The Spanish flu (1918-1919) was exceptionally severe, and death rates were higher than with any other influenza pandemic to date. An estimated 500 million people were infected during this pandemic. This flu is not likely to re-emerge. Instead, experts believe a pandemic will likely be caused by an influenza subtype to which there is little, or no, preexisting immunity in humans.

The 1918 flu, or “Spanish flu,” caused the highest number of knowninfluenza 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.

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 virusderived from an unknown source.

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.

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 pandemicemergence 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 pathogenicavian 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.

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 annualinfluenza vaccines contain an H1N1 component, a 1918-like H1N1 virus would not fit the current criteria for a new pandemic strain.

Two types of antiviral drugs, rimantadine (Flumadine^®) andoseltamivir (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.

Will the Swine Flu end up similar as the Spanish Flu?

Source: Reuters - 24 Apr 2009 21:03:59 GMT

MEXICO CITY, April 24 (Reuters) – The majority of the people killed in Mexico’s fatal flu outbreak were adults between 25 and 45 years old, a Mexican health official said on Friday.

Swine flu continues to affect Mexico, the United States and other countries around the world. The outbreak has confounded experts who wonder how the flu is spreading and what it will do next.

One possibility is that the flu could behave like the 1918 Spanish influenza—a terrifying prospect as estimates put the death toll from that flu strain at anywhere from 20 to 50 million, or more.

There are striking similarities between the two outbreaks. Swine flu, like the 1918 pandemic, began in the spring. Both outbreaks have also mostly been reported in healthy adults; the elderly and the very young, usually the hardest hit by the flu, have not been significantly affected.

In 1918, the flu did not immediately incite hysteria. According to PBS, a Spanish wire service announced late that spring that “A strange form of disease of epidemic character has appeared in Madrid,” but that “The epidemic is of a mild nature, no deaths having been reported.”

But the Spanish flu didn’t remain mild, to say the least. As summer became fall, the flu—which had at first seemed “benign as the common cold,” according to a Stanford University report—changed. People began to die around the world in huge numbers, most likely due to a mutation in its makeup, making it more virulent.

The Spanish flu became known for its high mortality rate compared to previous influenza outbreaks, and its quick onset. One anecdote from 1918 involved four women playing bridge late into the night; overnight, three of them died from the flu.

Could the current outbreak of swine flu follow a similar pattern? After being declared a somewhat low risk to the world population, could it become more virulent? Or will the flu behave more like it did during a1976 outbreak among 500 soldiers (only one of whom died) at Fort Dix, N.J. ?
According to a 2003 Washington Post article that compared the 1918 flu to SARS, both the Spanish flu and SARS appear to have arisen from animal sources and gained the ability to infect humans “through mutation or genetic reassortment.” Swine flu undoubtedly arose from an animal source—pigs—and now is able to spread not only from animal to human, but from human to human.

Time will tell how the swine flu will pan out, but if it follows the course of its distant relative, the 1918 influenza, the results could be disastrous. According to Bloomberg, which compared the two, “A flu pandemic that’s similar in scope to the 1918 pandemic could kill 71 million peopleworldwide and push the economy into a ‘major global recession’ costing more than $3 trillion.”