Can pandemics of the past act as playbooks for todays crisis?

The swings and roundabouts of the COVID pandemic feel new and strange, but this isn't the first time humanity has stared down a deadly viral disease.

The Spanish flu of 1918-19 has long stood out as a stark warning that devastating viruses can spring seemingly from nowhere and wreak havoc across the globe.

This pandemic killed as many as 50 million people worldwide over a period of approximately 18 months. Around 500 million people were infected - almost a third of the global population at the time.

So what was it that made the 1918 virus so deadly? Where did it come from? And where did it go?

Today's pandemic is mapped and documented in minute detail. We track its waves across the globe in real time, and our understanding of its internal machinery is developing rapidly.

But in 1918, the study of infectious pathogens was a new field.

How could scientists understand what had happened when the biggest puzzle piece, the virus itself, was missing?

It wouldn't be until 1999 that scientists were able to reconstruct the entire genome of the virus that caused the 1918 pandemic, using viral remnants preserved in the tissues of people who had died from the influenza.

Beginning in 2005, researchers at the Centres for Disease Control and Prevention (CDC) in the US used a pioneering technique called reverse genetics to grow copies of the 1918 virus's RNA genome inside human kidney cells. This zombie-like reanimated Spanish flu was then tested on mice.

By watching the virus in action, researchers were finally able to link its genetic structure with the patterns of disease.

It was not any single component of the 1918 virus, but instead "the constellation of all eight genes together", that made the Spanish flu "exceptionally virulent".

The CDC team were also able to establish, using gene sequencing and experiments in chicken eggs, that the 1918 virus likely first arose in birds before making the leap to humans.

So that covers off the first two big questions about the Spanish flu - where it came from, and why it was so deadly.

But what about the biggest - how did the pandemic end?

The Spanish flu pandemic certainly ended, but the virus that kicked it off did not. Instead, it gradually grew milder, morphing from its lethal beginnings into a much more placid sniffle.

This is a common progression for viruses, partly because the best evolutionary pathway optimises spread but leaves disease severity primarily to random chance.

This means that while nature selects for increased transmissibility, there's a good chance that lethality will fade.

But there is no absolute rule that dictates diminishing virulence, and there can certainly be surprising spikes along the way.

This was true of the Spanish flu, which ebbed and flowed in waves much like COVID has.

The scarily virulent 1918 form of Spanish flu is long gone, but its descendants still circulate as part of our seasonal flus. And while the general trajectory was towards becoming milder, the path wasn't linear - some later waves were significantly deadlier than their predecessors.

But if the Spanish flu's overarching pattern is followed by SARS-CoV-2, as many experts believe it will be, we can expect that while we'll never eradicate the virus, it won't always be a major public health threat.

People who lived through the Spanish flu years probably developed a degree of protection against its genetic "cousins".

Here is another lesson for current times: immunity is key. Each wave of the flu - or of vaccination - adds layers of immunity to the global population.

With COVID, whilst our immunity fades over a period of months following vaccination or infection, we seem to retain an increasing ability to ward off severe disease with each exposure.

As we wait for global immunity to build, public health measures remain vital in protecting us from the worst effects of the virus.

The importance of these measures was laid bare in the 1918-19 pandemic as well.

As the flu raged a century ago, people were asked to wear masks and adopt social distancing measures, just as we are today. And, just as today, a number of anti-mask advocates opposed these impositions.

But experts agree that these measures significantly dampened the death toll while immunity grew, and that they remain vital for us in the current pandemic for the same reasons.

When researchers resurrected and characterised the Spanish flu, they felt they'd taken significant steps towards safeguarding humanity against future outbreaks.

They believed their work would serve as a portent of nature's ability to produce future pandemics and help us to begin building our defences and public health capabilities.

But were we paying enough attention?

In a 2019 summary of their work on the Spanish flu, CDC researchers noted that despite advances in medicine and public health, "a severe pandemic could still be devastating to populations globally".

We will inevitably make it through this pandemic, though the length of the road ahead remains unclear. But when the next outbreak occurs, will we be any better prepared than we were this time?

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