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In 1918, the city of Philadelphia threw a parade that killed thousands of people. Ignoring warnings of influenza among soldiers preparing for World War I, the march to support the war effort drew 200,000 people who crammed together to watch the procession. Three days later, every bed in Philadelphia鈥檚 31 hospitals was filled with sick and dying patients, infected by the .

By the end of the week, more than that would claim as many as 100 million people worldwide. By the time Philadelphia鈥檚 politicians closed down the city, it was too late.

A different story played out in St. Louis, just 900 miles away. Within two days of detecting its first cases among civilians, the city closed schools, playgrounds, libraries, courtrooms, and even churches. Work shifts were staggered and streetcar ridership was strictly limited. Public gatherings of more than 20 people were banned.

The extreme measures鈥攏ow known as social distancing, which is being called for by to mitigate the spread of the novel coronavirus鈥攌ept per capita flu-related deaths in St. Louis to of those in Philadelphia, according to .

The concept of 鈥渇lattening the curve鈥� is now to epidemics, including the . Once a virus can no longer be contained, the goal is to slow its spread. Exponential growth in infections leaves health care systems . But with fewer people sick at once (and overall), services aren鈥檛 overwhelmed and deaths diminish. This buys time for doctors to treat the flood of patients and researchers to develop vaccines and anti-viral therapies.

But it wasn鈥檛 always this way, says Richard Hatchett, a physician and head of the Coalition for Epidemic Preparedness Innovations in London, who co-authored the 2007 paper. Social distancing interventions were not always trusted, he wrote in an email; they were widely ignored during flu pandemics in 1957 and 1968. But in the 2000s, several papers including Hatchett鈥檚 reanalyzed Spanish flu data to show the efficacy of distancing measures鈥攁nd the U.S. Centers for Disease Control and Prevention later incorporated them into their outbreak guidance.

But in the 2000s, several papers including Hatchett鈥檚 reanalyzed Spanish flu data to show the efficacy of distancing measures鈥攁nd the U.S. Centers for Disease Control and Prevention later incorporated them into their outbreak guidance.

The key to effective social distancing, though, is timing.

鈥淚 think the critical lesson from both the modeling and the historical work is that the benefits of multiple interventions are greatest if they are introduced early (before 1% of the population is infected) and maintained,鈥� wrote Hatchett, who has also directed medical preparedness in the Obama White House. Distancing measures are less effective once more people have contracted the virus, especially in cases where the vast majority of people are not sick enough to need medical attention.

This outbreak is one of those cases. Only 19% of confirmed cases of COVID-19, the disease caused by the new coronavirus, become severely or critically ill, . Those with mild symptoms (or none at all) may easily pass the virus on to , particularly those who are older or have pre-existing health conditions. 鈥淵ou can compare the outcomes in , and which used such interventions aggressively from the very start, with what happened in and what is happening now in and ,鈥� Hatchett wrote. 鈥淭here is no reason to expect the virus to behave differently in Europe and the U.S. than it has in Asia.鈥�

China and Italy may have waited too long; both were forced to take drastic steps weeks after the first cases were discovered. China鈥檚 government locked down nearly 60 million people in Hubei province while restricting travel for hundreds of millions of others. Now Italy is for 60 million residents, a .

In the U.S., these restrictions are just beginning. New York state has and deployed National Guard to disinfect buildings and deliver food in a 鈥渃ontainment zone鈥� in New Rochelle, just north of New York City.

But social distancing doesn鈥檛 have to be draconian. South Korea has adopted a modern version of the St. Louis model; the country never locked down its residents or quarantined entire cities, but has still managed to slow the spread of the new coronavirus. In recent days, new infections have , thanks in part to thousands of free daily tests and a that closed schools, canceled public events, and supported flexible working arrangements. 鈥淲ithout harming the principle of a transparent and open society,鈥� South Korea鈥檚 Vice Health Minister Kim Gang-lip in the South China Morning Post, 鈥渨e recommend a response system that blends voluntary public participation with creative applications of advanced technology.鈥�

This article was originally published by . It has been published here with permission.