Is Covid more deadly and contagious than seasonal flu?

David Spiegelhalter and Anthony MastersSun, 14 February 2021, 7:15 am

<img src="https://s.yimg.com/ny/api/res/1.2/ERM_GnbXp8OGR9Oqdus1LQ–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTU3Ng–/https://s.yimg.com/uu/api/res/1.2/vazXkD0v_f9eHVTzhNNong–~B/aD02MDA7dz0xMDAwO2FwcGlkPXl0YWNoeW9u/https://media.zenfs.com/en/theguardian_763/9f4753ad966065978e7f9d3d67de0724&quot; alt="<span>Photograph: Wiktor Szymanowicz/REX/Shutterstock
Photograph: Wiktor Szymanowicz/REX/Shutterstock

Covid-19 and influenza are both respiratory diseases, but there are important differences, which statistics can help us understand.

First, Sars-CoV-2 is more infectious than seasonal flu. We’re used to hearing about the reproduction number R, the average number of people whom someone with the virus will infect. In a population without immunity and policies such as social distancing, R for Sars-CoV-2 is now estimated to be around 3. New mutations have raised R further.

In comparison, reproduction numbers for seasonal flu are about 1.3, varying yearly. In the 1918 Spanish flu pandemic, R was higher, at roughly 1.8. That explains why the distancing and other measures being taken, which can bring R for Sars-CoV-2 from 3 to below 1, are enough to almost eliminate seasonal flu.

Second, the novel virus is more deadly. The proportion of all those infected by Sars-CoV-2 who die of the disease is estimated to have been about 1.1% in a high-income country in the first wave, although the risk in different age groups varies around this average.

Due to improvements in treatment, this rate will be lower in the second wave. In comparison, the World Health Organization states the fatality rate of standard flu is “usually well below 0.1%”, around a tenth as lethal as Sars-CoV-2.

Third, Covid-19 has had a greater mortal impact than seasonal flu. Flu-attributable deaths average around 10,000 each year in England, but with huge variability: only 4,000 in the winter of 2018-2019, but more than 22,000 in the preceding bad winter. These figures are not taken from death certificates – only about 800 a year put flu as the underlying cause of death – but are modelled estimates of the number of deaths in winter over what we would expect given the seasonal colder temperatures.

In contrast, registered deaths in England involving Covid-19 are more than 100,000 since the pandemic began – 55,000 of those since September; 90% of these have Covid as the underlying cause.

There is much more to learn about the novel coronavirus, but it is certainly worse than seasonal flu.

• David Spiegelhalter is chair of the Winton Centre for Risk and Evidence Communication at Cambridge. Anthony Masters is statistical ambassador for the Royal Statistical Society