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The Math Behind Social Distancing

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The math behind social distancing

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The Math Behind Social Distancing

As we wait for scientists and healthcare professionals to develop a vaccine for COVID-19, there is another, more readily available tool at our disposal.

Social distancing, defined as measures taken to reduce physical contact, is the first line of defense for containing an infectious disease like COVID-19. That’s because these infections spread when people cough, sneeze, or touch surfaces on which the virus resides.

To help us grasp the impact these measures can actually have, today’s infographic illustrates how a reduction in social exposure can theoretically contain the spread of infection.

Theoretical Potential

The calculations used to create today’s infographic come from Signer Laboratory, a stem cell research lab located in the Moores Cancer Center at the University of California San Diego.

Using a summation formula makes it possible to estimate the number of new infections over a 30 day period, across three scenarios.

Scenario5 Day Period30 Day Period
No social distancing practiced1 person infects 2.5* others406 people infected as a result
50% reduction in social exposure1 person infects 1.25* others15 people infected as a result
75% reduction in social exposure1 person infects 0.625* others2.5 people infected as a result

*For estimations only. It is not possible to infect only a fraction of another person.

To arrive at the figures reported above, Robert A.J. Signer, Ph.D., and his team made a number of key assumptions.

First, they estimated the basic reproduction number (R0) of COVID-19 to be 2.5, a figure supported by recent research. This means that, on average, an infected individual will spread the disease to 2.5 other people.

Next, they assumed that an infected individual will unknowingly spread COVID-19 over the median five day incubation period. After this period, the individual will begin to develop symptoms, immediately self quarantine, and no longer pose a threat.

Finally, they assumed a direct linear correlation between social interactions and R0. This means that when an infected person reduces their physical contact with others by 50%, they also spread the disease by an amount 50% less.

Timing is Everything

While the figures above are the results of mathematical estimations, researchers have actually studied social distancing from a variety of angles.

One study used simulations to determine the magnitude and timing of social distancing measures required to prevent a pandemic. The distancing measures simulated were:

MeasureDetails
School closureTeachers and students spent weekday daytime cycles at home, rather than at school.
Increased case isolationUpon becoming symptomatic, adults (90%) and children (100%) would self quarantine for the duration of the infection.
Workplace non-attendanceEach day, a person had a 50% chance of staying home instead of attending their workplace.
Community contact reductionIndividuals reduced their physical contact with community members by half, each day.
Combination of all fourAll four measures combined. 

The results, for a community of 30,000 people and an epidemic with R=2.5, are charted below. We can define the final illness attack rate as the share of people from an at risk population who ultimately catch the disease.

power of early social distancing

Results showed that when no action was taken, 65% of the population contracted the disease. However, if a combination of all four distancing measures were implemented instead, the attack rates were:

  • 45% (distancing begins after a 4 week delay)
  • 21% (distancing begins after a 3 week delay)
  • 7% (distancing begins after a 2 week delay)

What’s clear is that social distancing was significantly more effective when implemented with minimal delay—the final illness attack rate rose quickest beyond the third week. These findings draw a parallel to the visualizations in today’s infographic, which showed us just how quickly a disease can spread.

Social distancing interventions are important as they represent the only … measure guaranteed to be available against a novel strain of influenza in the early phases of a pandemic.

Kelso, J.K., Milne, G.J. & Kelly, H., BMC Public Health 9, 117 (2009)

We arrive at a similar conclusion when it comes to the types of distancing measures implemented. In the simulations, none of the four measures taken on their own were able to have a similar effect as when they were combined.

We All Have a Part to Play

With the global number of COVID-19 cases still rising, many governments have issued quarantine orders and travel bans.

The math supports these decisions—reducing our physical contact with others, even when we aren’t experiencing any symptoms, is crucial. Studies like the one summarized above also prove that taking action sooner, rather than later, can go a long way in reducing the spread of infection.

The key takeaway from all of this? Social distancing is a powerful disease control tool, but only if we all participate.

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Business

Pandemic Recovery: Have North American Downtowns Bounced Back?

