” the study says.and although an older population tends to be associated with lower transmission, in some situations it can lead to more deaths. this could also be driven by outbreaks at nursing homes, which have seen horrifying death rates.“this finding is consistent with the idea that once a county is affected by the pandemic, its nursing homes can quickly become powderkegs, and account for larger shares of countywide deaths.”

other factors

the researchers found that some widely hypothesized risk factors for covid-19 may not be that important. for example, the share of smokers and obese people does not seem to be a big driver of high mortality rates. even the quality of health care infrastructure seems to be much less important than other factors like the density of the city and the age of the population.although a person’s level of education itself seems to have little bearing on the severity of an outbreak, poverty and inequality seem to be big drivers. poverty has a strong bearing on the mortality rate and inequality correlates with more cases of covid-19.in the united states, race also matters a great deal. outbreaks are more severe in areas with more black and hispanic people, both in terms of the number of cases and deaths.“overall, these results confirm concerns that the covid-19 pandemic has a disparate effect on various racial groups,” the paper reads.all of these trends are important, but they aren’t the final story. for instance, the fact that new jersey is right beside new york is far more influential to its outbreak potential than any state-specific characteristics. likewise, the fact that hawaii is a remote island is a much better predictor than any other characteristic.the study also emphasizes one of the most vexing features of a virus outbreak, that so many of the activities that make for a happy life now put people at an increased risk.“taken together, being connected to many other people makes you vulnerable. ironically, what is very beneficial in normal times for the economy, makes you vulnerable in these abnormal times,” said desmet.• email: sxthomson@postmedia.com | twitter: stuartxthomsondon’t miss the latest on covid-19, reopening and life. subscribe to healthing’s daily newsletter coming out of covid.