A century after the world’s deadliest influenza pandemic killed an estimated 50 million people, scientists estimate a similar global outbreak could be nearly three times as deadly. To find out how prepared today's society is to handle such a dangerous public health issue, a team of researchers analyzed influenza studies to see what factors made the 1918 Spanish Flu was so virulent.

"Like the 1918 pandemic, the severity of any future outbreak will result from a complex interplay between viral, host and societal factors," said Dr Carolien van de Sandt in a statement. "Understanding these factors is vital for influenza pandemic preparedness."

The Spanish Flu was first detected in the Spring of 1918 and rocked the world in a series of waves again in Autumn and during the Winter season. Today, it is believed the flu originated in the Midwest of the US and spread through the country before hitchhiking on soldiers during the First World War and infected as much as one-third of the global population.

Publishing their work in Frontiers in Cellular and Infection Microbiology, the authors note that it is impossible to know where or how the next pandemic will emerge, but how these factors influenced the 1918 influenza season will help us to better prepare for the next.

For starters, the H1N1 strain itself was particularly deadly. For reasons we still don’t fully understand, the virus had certain mutations that made it more transmissible between people. Once it had infected a person, the virus was then able to spread to other tissues beyond just the respiratory tract making it more able to wreak havoc on its host. The 1918 virus most affected young adults, who are normally the most resilient. The authors note that older people were probably spared because they had more immunity to the strain after having developed immunities to other viruses. However, the seasonal flu typically kills the very old, and an aging population could be cause for more concern in a future pandemic.

The overall death rate was as high as 5 per 1,000 individuals, though that number varies greatly between countries. Population demographics between these countries played a huge role in how fatal the flu was. Underlying bacterial infections that would have already compromised a person’s immune system, such as strep and staph, were common and often left untreated. As antibiotic resistance becomes more common today, bacterial infections could see a similar spike, making people who have health issues or infections more susceptible to the flu.

Climate change may also influence the virus’ ability to kill more. Malnutrition during the 1918 pandemic left already weakened people at a higher risk to die from an infection. As the world’s changing environment could impact crop yields, malnutrition could again make an appearance and make vulnerable infection less likely to fight off infection.

So where does that leave us now?

“An understanding of past influenza virus pandemics and the lessons that we have learned from them has therefore never been more pertinent,” write the authors.

Providing emergency vaccines during future pandemics should address how these factors play together to create more vulnerable populations. Public awareness measures need to address transmission causes and preventions.

"Until a broadly-protective vaccine is available, governments must inform the public on what to expect and how to act during a pandemic," said van de Sandt. "An important lesson from the 1918 influenza pandemic is that a well-prepared public response can save many lives."