When people talk about the severity of Covid-19, they focus on the number of cases and deaths. However, the death rate ranges depending on the region of the world. Moreover, the risk of infection can increase or decrease depending on age and underlying medical conditions.
On November 2, 2020, Nature published an unedited manuscript titled, Age-specific mortality and immunity patterns of SARS-CoV-2. The paper utilizes age-specific data on deaths from Covid-19 to “investigate the consistency of infection and fatality patterns across multiple countries.”
Covid-19 and mortality
With the news outlets continuously announcing the elder populations’ higher risk of contracting the virus and the high number of deaths in nursing homes, many are aware that age increases risk of mortality due to the novel coronavirus. However, this does not necessarily mean that the number of deaths indicates the severity of Covid-19.
According to Dr. Megan O’Driscoll, “The level of transmission among the general population can be difficult to disentangle from large outbreaks in vulnerable populations, such as nursing homes and other long-term care settings.”
In several countries, including Canada, Sweden, and the United Kingdom, more than 20 per cent of all Covid-19 related deaths are in nursing homes. In other countries, such as South Korea and Singapore, very few deaths have been reported in nursing homes.
“Nursing homes are enclosed communities of people, and once the virus gets in, it can spread quickly, resulting in higher levels of infection than in the general population. We’re seeing an excessively large number of deaths from Covid-19 in this older age group, particularly in countries that have many nursing homes,” Dr. Henrik Salje from the University of Cambridge’s Department of Genetics and senior author of the report, told Science Daily.
Examining the total number of deaths may not accurately indicate “the underlying level of transmission,” but focusing on death data in the younger population will.
Age-specific mortality patterns
O’Driscoll and her team compared the relative number of deaths by age in 45 countries, using ages 55 to 59 as the age-reference group. They found that the relative risk of death for individuals under 65 was consistent, regardless of country or continent.
“Our model shows that the number of Covid-19 deaths by age, in people under 65-years-old, is highly consistent across countries and likely to be a reliable indicator of the number of infections in the population. This is of critical use in a context where most infections are unobserved,” says O’Driscoll.
However, the data also showed some inconsistencies in the death rate, which could be due to numerous factors, according to Henrik Salje, another researcher on the team. “It seems that people living in places such as Slovenia and Denmark have a low probability of death following infection with SARS-CoV-2, even after accounting for the ages of their populations. [This] is very different from what we’ve seen in New York, for example,” says Salje. “There are likely to be fundamental differences in the populations across countries and might include their underlying health [conditions].”
Potential explanations for this include variations in the percentage of high-risk individuals, variations in the seroprevalence [measurement of a pathogen in a population, Covid-19] studies, variations in care availability and quality, and variations in how Covid-19 deaths were reported.
The researchers acknowledged that “translating the number of Covid-19 deaths into estimates of the number of infections requires careful consideration of fatalities from outbreak events in highly vulnerable populations.” By using the age-specific data to predict the expected number of deaths in older individuals, countries can identify where more outbreaks in nursing homes could occur.