Enlarge /. WUHAN, February 5, 2020 – Patients infected with the novel coronavirus are seen in a makeshift hospital rebuilt from an exhibition center in Wuhan, central China's Hubei Province. Such containment methods can help countries bring the pandemic under control.
On Monday, the COVID-19 response team at Imperial College London released a report describing its efforts to model how various control efforts could change the pathway of the coronavirus pandemic in the United States and the United Kingdom. The report is published by a publication of the World Health Organization, but has been made available to the governments of these countries, where it has been reported to influence their reactions.
The report paints a bleak picture of millions of deaths in both countries if no action is taken as infections far exceed the capacity of their hospital systems to treat patients. However, it turns out that aggressive steps can hold the virus back until an effective vaccine can be developed, although this will mean that many aspects of society will be shut down for more than a year.
Before this causes panic, however, it is important to emphasize that these results are based on a model that is imperfect due to the incomplete information we currently have and that must be based on a number of assumptions. We'll go through all of these limitations before delving into the conclusions the Imperial College team has drawn from the model.
Epidemiology is the study of diseases at the population level. Instruments in this area include epidemiological models that are designed to show how infectious diseases can spread within a population. To understand this study, one has to understand how the model deals with virtual infections.
In this study, the model goes back to the level of the individual. They are distributed in one country – in this case the United Kingdom or the United States – based on high-resolution population data. Contacts with other people that allow the spread of an infectious agent are made in different places: within a household, at school, at work and in social environments such as shopping. The researchers used census data to determine typical household sizes and the age distribution of the local population. Average class size data was used to edit contacts in school, while job size and average commuting distances were used to model contacts at work.
Overall, this is an incredibly detailed model and has been used for peer-reviewed publications.
If the population is present, you need to add the corona virus. Here the researchers deal with incomplete information, since all of our data on the virus is necessarily incomplete. Although we know that each infected individual tends to infect about two others, we do not have an accurate measure of their infectivity, and even a small difference in this value will increase if it is extended to a national level.
While the authors are doing their best to base their assumptions on real data, all values describing the behavior of the coronavirus are uncertain. They used an incubation period of five days and assumed that the transmission would start 12 hours before the onset of symptoms. They also assumed things like the percentage of cases in the hospital that would require intensive care facilities (30 percent), the overall mortality rate (half of the intensive care cases, biased towards the elderly, etc.), giving the values they used the limited information again We have reported on the spread of the pandemic so far, but the irregular testing in most countries means almost certainly that some of them will fail.
From the beginning of January, the authors modeled infectious persons who entered the population for both the USA and Great Britain. The number of new arrivals doubled every five days when the pandemic broke out worldwide. The total number of new arrivals was calibrated using mortality data from both countries.
Using a pandemic working model in the UK and US, the researchers were able to try out different scenarios, from nothing to an increasingly draconian combination of boundaries for population movements. These are roughly divided into two categories. One of them is containment, an attempt to slow the spread so that the country's health system is not overwhelmed before previously infected people begin to guarantee herd immunity. And the second is suppression, which uses different forms of isolation to lower the transmission rate to the point where every infected person has less than a low chance of passing it on.
However, there is no fixed boundary between these categories. School closures would be an independent remedy, but can be part of a set of measures to try to suppress the virus. Overall, the researchers consider combinations of the following methods:
- Case isolation: infected people stay at home for a week.
- Quarantine at home: All household members of an infected person stay at home for two weeks.
- Social distancing of older people: Over 70s reduce social contacts (75 percent compliance assumed).
- Social distancing of all: Everyone except the students reduce social contacts (75 percent compliance is accepted).
- School closure: 75 percent of schools are closed.
Some of them have mixed effects. For example, social distancing efforts are believed to increase the number of transmission options within a household. And school closings are believed to increase contact between people in a neighborhood, as children may play more together. This is not entirely realistic – compliance with social distance would obviously improve if news of the collapse of the hospital system emerged – but it is not an inappropriate number for a model.
So what happens?
Bad things if nothing is done. In the United Kingdom, the epidemic peaked in mid-May, reaching levels well beyond the country's intensive care infrastructure. Around 22 people die per 100,000 people a day. That's about 14,000 people who die daily for a few weeks, with half a million deaths. In the United States, a younger and more diffuse population means that the peak is reached a little later and does not rise as high per capita, but the larger population means that over 2 million people end up dead.
Note that this does not include the additional deaths that are caused by people who need critical care for reasons other than the coronavirus.
Enlarge /. While the capacity of intensive care remains firm, most mediation methods allow enough cases to far exceed it.
When it comes to intensive care facilities, most mitigation strategies don't work very well. While everyone is flattening the curve to varying degrees, the closure of schools or the isolation of infected people still leaves hundreds of thousands of people in need of hospitalization without a place to go. Even a combination of measures (case isolation, quarantine at home and social distancing of older people) completely floods the medical system.
General social isolation is needed to really make an impact in the UK or US. In combination with household quarantine and case isolation, the need for hospital beds for intensive care will be exceeded for several months, but the surplus will only be minor and can be managed with emergency measures. The combination of school closings, case isolation, and general social distancing keeps peak infections low enough to ensure that intensive care capacity is not exceeded, and problems will gradually diminish after the peak in April.
The problem here is that the virus circulates in small amounts in the population in this scenario. The author's model, which lifted the restrictions in September, and until October seems to be a high point that competes with that caused by the fact that nothing is done at first. (Although less effective forms of suppression have a lower peak because so many people have been infected in the past few months.)
The impact of different constraints on cases compared to intensive care capacity in the UK.
The impact of two approaches to curbing infections in the United States. The less intrusive case enables cases that often exceed hospital capacity but reduce the recurrence of the disease after the restrictions are lifted.
The results alone suggest that the best way to minimize mortality is to maintain social distancing indefinitely until a vaccine is developed that provides adequate protection for herd immunity.
let's wait for it
The biggest test of this model is that some countries that have been aggressive with testing and suppression efforts have successfully limited the spread of the coronavirus and are beginning to lift their restrictions. If the authors' model is correct, countries such as China and South Korea are at risk of recurrence.
The researchers also see the possibility that the initial suppression strategy will give these countries a long-term advantage. With increased vigilance and a well-developed testing and isolation infrastructure, they should be able to identify newly arrived cases and situations in which the virus continues to circulate in the background. By tracking contacts and aggressive testing, it should be possible to keep the corona virus in check, a process that South Korea is now trying to try.
While the US and Britain have missed the boat to prevent the pandemic from developing in their countries, it is still possible that they will not have to take aggressive suppressive measures until a vaccine is developed. If the South Korean efforts are successful, they can provide a model for how the constraints on suppression can be relaxed. And add a new wrinkle to all epidemiological models of this coronavirus outbreak.
Correction: The derivation of one of the percentages has been clarified.