Near the end of World War One, a nasty flu started spreading around the world. The virus responsible for the disease, which became known as Spanish flu, infected over a quarter of the world’s population. With an estimated death toll of between 50 million and 100 million, it was one of the deadliest pandemics in human history.

In the midst of this pandemic, during September 1918, cities around the US were planning parades to promote liberty bonds, being sold to help pay for the war effort in Europe. In Philadelphia, Pennsylvania, where 600 soldiers were already infected with the flu virus, city chiefs decided to go ahead with their parade. Meanwhile, the city of Saint Louis, Missouri, opted to cancel their parade and introduce other measures to limit public gatherings. One month later, more than 10,000 people in Philadelphia had died of Spanish flu, while the number of fatalities in Saint Louis stayed below 700.

The parade was not the only reason for the difference in death rates, but the figures show the importance of measures now known as “social distancing” can have during pandemics. “Social distancing refers to a way of creating a barrier of physical distance between two or more people so that transmission of virus can be prevented or halted,” says Arindam Basu, associate professor of epidemiology and environmental health at the University of Canterbury, in New Zealand.

You might also like:

● Will hot weather change the coronavirus pandemic

● How long does the coronavirus survive on surfaces

● The right way to work from home

One analysis of the interventions made in several cities around the US during 1918 showed that those that banned public gatherings, closed theatres, schools and churches early had far lower peak death rates.

Just over 100 years later, the world is facing another pandemic, this time from a different virus – the Covid-19 coronavirus. Today the global population stands six billion higher than it did in 1918. While Covid-19 is different in many ways from the Spanish flu – particularly in terms of who it affects and its mortality rate so far – there is a very important lesson about the difference social distancing can make. It might still be one of the best ways of fighting this pandemic. (Read more about the other lessons we can learn from Spanish flu.)