A low IFR, or infection fatality rate, means that a very small percentage of everyone infected with a pathogen dies from it. IFR is calculated by dividing the total number of deaths caused by a disease by the total number of people infected, including those who never showed symptoms or sought medical care. An IFR of 0.1%, for example, means roughly 1 in every 1,000 infected people dies. Most familiar infectious diseases have IFRs well below 1%, which qualifies them as “low” in epidemiological terms.
Understanding what counts as low matters because IFR is one of the primary ways public health officials gauge how dangerous a pathogen truly is. It differs from the case fatality rate (CFR), which only counts confirmed, diagnosed cases in its denominator. Since many infections go undiagnosed, CFR almost always overstates the actual risk of death. IFR captures the fuller picture.
How IFR Is Measured
Getting an accurate IFR requires knowing two things: how many people actually died from the disease and how many people were actually infected. Death counts come from medical records and death certificates, but estimating total infections is harder. Many people recover without ever being tested, so researchers turn to seroprevalence studies, which test blood samples from a population for antibodies. If antibodies are present, the person was infected at some point, whether they knew it or not.
These studies adjust for the accuracy of the antibody tests themselves, the demographics of the sample, and whether the tests checked for multiple types of antibodies. The adjustments matter: a test that only detects one type of antibody will miss some past infections, pushing the IFR estimate artificially higher. The more complete the picture of total infections, the lower and more accurate the IFR becomes.
What Counts as Low
There is no universal cutoff, but context helps. Seasonal influenza kills roughly 290,000 to 650,000 people worldwide each year out of an estimated 1 billion infections, putting its IFR somewhere around 0.03% to 0.07%. That is generally considered low. Chickenpox causes over 4,200 deaths globally out of 40 to 50 million annual cases, yielding an IFR around 0.01%. Measles, by contrast, kills about 107,000 people out of 10.3 million cases each year, giving it an IFR near 1%, which is significantly higher.
For comparison, the 1918 influenza pandemic infected roughly 500 million people and killed at least 50 million, producing an IFR of approximately 10%. That is extraordinarily high. Most pathogens that circulate widely in human populations sit far below that threshold.
COVID-19 as a Shifting Example
COVID-19 illustrates how a single disease’s IFR can change dramatically over time. Early pandemic estimates placed the overall IFR in the range of 0.5% to 1% for the original virus. By 2024, nationwide data from Austria estimated the overall IFR at just 0.048%, a steep decline driven by widespread immunity from prior infections and vaccination, along with the dominance of less severe viral variants like Omicron and its descendants.
Even as vaccination rates dropped between 2023 and 2024, the fatality rate did not climb back up. Researchers attributed this to near-universal levels of natural or hybrid immunity in the population, combined with the reduced severity of currently circulating strains. A disease that began the pandemic with a relatively high IFR had, within four years, dropped to a level comparable to seasonal flu.
Age Changes Everything
A single IFR number for an entire population hides enormous variation by age. A large meta-analysis of COVID-19 data among people under 70 found the following median IFRs across age groups:
- Ages 0 to 19: 0.0003%
- Ages 20 to 29: 0.002%
- Ages 30 to 39: 0.011%
- Ages 40 to 49: 0.035%
- Ages 50 to 59: 0.123%
- Ages 60 to 69: 0.506%
The pattern is remarkably consistent: each decade of life multiplies the risk by roughly three to four times. Compared to people in their twenties, those in their sixties faced a risk 253 times higher. In 2024 Austrian data, nursing home residents over 85 had an IFR above 1%, while younger groups remained far below that. When someone describes a disease as having a “low IFR,” it is worth asking: low for whom?
Why a Low IFR Can Still Be Deadly
A pathogen does not need a high fatality rate to cause mass death. It just needs to infect enough people. If a virus with an IFR of 0.05% spreads to 100 million people, that is still 50,000 deaths. Transmissibility and IFR together determine the total toll, and highly transmissible pathogens with low IFRs can outpace less transmissible ones with higher fatality rates in raw death counts.
This dynamic also shapes how viruses evolve. Theoretical modeling shows that for a pathogen with a very low IFR, a mutation increasing transmissibility by just 1% would be favored by natural selection even if it simultaneously raised the fatality rate tenfold. In practical terms, the “cost” of killing a small number of additional hosts is trivial compared to the evolutionary advantage of spreading faster. This is why low-IFR pathogens can, in theory, become more dangerous over time, though other biological constraints usually prevent that from happening in practice. The common cold, for instance, has never evolved toward higher lethality despite ample theoretical room to do so.
What Drives IFR Down
Several factors push a pathogen’s IFR lower over time or make it low to begin with:
- Population immunity: Prior infections and vaccinations train the immune system to fight off severe disease, reducing the proportion of infections that turn fatal.
- Less virulent strains: Viral evolution sometimes favors variants that spread efficiently but cause milder illness, as seen with COVID-19’s Omicron lineage.
- Better treatment: Effective therapies introduced after a pathogen first emerges reduce the death rate among those who do get seriously ill.
- Healthcare capacity: When hospitals are not overwhelmed, patients receive the supportive care that prevents deaths. When healthcare systems collapse under surge conditions, the IFR climbs noticeably, even for the same pathogen in the same population.
All of these factors interact. A population with high vaccination rates, good hospital infrastructure, and a dominant mild variant will see a dramatically lower IFR than one facing a novel pathogen with no immunity and strained medical resources. The IFR of a disease is not fixed. It is a snapshot of a specific pathogen in a specific population at a specific moment in time.