By Philip Hill

One of the interesting features of the COVID-19 pandemic is the different incidence rates of the disease and the different mortality percentages, across countries. There are several possible reasons for this.

Firstly, the capacity and policies around testing for COVID-19 differ between countries. Across the world the number of tests that countries can do per million population differ markedly. For example, New Zealand has capacity for up to 8,000 tests per day for a population of five million. Liberia has capacity of only 300 tests per day for a similar-sized population. Policies around the indications for testing vary between countries and over time within countries. New Zealand, for example, changed its indications from a restricted set of symptoms and a travel history, to a broad array of symptoms and no travel history required, over time. Some countries, including New Zealand recently, have also been doing strategic testing in at-risk populations, regardless of symptoms.

Secondly, the structures and densities of populations vary. The main issue here is age structure. The median age of the population of New Zealand is 37 years. This compares to 47 in Italy, 45 in Spain, 41 in UK, 38 in China, 30 in Indonesia and 18 across sub-Saharan Africa. Given that SARS-COV-2, the virus that causes COVID-19, causes by far the greatest mortality in the elderly, and very few infections or deaths are in young children, one would expect high numbers of deaths in countries like Italy, and relatively few deaths in sub-Saharan Africa. Similarly, population densities vary greatly between countries. New Zealand has a population density of 18 people/square km, compared to three in Australia, 36 in the United States, 206 in Italy and 259 in the United Kingdom. However, it is important to note that vulnerable sub-populations, such as the elderly, are present in high-density care-facilities in large numbers in most Western Countries. Therefore, since these populations are the primary target for SARS-COV-2, there can be especially high numbers of deaths where it is allowed to enter care-facilities.

Thirdly, living conditions, other than in care-facilities, and social practices vary between countries and these may have an impact in infection rates. For example, since SARS-COV-2 spreads by droplets directly or indirectly through hands and surfaces, how people behave towards each other is likely to be important. In some cultures, people embrace and kiss when greeting each other. Handwashing practices also vary. The structure and size of housing differ between countries, with over-crowding likely to be an important factor. Temperature and humidity within households may be important. Also, in many Asian countries, it is routine practice to wear a mask when you have a respiratory infection.

Fourthly, the prevalence of underlying conditions varies within and between populations. For example, in New Zealand Pacific and Māori people have high rates of Diabetes, which is a known risk factor for COVID-19 death. However, because the majority of cases in New Zealand have arisen because of overseas travel, we have not yet seen a major COVID-19 epidemic in our Pacific and Māori populations.

And fifthly, modern intensive care facilities reduce the chances of COVID-19 death by around 50%, through ventilation and other supportive measures. New Zealand only has around three intensive care beds per 100,000 people (160 beds overall). This compares to seven in the United Kingdom, nine per 100,000 in Australia, 29 in the United States, but only around 0.1 in most low and middle-income countries. When hospitals become overwhelmed, death rates increase.

While it is difficult to draw too many conclusions at present about how different countries will fare in terms of total numbers of cases and total numbers of deaths by the end of their epidemics, it is possible to make a few observations from the data that are currently available. Number one: those countries with a high case mortality, such as the United Kingdom and the United States, without overwhelming their intensive care facilities, have clearly not diagnosed a large proportion of their actual cases of COVID-19. This is similar to the situation earlier in China – Wuhan, where only those who were unwell were diagnosed, had a case mortality of around 4%, whereas areas outside of Wuhan had a case mortality of only around 0.5%. It is also important to note that, since most people who die do so after the first two to three weeks, early case mortality data can show ‘artificially’ low percentages.

Second, Singapore has incredibly low case mortality percentages, largely because of the fact that a high proportion of the cases there have been in young migrant workers living in crowded accommodation, at an age that has very low mortality. Related to this, at least at this point in time, in sub-Saharan Africa, despite the spread of the virus across the continent for several months now, hospitals are not being overwhelmed with COVID-19 cases. This is likely to be due to the low median age of the population – large numbers of young people who may become infected but do not have serious illness and those who are under 15 years age do not tend to pass on their infections to their peers or to their adult contacts.  South Africa appears to have a more rapidly expanding epidemic at present. However, they may have higher testing capacity than other African countries, and their median age is 29. It may be that SARS-COV-2 will eventually penetrate the elderly African population and cause large numbers of deaths over a longer period than European and North American countries, but only time will tell. It is possible that the large young sub-populations, without becoming immune individually, will provide a type of ‘herd-blockade’ protection for the elderly population.

Therefore, a number of different factors could explain the large differences in infection rates and case mortalities between countries across the world. How these will play out in terms of the total numbers of cases and the total numbers of deaths in each country at the end of the pandemic remains to be seen.

Philip Hill is a Professor in the centre for international health at the University of Otago. 

See Also:

How can New Zealand control the course of the COVID-19 epidemic?

What is a virus? How do they spread? How do they make us sick?

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