Americas especially hard hit by 2009 swine flu, study finds

H1N1 swine flu caused a pandemic in 2009. A new study shows an unusual pattern of deaths.
H1N1 swine flu caused a pandemic in 2009. A new study shows an unusual pattern of deaths. Cynthia Goldsmith

A new look at the 2009 pandemic of H1N1 swine flu finds an unusual pattern — more people died in the Americas than in the rest of the world. What’s not clear is why.

The study, originally started to get a final estimate of global deaths from the H1N1 virus, shows about 200,000 people died. And it confirms most of them were young adults and children — just the opposite pattern from seasonal flu, which tends to kill people 65 and older.

But what was intriguing was the global pattern, says Lone Simonsen of George Washington University, who led the study published in the Public Library of Science journal PLoS Medicine.

“It was the whole of the Americas,” Simonsen told NBC News. “All over the Americas, we tended to measure a higher burden than in Europe.” For example, the death rate in South America was 20 times that seen in Europe.

Why? Some studies have suggested that people with diabetes were far more likely to die during the H1N1 pandemic, but Simonsen said the pattern doesn’t support that.

H1N1 was first detected in Mexico in 2009. It was a new version of the flu virus, a very distant descendant of the H1N1 flu that caused the deadly 1918 pandemic. Could the Americas have suffered because the virus started there and mutated into something milder as it spread?

No, says Simonsen — because most of the deaths in Mexico, for instance, came in the so-called third wave of the pandemic, months after it first started spreading and after it had gone around the world already.

It may have to do with small differences in viruses that infected people in earlier years — maybe something slightly different was circulating in Europe, she says.

The study is the latest to show that influenza can be baffling. There are several strains of flu virus, and it’s especially prone to mutation. That’s why it spreads every year, and why the vaccine must be changed annually to match what’s out there. Even so, small changes make the virus tricky and it can infect people who have been vaccinated.

Seasonal influenza is bad enough — it kills between 3,600 and 49,000 people in the United States in a year and between 250,000 and 500,000 people a year globally. But these are just estimates — often flu deaths aren’t reported as flu deaths, so scientists must estimate.

Simonsen’s team gathered officials in 20 countries representing a third of the world's population to really dig down into the numbers to get a very good look at the 2009 pandemic — the first flu pandemic the world had experienced since 1968.

“The World Health Organization reported 18,631 laboratory-confirmed pandemic deaths, but the total pandemic mortality burden was substantially higher,” her team wrote. “We estimate that 2009 global pandemic respiratory mortality was 10-fold higher than the World Health Organization’s laboratory-confirmed mortality count.”

This fits in with what other groups have reported, using different methods. 

It sounds like a mild year, but Simonsen says it wasn’t. “These are not deaths in age groups where you are expecting them,” she said. “These are deaths of young adults and children. It is different from seasonal influenza.” H1N1 killed 282 U.S. children in 2009-2010, the Centers for Disease Control and Prevention says.

CDC says H1N1 infected 61 million Americans during the pandemic and killed around 12,000. H1N1 is now part of the annual seasonal flu mix.

Studies suggest that people over 50 or 60 may have been protected from H1N1, perhaps because they were infected with something similar in their childhoods.

“Understanding the global mortality impact of pandemic influenza — who died, where, and when — is fundamental to understanding how pandemics emerge and evolve, and will help to guide responses to future pandemics,” Simonsen’s team wrote.

Researchers are keeping an eye on two evolving flu strains now — the H7N9 avian flu virus in China, and H5N1 bird flu, found in several Asian and Middle Eastern countries. Both jump only occasionally from birds to people, but they have extremely high death rates when they do and it’s feared if they mutate in just the right way, either or both could cause a pandemic.

But the emergence of H1N1 shows that the next pandemic can come from a completely unexpected place.