The age distribution of mortality from novel coronavirus disease (COVID-19) suggests no large difference of susceptibility by age

Abstract

Among Italy, Spain, and Japan, the age distributions of COVID-19 mortality show only small variation even though the number of deaths per country shows large variation. To understand the determinant for this situation, we constructed a mathematical model describing the transmission dynamics and natural history of COVID-19 and analyzed the dataset of mortality in Italy, Spain, and Japan. We estimated the parameter which describes the age-dependency of susceptibility by fitting the model to reported data, including the effect of change in contact patterns during the epidemics of COVID-19, and the fraction of symptomatic infections. Our study revealed that if the mortality rate or the fraction of symptomatic infections among all COVID-19 cases does not depend on age, then unrealistically different age-dependencies of susceptibilities against COVID-19 infections between Italy, Japan, and Spain are required to explain the similar age distribution of mortality but different basic reproduction numbers (R₀). Variation of susceptibility by age itself cannot explain the robust age distribution in mortality by COVID-19 infections in those three countries, however it does suggest that the age-dependencies of (i) the mortality rate and (ii) the fraction of symptomatic infections among all COVID-19 cases determine the age distribution of mortality by COVID-19.

Figure 1

The age distribution of mortality by COVID-19 in Italy reported on 13th May 2020, Japan reported on 7th May 2020, and Spain reported on 12th May 2020. Circle, square, and “+” denote Italy, Japan, and Spain.

Figure 2

The sensitivity of (a) age-dependency of susceptibility and (b) transmission coefficient β against age distribution of mortality when age-independent mortality was assumed. In panel (a), all parameters except the exponent parameter φ, describing the variation of susceptibility among age groups, were fixed and parameterized as R₀ = 2.9 in the setting for Spain. In panel (b), all parameters parameterized as the setting for Spain (φ = 12.3) were fixed except transmission coefficient β.

Figure 3

The sensitivity of transmission coefficient β against age distribution of mortality when it was assumed that age-dependent mortality was proportional to cCFR per age group. All parameters were fixed and parameterized as the setting for Spain except the transmission coefficient β.

Figure 4

The estimate of exponent parameter φ describing the variation of susceptibility among age groups using model 1 and assuming that mortality rate does not depend on age. True and broken lines represent the maximum likelihood estimates and 95% confidence intervals, respectively.

Figure 5

The estimate of exponent parameter φ describing the variation of susceptibility among age groups using model 2 and assuming that the fraction of infections that becomes symptomatic among all COVID-19 cases is 0.25. True and broken lines represent the maximum likelihood estimates and 95% confidence intervals, respectively.