All North American downtowns are facing a sluggish recovery, but some are still seeing more than 80% less foot traffic than pre-pandemic times

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Pandemic Recovery: Have Downtowns Bounced Back?

As we continue on our journey towards recovery from the impacts of the pandemic, North American offices that sat empty for months have started to welcome back in-person workers.

This small step towards normalcy has sparked questions around the future of office life—will office culture eventually bounce back to pre-pandemic levels, or is remote work here to stay?

It’s impossible to predict the future, but one way to gauge the current state of office life is by looking at foot traffic across city centers in North America. This graphic measures just that, using data from Avison Young.

Change in Downtown Office Traffic

According to the data, which measures foot traffic in major office buildings in 23 different metropolitan hubs across North America, remains drastically below pre-pandemic levels.

Across all major cities included in the index, average weekday visitor volume has fallen by 73.7% since the early months of 2020. Here’s a look at each individual city’s change in foot traffic, from March 2, 2020 to Oct 11, 2021:

CityCountryChange in Foot Traffic
Austin🇺🇸-51.70%
Calgary🇨🇦-54.50%
Boston🇺🇸-54.90%
New York🇺🇸-60.50%
San Francisco🇺🇸-60.80%
Edmonton🇨🇦-62.20%
Houston🇺🇸-67.90%
Chicago🇺🇸-68.10%
Vancouver🇨🇦-68.20%
Los Angeles🇺🇸-68.60%
Philadelphia🇺🇸-69.00%
Washington, DC🇺🇸-69.40%
San Francisco Peninsula🇺🇸-70.00%
Denver🇺🇸-73.50%
Nashville🇺🇸-75.60%
East Bay/Oakland🇺🇸-76.10%
Atlanta🇺🇸-77.50%
Dallas🇺🇸-79.80%
Montreal🇨🇦-80.30%
Toronto🇨🇦-81.20%
Miami🇺🇸-82.20%
Silicon Valley🇺🇸-82.60%
Ottawa🇨🇦-87.70%

The Canadian city of Calgary is a somewhat unique case. On one hand, foot traffic has bounced back stronger than many other downtowns across North America. On the other hand, the city has one of the highest commercial vacancy rates in North America, and there are existential questions about what comes next for the city.

Interestingly, a number of cities with a high proportion of tech jobs, such as Austin, Boston, and San Francisco bounced back the strongest post-pandemic. Of course, there is one noteworthy exception to that rule.

A Tale of Two Cities

Silicon Valley has experienced one of the most significant drops in foot traffic, at -82.6%. Tech as an industry has seen one of the largest increases in remote work, as Bay Area workers look to escape high commuter traffic and high living expenses. A recent survey found that 53% of tech workers in the region said they are considering moving, with housing costs being the primary reason most respondents cited.

Meanwhile, in a very different part of North America, another city is experienced a sluggish rebound in foot traffic, but for very different reasons. Ottawa, Canada’s capital, is facing empty streets and struggling small businesses that rely on the droves of government workers that used to commute to downtown offices. Unlike Silicon Valley, where tech workers are taking advantage of flexible work options, many federal workers in Ottawa are still working from home without a clear plan on returning to the workplace.

It’s also worth noting that these two cities are home to a lot of single-occupant office buildings, which is a focus of this data set.

Some Businesses Remain Hopeful

Despite a slow return to office life, some employers are snapping up commercial office space in preparation for a potential mass return to the office.

Back in March 2021, Google announced it was planning to spend over $7 billion on U.S. office space and data centers. The tech giant held true to its promise—in September, Google purchased a Manhattan commercial building for $2.1 billion.

Other tech companies like Alphabet and Facebook have also been growing their office spaces throughout the pandemic. In August 2021, Amazon leased new office space in six major U.S. cities, and in September 2020, Facebook bought a 400,000 square foot complex in Bellevue, Washington.

Will More Employees Return or Stay Remote?

It’s important to note that we’re still in the midst of pandemic recovery, which means the jury’s still out on what our post-pandemic world will look like.

