COVID-19 has the potential to be as deadly as the 1918 flu

The Walter Reed Hospital flu ward in Washington D.C. treated patients during the 1918 flu pandemic.

The COVID-19 pandemic has the potential to become more deadly than the 1918 flu pandemic, a new study suggests.

Remembered as the deadliest pandemic in recent history, the 1918 influenza pandemic infected one-third of the world’s population and killed at least 50 million people, 675,000 of them in the U.S., according to the Centers for Disease Control and Prevention (CDC). At the time, there were no vaccines and no antibiotics to treat secondary infections that inevitably popped up in flu patients. However, society employed measures like the ones being used today to curb the spread, such as imposing quarantines and endorsing good personal hygiene, according to the CDC.

Over a century later, the world has found itself in the grip of another pandemic, this one caused by the coronavirus SARS-CoV-2 — and it’s still not clear just how deadly it is. The virus has now infected at least 22.2 million people and has claimed at least 783,525 lives worldwide, according to the Johns Hopkins dashboard.

To compare the current pandemic with last century’s, a group of researchers focused on a sliver of the world that was hard hit by both viruses. In the spring, SARS-CoV-2 hammered New York City, causing more than 19,000 known deaths — and more than 4,600 probable deaths or those that were likely caused by COVID-19 but there aren’t positive laboratory tests to confirm, according to the NYC Department of Health and Mental Hygiene.

Using data from the CDC, the New York Department of Health and Mental Hygiene and the U.S. Census Bureau, the researchers compared the mortality rate in New York City during the early COVID-19 outbreak with the mortality rate during the peak of the 1918 H1N1 influenza pandemic.

The researchers specifically analyzed deaths from all causes in New York City in October and November of 1918 — the peak of the influenza pandemic in the city — and compared them with all-cause deaths in the same months since 1914. They then calculated all-cause deaths for March 11 to May 11 of this year in New York City, when the COVID-19 outbreak peaked and subsided in New York. The time periods they compared were each 61 days long.

The researchers chose to compare the early outbreak in NYC with the peak of the 1918 flu, rather than with the milder influenza wave that hit in the spring of 1918, so that “people can get context for how serious,” this modern outbreak is, said lead author Dr. Jeremy Faust, an emergency physician at the Brigham and Women’s Hospital in Boston and instructor at Harvard Medical School.

They found that during the peak of the 1918 influenza outbreak in New York City, a total of 31,589 all-cause deaths (this included death from any cause) occurred among the 5.5 million residents that lived there at the time. The all-cause mortality in the peak of the influenza pandemic in 1918 was 2.8 times higher than during the same months in previous years.

In contrast, for the early 2020 COVID-19 outbreak in New York City, they found that 33,465 deaths from all causes occurred among 8.28 million residents between March 11 and May 11. The all-cause mortality in those months of 2020 were 4.15 times higher than those months between 2017 and 2019.

That means that in the peak of the 1918 influenza pandemic in NYC about 287 per 100,000 people died a month from any cause in NYC, whereas during the early COVID-19 outbreak, about 202 per 100,000 people died a month in the city. So the all-cause mortality during the spring of 2020 was 70% of the all cause mortality during the fall of 1918. “When we do that, we see that COVID-19 really does have the potential and has already unfortunately caused per capita death rates that were in the same ballpark,” Faust told Live Science.

But there’s another way to look at the deaths related to each pandemic: comparing deaths during a pandemic to the baseline that you’d expect during a particular time. There were more “excess deaths” during the 1918 flu than the early COVID-19 outbreak. But in relative terms, the COVID-19 outbreak in the spring actually looks worse, because the numbers quadrupled from pre-pandemic times (from a baseline of around 50 deaths per 100,000 people per month), whereas in the peak of the 1918 flu, the numbers less than tripled (from a baseline of around 100 deaths per 100,000 people per month).

“Its a bigger shock to our system, but that’s a little bit unfair because we started off at a lower death rate,” than there was in 1918, due to advances in hygiene, medicine, public health and safety, Faust said. Really, we don’t yet know if the 1918 pandemic or the COVID-19 pandemic is more deadly, he added. Maybe what happened in New York in the spring was a “freak thing,” before interventions such as masks and shutdowns took hold; or maybe the numbers will slowly creep up to match the death tolls seen in the 1918 flu until an effective vaccine is found.

One limitation of the study is that it’s not possible to directly compare how infectious and harmful the two viruses are to people and it’s unknown how many deaths from SARS-CoV-2 were prevented because of modern interventions that weren’t available a century ago, the authors wrote.