The 1918 Influenza pandemic spread during 1918-1919, was the most severe in recent history.
It is estimated that about 500 million people i.e 1/3rd of the world's population infected with this virus. The number of deaths estimated to be at least 50 million.
The death toll due to COVID19 is 3.5 lakhs while you are reading this article ie 0.35million.
Compare this death toll with that of the Spanish flu which is around 50 million. Now you may understand the intensity of the 1918 flu.
About the Flu
The 1918 flu was caused by the H1N1 virus.
An unusual characteristic of this virus was the high death rate it caused among the healthy adults 15-34 years of age.
Due to this pandemic, life expectancy was suddenly dropped to 47.2 years.
Reason for High Deaths
Besides the high virulence(severity) of the virus itself, many additional factors contributed to the virulence of 1918 pandemic.
In 1918, the world was still engaged in World War-1. Movement and mobilisation of troops placed a large number of people in close contact and living spaces were overcrowded.
Health services were limited and as much as 30% of US physicians were deployed to military service.
No diagnostic tests, no Vaccines, not even Antibiotics(penicillin was discovered in 1928.)
In fact, doctors didn't know that the Influenza viruses existed. Many health experts thought that the 1918 pandemic was caused by a bacterium called "Pfeiffer's bacillus".
Discovery
As per the CDC, John Hultin, a 25-year-old Swedish microbiologist in 1951 conducted an expedition to Brevig Mission to find out the 1918 virus.
He was able to obtain some lung tissue from five dead bodies at a graveyard.
However, he was unable to retrieve the 1918 virus from the initial attempt.
In 1997, Taubenberger, a CDC microbiologist and co were able to extract the RNA of the 1918 virus.
In February 1999, a journal entitled "Origin and Evolution of the 1918 "Spanish" Influenza virus Hemagglutinin gene" was published.
In this, the authors tried to describe their effort to sequence the 1918 virus Hemagglutinin(HA) gene.
HA gene= Contains information about HA surface protein.
HA surface protein allows the Influenza virus to enter and infect a healthy respiratory tract cell.
In 1999, the authors succeed to determine the sequence of the full-length HA gene.
This HA gene sequence was noted to have mammalian(mammals) sequences opposed to avian(birds) adaptations.
This virus was placed in a mammalian clade in accordance with their evolutionary development.
A follow-up paper published in June 2000, entitled "Characterization of the 1918 "Spanish" Influenza Virus Neuraminidase(NA) gene".
NA gene= Contains information about NA surface protein.
NA surface proteins allow an Influenza virus to escape an infected cell and infect the other cells after replication. Therefore it plays an important role in spreading the virus.
The authors were able to code the entire sequence of the NA gene. It shared many sequences and structural characteristics with mammalian and avian strains.
Phylogenetic analysis suggested that NA gene of this virus intermediately located between mammals and birds, suggesting that the virus strain evolving into mammals from birds(introduced into mammals from birds just before 1918 leading to a pandemic).
Therefore the ultimate source of the 1918 virus was avian in nature.
Virulence
During the reconstruction of the 1918 virus, they found that it was highly lethal and HA gene played a large role in its severity.
IRAT- Influence Risk Assessment Tool
Measures the risk of a novel virus in humans that would result in human to human transmission.
Used to find the potential of the virus impacting public health, if the virus gains the ability to spread efficiently.
Thus, IRAT helps for Pandemic preparedness.