What the models predict about Covid-19

2020-05-22 19:26

A week ago, many were complaining about the lack of published models by government to explain the decision-making process around the Covid-19 coronavirus pandemic.

On Thursday, the department of health hosted a Zoom call in which more models were presented than could fit on a catwalk in Milan. Some models are useful.

If modellers carefully explain their assumptions and present multiple scenarios, we can get a better understanding of the pandemic’s possible trajectories, or the potential of different interventions to reduce the number of infections.

But models, especially because they are surrounded by fancy equations, can give a false sense of certainty. No one truly knows how the pandemic will play out.

In contrast to every other species, humans have an insatiable desire to know the future. But we can’t. We can only make educated guesses based on the limited information at our disposal, and when it comes to Covid-19, that information is still very limited.

Some common uncertainties that stand out in the models are the rate of asymptomatic infections, how infectious the virus is, how effective various interventions are, and the death rate. A number of the models suggest that South Africa will not have enough ICU beds for everyone who will need one.

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Estimates of Covid-19-related deaths range from a few thousands to several hundreds of thousands. Here is a brief summary of each model and a link to more information about it:Government’s primary model.

The South African Covid-19 modelling consortium mainly consists of researchers from Wits Cape Town and Stellenbosch universities, as well as the National Institute of Communicable Diseases.

It is the main model that the health department is using. The model makes projections under optimistic and pessimistic scenarios with epidemic peaks in August and July, respectively.

It projects six months into the future.Under the two scenarios, this model estimates between 34 000 and 50 000 deaths by November, although the modellers stress the significant uncertainty about these numbers. 

“These projections are subject to considerable uncertainty and variability. Estimates will change and improve as the epidemic progresses and new data become available.”

Covid-19 will require R26 billion to R32 billion over the next six months
Marcus Low and Nathan Geffen

Even in an optimistic scenario, ICU capacity is exceeded by many thousands of beds in August to September. While they do not factor in the additional deaths that this will cause, it might be considerable given that ICU bed capacity is about 3 300 and that the need for ICU beds may exceed 20 000 to 30 000 at the peak of the virus.

The modellers estimate that dealing with Covid-19 will require R26 billion to R32 billion over the next six months, of which R10 billion to R15 billion will accrue to the national health department. 

South Africa’s health budget for this year is about R230 billion, and the national department has allocated R20 billion to Covid-19. Note that government was originally using a very simple, but possibly no less prescient, model developed by some members of this consortium, that we described here.

Actuarial Society of SAOn April 29, the Actuarial Society of SA warned that as many as 48 000 people could die of Covid-19 in the next four months if “government does not maintain a strict approach to flattening the curve”.

It also projected that the need for ICU beds may peak at 10 000, significantly more than the estimated 3 300 capacity. The society is open about the significant uncertainty in its model and has indicated that it is being updated, with new projections to be shared soon. 

(This PowerPoint presentation of the ASSA model, dated 21 May, may indeed be an update, but we’re struggling to read the graphs.)

Cannot predict the future

A model [] by Alex van Den Heever of the Wits School of Governance looks at strategies for keeping the epidemic suppressed, or, to put it technically, to get R below 1 (see this explanation of R).

His model is not concerned with trying to predict the future. Some of the interventions he considers are testing and contact tracing, lockdown, closing borders, social distancing and stringent workplace protocols.

He also considers the effect of cold and warm weather (other coronaviruses transmit a bit less efficiently in warm weather, so this is a reasonable assumption for Covid-19).

Van den Heever has argued that if government is using a strategy that assumes R goes above 1 (like the one described above), then it has effectively given up because the epidemic will get out of control.

His model, on the other hand, envisages the switching off and on of a selection of interventions as the epidemic is suppressed here and resurges there.

Vital to this strategy is good contact tracing, but Van den Heever has expressed frustration at the poor quality of government’s test and trace strategy so far.He has described his model in detail here.

Compartmental modelling 

Most of the models of the pandemic are what are called compartmental ones; they divide the population into compartments (susceptible, exposed, infectious, recovered and so on) and then describe how people move between these compartments with a set of equations.

A different kind of model is the SABCoM one developed by researchers at the University of Cape Town. This simulates how people move around and transmit or contract Covid-19.

The creators of this model deserve credit for making the computer code of their model available online. They have applied their model to each ward in Cape Town, but, as with Van den Heever’s model, rather than predicting the future, the main aim appears to be to see what the effect of different interventions are on the pandemic.

Alternatives to lockdown exist, they write, but must be done properly. In particular, they emphasise the importance of effective contact tracing.

Other models

Auditing and consulting firm Deloitte has developed a model for Business for South Africa (B4SA) to assist B4SA in its engagement with government. The model projects the need for, among other equipment, hospital beds, oxygen, ventilators and N95 masks.

The London School of Hygiene and Tropical Medicine has produced a model of the South African pandemic that estimates 310 000 deaths if there are no interventions, which is more than the HIV-Aids-related death rate in South Africa in 2005, the country’s worst year of the HIV epidemic.

Introducing social distancing and “shielding” high-risk people from infection can reduce the number of deaths to below 100 000 if implemented very effectively. Charles Simkins explains the model well on Politicsweb.

A group called Pandemic Data Analysis estimates only 20 000 deaths with, as they write, “a sense that we might bring it down or decide that it is the top of our range”.

Researchers at the University of East Anglia in the UK have used Chinese and Italian data to estimate the number of deaths in many countries for different infection rates, corrected for age distribution, including South Africa. We’ve previously explained their work.

 *This article was produced by Spotlight – health journalism in the public interest. Sign up for our newsletter and stay informed.

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May 31 2020