Will different cities and industries eventually recover in different ways, or are we approaching the realities of “new normal” foot traffic in North American city centers?

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Healthcare

How Does the COVID Delta Variant Compare?

How does the COVID-19 Delta variant compare with the original disease? Here are the key differences according to consolidated studies.

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How Does the COVID Delta Variant Compare Social

How Does the COVID Delta Variant Compare?

In late 2020, a variant of COVID-19 was detected in India that began to quickly spread.

Soon after it received the label “Delta,” it started to become the predominant strain of COVID-19 in countries of transmission. It spread faster than both the original disease and other variants, including “Alpha” that had taken hold in the UK.

Now the COVID-19 pandemic has essentially become the Delta pandemic, as the variant accounts for more than 90% of global cases.

But how does the COVID-19 Delta variant differ from the original disease? We consolidated studies as of September 2021 to highlight key differences between COVID-19 and the dominant variant. Sources include the CDC, Yale Medicine, and the University of California.

COVID-19 vs COVID-19 Delta Variant

At first glance, infections caused by the Delta variant are similar to the original COVID-19 disease. Symptoms reported from patients include cough, fever, headache, and a loss of smell.

But studies showed that the difference was in how quickly and severely patients got sick:

  • Spread rate: How quickly the infection spreads in a community (based on the R0 or basic reproductive number). The Delta variant spread 125% faster than the original disease, making it potentially as infectious as chickenpox.
  • Viral load: How much of a virus is detectable in an infected person’s blood, with higher loads correlating with more severe infections. Delta infections had a 1000x higher viral load.
  • Virus detectable: How long after exposure a virus is detectable in an infected person’s blood. Delta infections were found to be detectable four days after exposure, faster than the original disease (six days).
  • Infectious period: How long an infected person has the capability to pass on the virus to other people, from the first time they were exposed. Delta infections were contagious for longer than traditional COVID-19 infections, at 18 days compared to 13 days.
  • Risk of hospitalization: How much more or less likely is an infection going to require hospitalization for treatment? Infections caused by the Delta variant were twice as likely to cause hospitalization compared to the original disease.

One other important finding from studies was that the existing COVID-19 vaccines helped against Delta infections.

The CDC found that approved vaccines reduced the rate of infection by 5x and the rate of hospitalization by 29x in a breakthrough case. They also found that overall efficacy against infection can wane over time, however, and at-risk people might require a booster vaccine.

What About Other COVID-19 Variants?

Delta is just one of many COVID-19 variants tracked by health officials, but it’s the one we know the most about.

That’s because reliable statistics and information on diseases requires thousands of cases for comparisons. We know a lot about Delta (and the once-dominant UK strain Alpha) because of how widespread they became, but there haven’t been enough cases of other variants to reliably assess differences.

As of September 2021, WHO was tracking 20 COVID-19 variants around the world with different classifications based on potential severity:

  • 14 Variants under monitoring (VUM): Variants that are deemed to not pose a major global health risk, or no longer pose one.
  • 2 Variants of Interest (VOI): Variants that affect transmissibility, virulence, mutation, and other virus characteristics, and are spreading in clusters.
  • 4 Variants of Concern (VOC): Have similar characteristics to VOI but are further associated with a global risk.

Most of the current variants of interest and concern were first identified and labeled in late 2020, though 2021 variants are showing up as well.

LabelDesignationDocumented OriginEarliest Identified Date
AlphaVariant of ConcernUKSeptember 2020
BetaVariant of ConcernSouth AfricaMay 2020
GammaVariant of ConcernBrazilNovember 2020
DeltaVariant of ConcernIndiaOctober 2020
LambdaVariant of InterestPeruDecember 2020
MuVariant of InterestColombiaJanuary 2021

Should you be worried about all of these variants? For the most part, a lack of cases to provide clear information also reflects that they’re equivalent to or weaker than traditional COVID-19 infections.

But it’s important to note that our understanding of diseases and variants becomes more nuanced and accurate over time. As research continues over a longer timeline and over a wider database of cases, expect information on COVID-19 variants (and any disease) to become more concrete.

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