Are Covid-19 death rates correlated to the intensity of previous flu seasons?

From the start of the Covid-19 outbreak early this year, researchers have tried to determine if and how the virus is different from the ordinary flu. While this research is ongoing, the Cambridge Judge Business School has found strong evidence that the death rate from Covid-19 is significantly negatively correlated to the intensity of the last two flu seasons in 32 European countries.

While Covid-19 was initially presented as a potential “killer virus” and understandably led to unprecedented lockdown and quarantine measures, it quickly became clear that despite a high infection risk, the death rate was relatively modest.

Research by the Cambridge Judge Business School has found that the death rate from Covid-19 is also significantly negatively correlated to the intensity of the last two flu seasons in 32 European countries. Taking the example of the UK, which has about 20,000 excess deaths from influenza in a typical year, the last two years were very mild, with about 1,700 excess deaths in 2018/19. A working paper by Chris Hope notes that “this implies that there were over 30,000 people alive in the UK at the start of the Covid-19 pandemic who would have been expected to die in the previous two flu seasons. These people are likely to have been predominantly elderly, and in poor health […]”.

This finding has led Dr Hope to study a possible link between Covid-19 deaths and the intensity of the previous two flu seasons. Based on weekly data by the European Influenza Surveillance Network for 32 countries, he concludes that the results suggest that the relationship between Covid-19 deaths rates and previous flu intensity would be worth further and fuller investigation. For example, one finding is that Belgium, which has the highest death rate of the 32 countries, had the lowest flu intensity apart from Scotland. Its excess death rate of 36 per 100,000 above the trend line is the highest of the 32 countries.

The question of the correlation between flu and Covid-19 is also discussed at national health institutes. Based on recent Swedish data, the country’s state epidemiologist Anders Tegnell is also of the opinion that there is a strong connection between a low excess mortality rate from the flue and high excess mortality from Covid-19, and vice versa. While Tegnell claims that the relatively high flu death rate in Norway over the past two winters explains the country’s low Covid death rate compared to Sweden, his Norwegian counterparts do not share this analysis and argue instead that the differences in the death rate in Norway and Sweden “stem primarily from the fortunate timing of Norway’s lockdown restrictions, which had stopped the spread of the virus at an early stage”.

While the Cambridge study was only very much a first attempt to investigate any link between the Covid-19 death rate and flu intensity, and despite evident shortcomings such as the fact that the recording of Covid deaths across countries is not straightforward, Dr Hope has concluded that further investigation should be able to determine whether the relationship is as significant as this first analysis suggests.