Because influenza (flu) viruses circulate in the population all the time, we can sometimes forget that these cause a large number of deaths. Researchers estimate that approximately one in a thousand (0.1%) of all people infected with common, seasonal flu die of the disease. This may sound like a low risk for any one infected individual, but it adds up to thousands of flu-related deaths every year in the UK and hundreds of thousands worldwide.
According to recent studies, the new coronavirus causes a death rate six to 16 times higher than common flu, killing about 0.6–1.6% of those infected. The risk of dying from COVID-19, as with common flu, increases with age – up to 3% or more in the over 60s, although there is still much uncertainty around these estimates.
Estimating fatality rates
It is surprisingly complicated to work out the chance of dying from a virus. New data could still change estimates of the fatality rate – the proportion of infected people who die.
The first issue is that we do not know precisely how many people infected with the new coronavirus have mild illnesses that go unreported. It is hard to identify patients with mild disease and almost impossible to identify those with no symptoms. So, if the number of people infected is higher than estimated, then the overall percentage who die will be lower. Similarly, if the number of people infected is lower than estimated, then the percentage of fatalities may be higher.
Ideally, a large representative fraction of the population would be tested for coronavirus infection, but these surveys have not yet been done. This is because the tests necessary to survey populations are not yet readily available in sufficient reliability and quantity. These tests measure the antibodies that develop as part of the protective immune response to an infection, so can be used to count the number of people who have had the infection and recovered. In the meantime, researchers are using small studies in specific settings, such as passengers on repatriation flights or a cruise ship, where testing was carried out on every individual to be able to accurately account for mild cases.
This is the main reason why current estimates vary between studies. For example, one study estimated an 18% case fatality rate among the over 80s in China, assuming all symptomatic cases in this age group were identified. Another estimated 8%, assuming that more cases were missed. Evidence from the Diamond Princess cruise ship with better case identification gives 8% case fatality rate in the over 80s, although this was measured among only a small number of patients on the ship. Those few patients may not be enough to be able to multiply reliably to large populations. Similarly, people who choose to go on cruises may not be representative of the average population of the same age.
Another issue occurs when a patient who is already severely ill with a different disease contracts the virus and dies. We do not know if they died as a result of the virus or would have died anyway from the original illness. A severe infection may bring forward the death of people whose life expectancy was already very short. In the case of flu, where scientists have much more data and sophisticated models, deaths are estimated by looking at whether the total number of deaths in a population is higher when there are more flu infections.
We do not yet have enough data on the new coronavirus yet to use such methods, although emerging evidence shows there is a relatively high death rate from coronavirus among infected people who also have other diagnosed health conditions, such as heart disease.
A new virus
An important difference between the new coronavirus and the common flu virus is that people have some level of immunity to common flu. This results from either previous infection or because of the flu vaccine. However, our immune systems have not been exposed to the new coronavirus before.
It has been suggested that other coronaviruses causing milder diseases that have been in circulation for a while might give some cross-protective immunity. This is not yet known.
The new coronavirus appears to have a fatality rate closer to the worldwide pandemic flu in 1918 that killed an estimated 1.0–1.5% of those infected, after accounting for milder undetected cases. Unusually, the 1918 virus caused a higher proportion of deaths among young adults. The 2009 ‘swine flu’ pandemic had a relatively low overall death rate of 0.01–0.08%, which is why strict control measures were not considered at that time.
The majority (around 80%) of people who become infected with the new coronavirus will only suffer from minor illness. But the fatality rate following infection is still high compared to other pandemic diseases, adding up to a large number of potential deaths across the world. More precise estimates of fatality rate in different age groups, and those with underlying health conditions, are critical to planning protection for those most vulnerable.