Age-specific mortality and immunity patterns of SARS-CoV-2

TLDR

Estimating COVID‑19 pandemic size and severity is hampered by inconsistent data, under‑reporting of deaths, and variable nursing‑home mortality reporting across countries. The study uses age‑specific death data from 45 countries and 22 seroprevalence studies to assess consistency of infection and fatality patterns. We combine age‑specific death counts with seroprevalence results in a simple modelling framework to estimate infection fatality ratios and infection prevalence. The analysis shows that death distributions in individuals under 65 are consistent across countries, the infection fatality ratio rises log‑linearly with age above 30, about 5 % of populations were infected by September 2020 (higher in Latin America), and the model can estimate infection prevalence from age‑specific death data.

Abstract

Estimating the size of the coronavirus disease 2019 (COVID-19) pandemic and the infection severity of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is made challenging by inconsistencies in the available data. The number of deaths associated with COVID-19 is often used as a key indicator for the size of the epidemic, but the observed number of deaths represents only a minority of all infections1,2. In addition, the heterogeneous burdens in nursing homes and the variable reporting of deaths of older individuals can hinder direct comparisons of mortality rates and the underlying levels of transmission across countries3. Here we use age-specific COVID-19-associated death data from 45 countries and the results of 22 seroprevalence studies to investigate the consistency of infection and fatality patterns across multiple countries. We find that the age distribution of deaths in younger age groups (less than 65 years of age) is very consistent across different settings and demonstrate how these data can provide robust estimates of the share of the population that has been infected. We estimate that the infection fatality ratio is lowest among 5–9-year-old children, with a log-linear increase by age among individuals older than 30 years. Population age structures and heterogeneous burdens in nursing homes explain some but not all of the heterogeneity between countries in infection fatality ratios. Among the 45 countries included in our analysis, we estimate that approximately 5% of these populations had been infected by 1 September 2020, and that much higher transmission rates have probably occurred in a number of Latin American countries. This simple modelling framework can help countries to assess the progression of the pandemic and can be applied in any scenario for which reliable age-specific death data are available. The relative risk of COVID-19-associated death for younger individuals (under 65) is consistent across countries and can be used to robustly compare the underlying number of infections in each country.