When the novel H1N1 flu virus began to appear in North America and Europe in Spring 2009, it contained some worryingly familiar signs to flu experts. The new strain appeared suddenly in a season when flu typically declines, spread at a rapid pace, and seemed to disproportionately affect the young more than the old. The last influenza to display those features was the notorious 1918 flu, which killed as many as 100 million people around the world before burning out a year later.
“It was the most devastating infectious diseases episode in world history,” said Michael David, Instructor of Medicine at the University of Chicago Medical Center. “In numbers, it was probably 10 to 100 times more severe in terms of the absolute number of people killed than were killed in The Black Death.”
Of course, last year’s H1N1 pandemic was nowhere near as deadly, causing only an estimated 12,500 deaths in the United States despite approximately 60 million infections. The low mortality among elderly populations from H1N1 may have actually made the 2009-10 flu season less deadly than usual, as the Centers for Disease Control and Prevention estimate a yearly average of roughly 36,000 influenza deaths. But comparing 1918 to 2009 still reveals interesting similarities, David said in his October 14 talk at the Department of Pediatric Grand Rounds.
In the spring, when the virus first showed up on public health radar as a novel strain with all the right ingredients for a pandemic (jumped from animal to human, easily transmissable), the worst case scenario of 1918 couldn’t be ruled out. Like the 2009 strain, the 1918 influenza also made a relatively modest appearance in the spring, David said – graphs of the pandemic’s death rate revealed a small spike in the summer. Come October, that mild hill was overwhelmed by the shocking spike of influenza deaths that raged across the United States and Europe. In 8 weeks, 25 million people were infected with the virus, and some 600,000 died – in the U.S. alone.
“That’s more than the number of soldiers that were killed on both sides in the U.S. Civil War,” David said. “It’s something that’s really hard for us to grasp with our imaginations.”
[If you have a JAMA subscription, you can read this 1918 first-hand account of the pandemic at Cook County Hospital in Chicago – “During the past five weeks, more than 2,000 patients were admitted to the hospital. The disease is highly contagious and the mortality among our patients has totaled 31 percent. The epidemic has seriously crippled the medical and most especially the nursing staff of our hospital.”]
In addition to its ferocious spread and mortality rate, the 1918 influenza was also unusual for the victims it chose: 20 percent of the deaths were in children under the age of 5, and 15 percent were between 20 and 25 years old. The line formed by these two mortality peaks combined with deaths in the elderly formed the pandemic’s characteristic “W curve” (pictured above), in contrast to the usual “U curve” seen when only the very young and old die from influenza.
Despite the far lower mortality rate in 2009, the “W curve” was echoed in the “attack rate” of the 2009 pandemic, David said. Children came down with the virus far more than older groups, while the elderly population normally most affected the seasonal flu had a relatively easy fall and winter. In Chicago, children between the age of 5 and 14 were 14 times more likely to get sick with the H1N1 flu than those above the age of 60.
“We saw this familiar pattern that is suggestive of the phenomenon we saw in 1918,” David said. “This was a very unusual season in 2009.”
So what saved us from a re-run of 1918? The scientific jury is still out, though a number of factors have been proposed by scientists. Older people may have been exposed to similar viruses in their youth, offering partial immunity; a Finnish study found that 96% of people born between 1909 and 1919 had antibodies against the 2009 H1N1 virus. Advances in disease surveillance, antiviral medication, public health measures and vaccination may have softened the blow, David said, but vaccination rates among children for H1N1 were distressingly poor in 2009: only 30 percent. Given that schoolchildren were determined to be “the engine driving the fall ’09 wave,” that’s a worrisome figure in case the next pandemic is more 1918 than 2009.