Researchers have been trying to figure out why large numbers of COVID-19 cases occur in some areas but only a few cases in others. Here in New Brunswick, we are getting about 1 new case a week (almost all travel related) while, next door in Quebec, there are over a thousand new cases daily. COVID-19 sometimes spreads to many people all at once and other times it is only passed to 1 or 2 people...or none at all. This is different than the flu, where it usually spreads evenly through a community and you’re almost guaranteed to catch it if you get too close to someone who has it (barring having been vaccinated, of course...got mine this week!). Researchers think this difference may be an important key to what we need to do to control this novel coronavirus.
You may have heard references to the “R0” measurement (pronounced “R-zero” or “R-naught”), sometimes also called the basic reproduction rate. This number indicates the average number of people who are infected by each new case of an infection. If each sick person infects 1 other person, the R value would be 1 and the number of cases would remain constant, with each case replacing itself. If less than 1 is infected on average, the number of cases will reduce over time; if more than 1, the number of cases will grow.
The R0 factor has been called a key factor in understanding what is happening with the pandemic, whether it is getting better or worse. But the problem with the R0 system, is that it is based on averages, while COVID-19 spreads mostly in batches at “super spreading” events, so it doesn’t show the true picture of what is happening.
But there is another type of measurement for disease spread that you may not have heard about: the “k” value, a measure of dispersion or how a disease spreads. Does it spread in a steady manner or in big bursts? The k factor, although complicated, captures viral behaviour that alternates between super infectious and noninfectious. Data collected in the first 9 months of the pandemic suggest that COVID-19 is an “overdispersed” pathogen, one that spreads in clusters. We need to take this into consideration when we assess the risk we will encounter from any given activity we are thinking of attending.
A study done in New Zealand found that only 19% of 277 separate introductions of virus resulted in more than one new case, while analysis of outbreaks in other countries have shown a single person to have infected as many as 5,000 known cases – that one occurred at a megachurch service in South Korea. The scientific term for this type of disease spread is “stochastic” – dominated by a small number of individuals and driven by super-spreading events. Even if the virus spread in a population is relatively under control, just a few super-spread events can turn that around quickly, as we are seeing in Quebec and Ontario right now.
The key to controlling spread
Multiple studies have suggested that as few as 10 to 20% of infected people may be responsible for 80 to 90% of transmission of COVID-19 through super spreading events. These include long-term care facilities, meat-packing plants, fish factories, cruise ships, family gatherings, house parties and night clubs. Researchers have realized that, if they want to get the pandemic under control, they need to prevent these super-spreading events that are driving the increase in cases. This is now believed to be the key to controlling the coronavirus.
This concept of unevenness is sometimes referred to as the 80/20 Rule or the Pareto Principle (named after economist, Vilfredo Pareto, who first noticed the phenomenon) , where approximately 80% of effects are often generated by about 20% of the causes. In addition to occurring in nature, we also see it in various human activities like 80% of business coming from 20% of customers or 80% of taxes being paid by 20% of citizens. If you’re interested in reading more about the 80/20 Rule, Wikipedia has a good explanation.
The three C’s of superspreading events…
So, what made events that resulted in many cases, different from similar gatherings that resulted in no new cases? Researchers found 3 factors in common, referred to as the 3 C’s:
Crowd – Many people at an event increase the chance that one will be carrying the virus. Holding an event in a community where the virus is less controlled also increases the chance someone who is highly infectious will be included in the group.
Close – Not being able to maintain a distance 2 meters or 6 feet from others increases the chance of viral spread, as we all know now. Prolonged contact is also known to increase the chance of the virus passing from person to person. It is believed that a contact time of 15 minutes or longer of contact increases the chance of passing on the virus.
Closed space – An indoor venue with poor ventilation allows tiny droplets containing viruses to hang in the air as an aerosol for much longer. Singing and loud talking (for example, talking over loud music in a bar) readily generate these aerosols, but normal talking and even just breathing also generate droplets. A mask of any kind, even a homemade cloth one, will trap these particles and reduce the chance of viral spread. Masks generally were not being worn at super-spreading events.
Backward contact tracing
Since this coronavirus spreads differently, we should be looking at contact tracing and testing differently, too, researchers say. Because only 20% of cases or less are responsible for most of the spread, it is important to identify those 20%. Statistically, It’s more likely that the person who gave the virus to a new case will be a super-spreader than the case at hand.
This suggests, especially if tests are limited, that it would be most efficient to trace the source of an infection and then test the source’s contacts, rather than try to test everyone’s contacts and not be able to keep up. If more than 1 or 2 cases are found amongst the contacts of a source, then there’s a good chance that a super-spreader may have been found. In lieu of testing all of the source’s contacts, isolation can simply be recommended when sufficient tests are not available. Tracing back to the source then checking their contacts is called “backward contact tracing”.
Using this information
This weekend is Thanksgiving in Canada, and many families will want to celebrate together. In some provinces, where the virus is spreading quickly and many new cases are being reported daily, governments are telling people to stay at home… and instead to share the holiday remotely, by video chat or phone call, instead.
But what about our “Atlantic bubble” where we have had very few new cases daily here in Atlantic Canada and no community spread for months? This week, we had an outbreak in a nursing home in a nearby city...15 cases there plus 2 family members who work at Costco and St. Hubert's restaurant, plus 3 other separate cases that were travel related and already in quarantine. They are still trying to identify the source of the nursing home outbreak (backward contact tracing!) and the extent of the spread, and have finally mandated that masks be worn in all public places in our province.
So, although it might seem to be fairly safe to gather in small groups, we can’t let our guard down. The pandemic is still at our doorstep and it's looking like the second wave is beginning its spread around the globe. We can’t forget that the virus is potentially everywhere…and we need to continually improve our systems and practices until we stop the spread and protect the most vulnerable.
References:
This Overlooked Variable Is the Key to the Pandemic – Medium.com
Stochasticity and heterogeneity in the transmission dynamics of SARS-CoV-2 – B.M.Althouse, et al, Institute for Disease Modeling
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