Disease IFR Infection Fatality Rate

Understanding the Infection Fatality Rate (IFR)

The infection fatality rate tells you what share of everyone who got infected ended up dying from the disease. That includes people who never had symptoms and people who were never tested. You compute it as IFR = deaths / total_infected × 100%. Since the denominator counts asymptomatic and unreported cases too, the IFR is the closest thing we have to an infection’s true biological lethality.

Pinning down the IFR is much trickier than the CFR, because you first have to know how many people were really infected. The usual routes are seroprevalence studies that measure antibodies in random population samples, retrospective cohort analyses, or models that pair surveillance data with excess mortality. For COVID-19 the global IFR has been put at 0.5–1%, though age changes everything: under 30 it sits below 0.01%, and past 80 it climbs above 5%.

Applications

The IFR feeds cost-benefit analyses of public health interventions like lockdowns, vaccination campaigns and NPIs, along with pandemic preparedness models and long-range death projections. When you want to compare lethality between countries, it’s the better yardstick, since testing capacity doesn’t skew it the way it skews other measures. For reference: the 1918 flu IFR was around 2%, H1N1 in 2009 around 0.02%, and pre-vaccine COVID-19 around 0.5–1%.

FAQ

Why is IFR always lower than CFR? The IFR's denominator takes in every infection, and most of those are mild or have no symptoms at all. The CFR only counts confirmed cases, which lean heavily toward the people who got sick enough to be tested.

How do scientists measure total infections? The main tool is seroprevalence surveys, which test antibodies across representative population samples. On top of that come statistical models that correct for waning immunity and for how sensitive the tests actually are.

Does IFR change over time? It does. Vaccination, better treatments and stronger healthcare push it down, while new variants or an overwhelmed system push it back up. After vaccines became widespread, the COVID-19 IFR fell by something like ten times.