The
1918 flu, or "Spanish flu," caused the highest number of known
influenza deaths. More than 500,000 people died in the United States, and up to 50 million people may have died worldwide. Many people died within the first few days after infection with the Spanish flu, and others died of related complications. Nearly half of those who died were young, healthy adults.
The Impact of the Spanish Flu
An estimated one-third of the world's population (500 million people) were infected and had
flu symptoms during the Spanish flu. The Spanish flu was exceptionally severe, and death rates were higher than with any other
influenza pandemic. It is estimated that the total number of deaths from the Spanish flu ranged from 50 million to 100 million people.
The specific virus that caused the Spanish flu was the influenza A (H1N1) virus, which appears to be an avian-like
influenza virus derived from an unknown source.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to answer a most basic question about the Spanish flu
pandemic: "Why was it so fatal?" No virus from 1918 had been isolated, but all of its apparent descendants caused substantially milder forms of human disease. Examination of mortality data from the 1920s suggested that within a few years after the Spanish flu, influenza epidemics had settled down, with substantially lowered death rates.
The Progression of the Spanish Flu
Before and after 1918, most
influenza pandemics developed in Asia and spread from there to the rest of the world. To the contrary, the Spanish flu
pandemic spread more or less simultaneously in 3 distinct waves during a 12-month period from 1918–1919, in Europe, Asia, and North America (the first wave was best described in the United States in March 1918).
The first pandemic
influenza wave appeared in the spring of 1918, followed in rapid succession by much more fatal second and third waves in the fall and winter of 1918–1919, respectively. The Spanish flu pandemic had another unique feature, the simultaneous (or nearly simultaneous) infection of humans and swine.
The curve of influenza deaths by age has, historically, for at least 150 years, been U-shaped, with death peaks in the very young and the very old, with a comparatively low number of deaths at all ages in between.
In contrast, age-specific death rates in the Spanish flu pandemic showed a distinct pattern that has not been documented before or since: a "W-shaped" curve, similar to the familiar U-shaped curve, but with the addition of a third (middle) distinct peak of deaths in young adults who were between 20 and 40 years of age.
Influenza and pneumonia death rates for those 15 to 34 years of age in 1918–1919, for example, were 20 times higher than in previous years. Overall, nearly half of the influenza-related deaths in the 1918 pandemic were in young adults who were 20 to 40 years of age, a phenomenon unique to that pandemic year.
The Spanish flu pandemic is also unique among influenza pandemics in that absolute risk of influenza death was higher in those who were younger than 65 years of age than in those who were greater than 65 years of age. People who were younger than 65 accounted for 99 percent of all excess influenza-related deaths in 1918–1919. In comparison, those who were older than 65 accounted for 36 percent of all excess influenza-related deaths in the 1957 H2N2 pandemic and 48 percent in the 1968 H3N2 pandemic.
Like the Spanish
flu virus,
H5N1 is an avian virus, although a distantly related one. The evolutionary path that led to
pandemic emergence of the Spanish flu is entirely unknown, but it appears to be different in many respects from the current situation with H5N1.
There is no historical data, either in 1918 or in any other pandemic, for establishing that a pandemic "precursor" virus caused a highly pathogenic outbreak in domestic poultry, and no highly pathogenic
avian influenza (HPAI) virus, including H5N1 and a number of others, has ever been known to cause a major human epidemic, let alone a pandemic.
Even with modern antiviral and antibacterial drugs, vaccines, and prevention knowledge, the return of a pandemic virus equivalent in pathogenicity to the Spanish flu would likely kill more than 100 million people worldwide. A pandemic virus with the (alleged) pathogenic potential of some recent H5N1 outbreaks could cause substantially more deaths.
Is the Spanish Flu Possible Again?
It is impossible to predict with certainly, but the probability of the Spanish flu virus re-emerging from a natural source appears to be remote.
Influenza experts believe that a pandemic is most likely to be caused by an influenza subtype to which there is little, or no, preexisting immunity in the human population. There is evidence that some residual immunity to the Spanish flu virus, or a similar virus, is present in at least a portion of the human population. Since contemporary H1N1 viruses circulate widely and the current annual
influenza vaccines contain an H1N1 component, a 1918-like H1N1 virus would not fit the current criteria for a new pandemic strain.
Prevention and Treatment of the Spanish Flu
Two types of
antiviral drugs,
rimantadine (
Flumadine®) and
oseltamivir (
Tamiflu®), have been shown to be effective against influenza viruses similar to the
1918 flu virus. Vaccines containing the Spanish flu HA or other subtype H1 HA proteins were effective in protecting mice against the Spanish flu virus. In fact, the current influenza vaccine also provided some level of protection against the Spanish flu virus in mice.
