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Compare under-65 death rates to understand global Covid-19 infections, say University of Cambridge researchers

Focusing on the number of deaths in those under the age of 65 would give us a better idea of the true level of transmission of the Covid-19 virus across different countries, researchers say.

Comparing total death rates could give us a misleading picture, according to scientists at the the University of Cambridge and the Institut Pasteur, because of large differences in the data on deaths among older people around the world.

The pandemic is thought to have claimed the lives of 1.2 million people around the world by the end of October, according to official data, The true figure may be considerably higher, however (42958428)
The pandemic is thought to have claimed the lives of 1.2 million people around the world by the end of October, according to official data, The true figure may be considerably higher, however (42958428)

Large Covid-19 outbreaks in European nursing homes, and the potential for missing death data in some Asian and South American countries, render cross-country comparisons of the pandemic inaccurate, they argue.

“Simply comparing the total number of deaths across countries can be misleading as a representation of the underlying level of transmission of SARS-CoV-2. Most deaths are in older people, but they are the least comparable across countries,” said Megan O’Driscoll, a PhD researcher at Cambridge’s Department of Genetics and first author of the paper published in Nature on Monday.

In some countries, such as the UK, Canada and Sweden, the virus has disproportionately affected nursing home residents, who account for more than 20 per cent of reported coronavirus-related deaths.

“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,” said senior author Dr Henrik Salje, also from the Department of Genetics..

“It’s not just that residents are older than the general population, they are also generally more frail, so a 70-year old living in a nursing home is often more likely to die of Covid-19 than a 70-year old in the general population.

“To reduce the overall number of Covid-19 deaths it is vital to protect vulnerable elderly communities.”

Some countries in Asia and South America have reported far fewer Covid-19 deaths among older people than expected, possibly because they are less likely to be investigated as countries battle to control the virus.

The researchers created a new model by integrating age-specific Covid-19 death data from 45 countries with 22 national-level seroprevalence surveys, which give an indication of infection rates by monitoring the proportion who have developed antibodies against the virus in their blood.

“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,” said Megan.

The model can be used to predict the likelihood of people dying from Covid-19 following infection based on their age, and can work in reverse to estimate a country’s infection numbers based on coronavirus-related deaths in certain age groups.

It shows that by September 1, an average of 5 per cent of the population of a country had been infected with SARS-CoV-2. However, some places had much higher rates, especially in South America, where the model suggested that more than half the population of Peru has been infected.

But other widespread co-morbidity factors are also at play, making comparisons even more challenging.

“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, which is very different to what we’ve seen in New York, for example. There are likely to be fundamental differences in the populations across countries, which might include their underlying health,” said Dr Salje.

One strong pattern seen across countries is in the five to nine-year-old age group, which consistently has the lowest probability of death following SARS-CoV-2 infection.

This research was funded by the University of Cambridge Covid-19 Rapid Response Grant.

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