Recent estimates (including our analysis led by @timwrussell: https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2020.25.12.2000256)">https://www.eurosurveillance.org/content/1... have suggested 0.5–1% of COVID-19 infections may potentially be fatal overall. Some have interpreted this to mean COVID could be similar to flu, but they& #39;re making a crucial mistake... 1/

To make a comparison with influenza, we need to look at all influenza *infections* (including asymptomatic ones), then compare with deaths. Remember, in epidemiology, always make sure you& #39;re comparing what you think you& #39;re comparing: https://twitter.com/AdamJKucharski/status/1241089579589009408?s=20">https://twitter.com/AdamJKuch... 2/

So how often do people get infected with influenza (with or without symptoms)? A few years ago, I looked at this with @SRileyIDD & others – using serological data we estimated 40-50% of younger groups were infected each year, and 15-20% of older groups. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002082">https://journals.plos.org/plosbiolo... 3/

Early data from a very detailed study in South Africa found similarly high values - 369/1118 (33%) of participants had a PCR confirmed flu infection during the year https://isirv.org/site/images/conferences/Optionsx/Options%20X_Abstracts%20_Oral%20and%20Poster.pdf">https://isirv.org/site/imag... 4/

So if we& #39;re calculating risk infection is fatal, we need to compare total flu infections - potentially 1/3 of population - with annual deaths. As rough estimate, this suggests 0.02-0.05% of flu infections are fatal, i.e. 10x lower than COVID (Data: https://www.cdc.gov/flu/about/burden/index.html)">https://www.cdc.gov/flu/about... 5/

More on interpreting fatality risk for the 2009 flu pandemic based on estimates of symptomatic cases and infected individuals: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3809029/">https://www.ncbi.nlm.nih.gov/pmc/artic... 6/6/