As the threat of a Covid-19 pandemic emerged earlier this year, many felt a sense of apprehension about what would happen when it reached Africa. Concerns over the combination of overstretched and underfunded health systems and the existing load of infectious and non-infectious diseases often led to its being discussed in apocalyptic terms.
However, it has not turned out quite that way. On September 29, the world passed the one million reported death mark (the actual figure will, of course, be higher). On the same day, the count for Africa was a cumulative total of 35,954.
Africa accounts for 17 per cent of the global population but only 3.5 per cent of the reported global Covid-19 deaths. All deaths are important, we should not discount low numbers, and of course, data collected over such a wide range of countries will be of variable quality. Still, the gap between predictions and what has happened is staggering. There has been much discussion on what accounts for this.
As leads of the Covid-19 team in the African Academy of Sciences, we have followed the unfolding events and various explanations put forward. In many African countries, the emerging picture has been higher, but the severity and mortality much lower than initially predicted based on China and Europe's experience.
We argue that Africa's much younger population explains a huge part of the apparent difference. Some of the remaining gaps are probably due to the under-reporting of events. Still, there are many other plausible explanations—these range from climatic differences, pre-existing immunity, genetic factors and behavioural differences.
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Importance of age.
Given the enormous variability in conditions across a continent – with 55 member states – the exact contribution of any one factor in a particular environment is likely to vary. But the bottom line is that what appeared at first to be a mystery looks less puzzling as more and more research evidence emerges.
The most obvious factor for the low death rates is the population age structure. Older people over 65 years and with pre-existing health issues are more susceptible to dying from the virus than those in their 20s.
As of September 30, the UK had reported 41,980 Covid-19 specific deaths while Kenya, by contrast, had reported 691. The UK population is around 66 million with a median age of 40 compared with Kenya's population of 51 million with a median age of 20 years.
If the median age was as that of the UK. However, if one also corrects for population structure (assumes that the age-specific death rates in the UK apply to Kenya's population structure), we would expect around 5,000 deaths. There is still a big difference between 700 and 5,000; what might account for the remaining gap?
One possibility is the failure to identify and record deaths.
As with most countries, Kenya initially had a limited testing capacity and specific death registration was challenging. However, Kenya quickly built up its testing capacity. The extra attention to finding deaths makes it unlikely that this size gap can be fully accounted for by missing information.
There has been no shortage of ideas for other factors that may be contributing.
A recent sizeable multi-country study in Europe reported significant declines in mortality related to higher temperatures and humidity. The authors hypothesised that this might be because the mechanisms by which our respiratory tracts clear virus work better in warmer, more humid conditions; hence people may be getting fewer virus particles into their system.
However, a systematic review of global data – while confirming that warm and wet climates seemed to reduce the spread of Covid-19 – indicated that these variables alone could not explain most disease transmission variability. It's important to remember that there's considerable weather variability throughout Africa. Not all climates are warm or wet and, if they are, they may not stay that way throughout the year.
Other suggestions include the possibility of pre-existing protective immune responses due either to previous exposure to other pathogens or to BCG vaccination, a vaccine against tuberculosis provided at birth in most African countries. An extensive analysis – which involved 55 countries, representing 63 per cent of the world's population – showed significant correlations between increasing BCG coverage at a young age and better outcomes of Covid-19.
Genetic factors may also be important. A recently described haplotype (a group of genes) associated with increased risk of severity and present in 30 per cent of south Asian genomes and 8 per cent of Europeans is almost absent in Africa.
The role of these and other factors – such as potential differences in social structures or mobility – are subject to an ongoing investigation.
An important possibility is that African countries' public health response, prepared by previous experiences (such as outbreaks or epidemics), was simply more effective in limiting transmission than in other parts of the world.
However, in Kenya, it's estimated that the epidemic peaked in July with around 40 per cent of the population in urban areas infected. A similar picture is emerging in other countries. This implies that measures put in place had little effect on viral transmission per se, though it does raise the possibility that herd immunity is now playing a role in limiting further transmission.
At the same time, there is another significant possibility: The idea that viral load (the number of virus particles transmitted to a person) is a key determinant of severity.
It has been suggested that masks reduce viral load and that their widespread wearing may limit the chances of developing severe disease. While WHO recommends mask-wearing, uptake has been variable and has been lower in many European countries than many parts of Africa.
So is Africa in the clear? Well, obviously not. There is still plenty of viruses around and we do not know what may happen as the interaction between the virus and humans evolves.
However, it seems clear that the secondary effects of the pandemic will be Africa's real Covid-19 challenge.
Marsh is a professor of Tropical Medicine, University of Oxford. Mr Alobo is programme manager for Grand Challenges Africa, African Academy of Sciences.