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density also favors the spread of infectious diseases transmitted from person to person due to the greater proximity and
the greater number of contacts between people. This is indeed what was observed empirically: A positive relationship
between incidence and population density, although not totally monotonic. The apparent accident in the second category
(density of 40-79 inhabitants per km²) was mainly due to the exceptional case of South Africa; otherwise, the relationship
would be monotonously increasing (Table 1 and Figure 5).

3.6.2. Urbanization
The same pattern was found with urbanization, and for the same reasons as for density: The relationship was positive and
strong, the most urbanized countries having more viruses, and here the relationship was monotonous with a wide gradient
from 1 to 17 (Table 1 and Figure 5).
3.6.3. Population concentration

The relationship with the population concentration index, which measured the geographical distribution of the population
in the territory, was U-shaped, which was difficult to explain by purely epidemiological criteria (Table 1).

3.6.4. Fertility level
The relationship between infectious disease and fertility level is often negative, with countries further behind in the
fertility transition having more difficulty in controlling infectious diseases. But here the relationship was reversed, with
a wide gradient between countries: When the total fertility rate (TFR) was >5 children per woman (10 countries), the
incidence was low (108 on average), but much higher (2233) in the opposite case, when TFR was <3 children per woman
(10 countries). Hence, the more countries are advanced in the fertility transition, the more COVID-19 they have, and
gradients were wide, ranging from 1 to 16 (Table 1).
3.6.5. Mortality level

The same pattern was found for mortality level, here calculated as the under-five death rate per 1000 births. The more
advanced in the health transition, the higher COVID-19 incidence. The gradients were weaker and less regular than for
fertility, but they remained important, with variations from 577 to 2372 per million, that is, a gradient from 1 to 4 (Table 1).
3.6.6. Age structure of the population

The relationship with the age structure was also complex and not monotonic. The average age of the population and
the proportion under age 20 were used, but the second parameter was somewhat less correlated with incidence. Large
differences existed between countries, ranging from 175 in very young countries (average mean age 20-21 years, that
is, 11 countries) to 3807 in older countries (average mean age 26-29 years, that is, 6 countries), but incidence fell to 809
in the oldest countries (average mean age of 30 years and over, or 7 countries). Here again, the relationship was rather
reversed, although not confirmed in the most advanced countries. It should be remembered that the age structure is the
result of past fertility and mortality levels, and therefore that one could expect an inverted and strong relationship with the
mean age of the population (Table 1 and Figure 5).

3.6.7. Progress of the demographic transition
The five-category demographic transition indicator summarized the relationships with fertility, mortality, and age
structure. The incidence gradient, according to this indicator, was close to that noted with the age structure: Monotonic
and fairly regular for the first four categories, and different for the highest category (very advanced transition). This
category grouped countries with demographic regimes different from others: Islands (Cape Verde, Mauritius, Reunion,
and Seychelles) and Maghreb countries (Libya, Morocco, and Tunisia) (Table 1).

3.7. Relationship with Economic Factors

3.7.1. GDP per capita
The relationship with per capita income (GDP in purchasing power parity and in constant dollars) was also inverted and
complex: The poorest countries (<$2000, that is, 20 countries) had less COVID-19 (average incidence = 274), middle-
income countries ($8000-$15,999, or 7 countries) had more than 10 times as much (average incidence = 3616), but

International Journal of Population Studies | 2020, Volume 6, Issue 2 9