April 9, 2020
The age affected structure doesn’t fit with pandemic theory
Carl Heneghan, Tom Jefferson
“It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”
― Sir Arthur Conan Doyle, Sherlock Holmes
Pandemic, as the definition goes, is the worldwide spread of a new disease. Most of the historical analysis point to those in younger age groups being disproportionately affected in a pandemic. As opposed to seasonal outbreaks where older people are more likely to be affected.
In this current pandemic, the age structure of those most affected reveals a tension between whether COVID-19 is operating more like a seasonal viral effect or is similar in its effect to previous pandemics
The US Centre for Disease and Control Prevention (CDC) estimated that 150, 000 to 575,000 people died from (H1N1) pandemic virus infection in the first year of the outbreak.
80% of the virus-related deaths were estimated to occur in those < 65 years of age.
In seasonal influenza epidemics, about 70% to 90% of deaths occur in people ≥65.
Points later confirmed by the WHO:
However, typical seasonal influenza causes most of its deaths among the elderly while other severe cases occur most commonly in people with a variety of medical conditions.
By contrast, this H1N1 pandemic caused most of its severe or fatal disease in younger people, both those with chronic conditions as well as healthy persons, and caused many more cases of viral pneumonia than is normally seen with seasonal influenza.
In a pandemic, the proportion of deaths among the young should increase (See here and here), and several studies have further demonstrated this age shift in deaths to younger populations. In a 1918 scenario, the major prevention strategies are to target younger people to minimize the loss to expected years of life. An analysis of three other pandemics over the last century also suggests pandemic viruses cause excess mortality in ‘unusually young populations.’
An analysis of countries and US states or major cities with at least 250 COVID-19 deaths as of 4th April reports that individuals aged <65 accounts for less than 10% of all COVID-19 deaths.
People <65 years old had 34- to 73-fold lower risk than those ≥65 years old in the European countries
People aged < 65 had 13- to 15-fold lower risk in New York City, Louisiana and Michigan.
The WHO further stated that older people are at highest risk: over 95% of deaths occurred in those > 60 years. More than 50% of all deaths were people > 80 years or older.
The data support the theory that the current epidemic is a late seasonal effect in the Northern Hemisphere on the back of a mild ILI season. The age structure of those most affected does not fit the evidence from previous pandemics.
The outbreak does, however, fit with the WHO’s definition of a Pandemic. This definition does not help explain the age structure of those most affected, and how this differs from that of seasonal outbreaks. The definition of a pandemic remains elusive.
What would Sherlock have made of all this?