What do the South African Covid-19 numbers mean?

Most of the Covid-19 numbers we have paint only part of the image and can easily be misinterpreted. Here are some tips for understanding the numbers.

Confirmed cases versus real cases

Every day, the Minister of Health, Dr. Zweli Mkhize, provides the total of confirmed cases and the increase in confirmed cases from the previous day. But how much can we really say about these numbers?

Confirmed cases are only a fraction of the total number of cases in the country. Perhaps the actual number of cases is double or perhaps it is 10 times the number of reported cases; we just don’t know. As the evidence expands, the image may get a little clearer. Unfortunately, in the last 10 days, the number of tests decreased from a maximum of 8,066 on March 27 to 1,225 on April 6. (Hopefully this will improve in the coming days if the government successfully implements the GeneXpert 45-minute test machines.)

The number of confirmed cases depends on the number of tests performed and how well those tests are conducted. All things being equal, a sudden increase in test capacity could result in a sudden increase in confirmed cases. This would not mean that there is definitely an explosive growth in the number of cases; only we would find more of them.

Similarly, over time it may be easier or more difficult to know who to evaluate. Most of South Africa’s first cases of Covid-19 were people traveling from abroad, where they probably contracted the virus. It made sense to focus the tests on people with symptoms of Covid-19 and who had traveled to affected countries. South Africa’s travel ban went into effect on March 18, effectively shutting down this source of new infections. Now, as recent trips abroad have become a less important indicator of a possible infection, it is increasingly difficult to know who to evaluate.

All of this probably explains why daily increases have not been so high in recent days. Every day from March 23 to 27, over 100 new cases were reported in South Africa; From March 28 to April 6, daily increases were less than one hundred. In addition to all of this, there will also be random fluctuations in the data. By chance, you can find 80 confirmed cases one day and 60 the next day. In itself, a decrease in confirmed cases like this doesn’t mean anything. For all these reasons, it is best to be cautious about how you interpret daily changes in confirmed cases.

Making sense of mortality rates

Calculating the death rate is not as simple as dividing the number of deaths by the number of cases. For example, as of April 7, 13 deaths were reported in 1,749 cases in South Africa, yielding a mortality rate of 0.74%. But it is much more complicated than that. For one, most of the 1,749 people who confirmed having Covid-19 on April 7 were still infected, and some of them may still die.

By the time South Africa had its 1,000th case, the country had no Covid-19 deaths. All our deaths have come later. Similarly, Germany also had no deaths when it had its 1,000th case. Over time, the death rate in that country has increased. Now it is around 2%.

The death rates generally cited in the media, and above, are case fatality rates (CFR): the number of deaths divided by the number of cases (multiply by 100 to convert to a percentage). Since the CFR only uses known cases and known deaths, it is heavily influenced by how well our Covid-19 tests and cause-of-death systems work.

The UK CFR rate is remarkably high, over 10% at the time of writing. Germany is only 2%. This tells us more about how widely each country has tested the virus than about the lethality of the virus in the two countries. The UK has left testing on a large scale too late.

Differences in evidence across countries have been well reported, but accurate tracking of Covid-19 deaths (the other part of the CFR calculation) is also not straightforward. the Economist reports that in some parts of Europe the number of expected deaths in recent weeks has increased dramatically, and not all of these deaths have been attributed to Covid-19. This could suggest that people are dying of Covid-19 without being diagnosed with it (perhaps even at home), or that Covid-19 is putting so much pressure on health systems that people with other health problems would normally be saved by the system dying. It will be a while before we understand this correctly.

While some CFR differences refer to how well cases and deaths are measured, there are also real differences between how deadly Covid-19 is in different settings. Countries with larger populations are likely to have more deaths from Covid-19. But so could countries with younger populations but with poorer health systems or less healthy populations. Treatment capacity is also important because people who might otherwise be saved may not be saved if the ability to provide intensive care is not available.

How infectious is the coronavirus that causes Covid-19?

We don’t know. But the available data suggests that this is a highly infectious virus. At the start of epidemics in China and Europe, the average infected person was infecting at least two, perhaps even three or four other people before recovering or dying. And they in turn were infecting two, three or four people. This resulted in the epidemic growing very rapidly.

What we must do to end the epidemic is to reduce the average number of times that an infected person infects another person below one. Then the epidemic will fade away.

The rate of new infections depends on three things: how many people the average person comes in contact with daily, how long people remain infectious, and how easily the virus is transmitted between two people in contact.

Suppose you are infected and come into contact with 10 uninfected people daily and the risk of infecting any one of them is 5% (these numbers are completely made up; we have no idea what they really are). Multiply these two numbers and you get 50% risk of infecting one person per day. If it’s infected for four days, it will infect two people (it’s actually more complicated than that, but it’s close enough).

The objective of blocking is to reduce the average number of people with whom we come into contact. If the same person, instead of being in contact with 10 people a day, is in contact with only two per day, then for four days that person might not infect anyone.

The goal of getting people to wash their hands frequently and reduce how often they touch their faces is to reduce the risk of contact infection.

By reducing both the number of contacts and the risk per contact, we can control the epidemic. DM / MC

Low is pursuing a doctorate in infectious disease models at the University of Cape Town. Geffen is the director of the Center for Mass Communication Science and Technology at Stellenbosch University. This article is co-edited by Highlight and GroundUp.