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H1N1 flu pandemic was deadliest for younger adults

A vendor throws a chicken into a bucket in the process of slaughtering them at a poultry market in Wuhan, Hubei province.

The first analysis of the complete U.S. death records for the 2009-2010 H1N1 influenza pandemic was published Wednesday. It confirms what was widely observed at the time: The pandemic was deadliest for people under the age of 65.

In fact, among people over the age of 65, the flu-related death rate actually declined during the pandemic.

The University of California, Irvine, researchers who authored the study say the most likely reason older people were protected from H1N1 was because their generation had been exposed to related influenza viruses that were in circulation between 1918 and 1957. That earlier exposure gave them immunity.

Usually, of course, it’s the elderly who must be most concerned about coming down with the flu. During normal influenza seasons, they are at greatest risk of dying from the illness.

The study’s authors also note that the mortality rates of other influenza pandemics of the past century — 1918-1919, 1957, and 1968-1969 — also skewed younger than seasonal influenza.

They recommend that vaccination efforts target younger people during future pandemics.

Study details

For the study, the UC-Irvine researchers used the National Center for Health Statistics’  final mortality data for the 2009-2010 H1N1 pandemic, which were released last August.

According to the data, 53,692 Americans died of complications of pneumonia and flu in 2009, and 50,003 died from those illnesses the following year. That made those combined illnesses the eighth leading cause of death in the United States in 2009 and the ninth leading cause in 2010.

Given that some 20 percent of Americans became infected with H1N1, this particular pandemic’s death rate was relatively mild. The UC Irvine researchers estimate that during the pandemic there were 2,634 “excess” flu-related deaths — ones above those that would be expected in a normal flu season.

But a further analysis revealed that those deaths had occurred disproportionally among younger adults. Among people aged 25 to 64, the excess death rate was 1.325 per 100,000. Among those aged 65 and older, the excess death rate was 0.228 per 100,000.

In addition, for reasons unknown, the H1N1 death rates was higher for women under the age of 55 than for men.

“One of the biggest challenges of pandemic preparedness is rapid formulation and manufacture of a strain-specific vaccine,” the study’s authors conclude. “Our analysis suggests that younger ages (25-54) should be prioritized in the event of a pandemic. … The mortality data for the 2009 pandemic do not provide the last word, but do suggest that age-targeted vaccination is a strategy well worth considering.”

China’s ‘bird flu’

Interestingly, the current H7N9 “bird flu” that has so far struck at least 131 people in China (36 fatally) appears to have an age-distribution that is more similar to seasonal flu than to a pandemic, according to a study published Wednesday in the New England Journal of Medicine. But, write the study’s authors, that finding may be because “retired persons have more opportunities to shop in live-animal markets and are therefore more likely to be exposed to live poultry.”

So far, almost every person in China who has contracted H7N9 was in direct contact with poultry before becoming ill. That may change, of course, as the virus mutates.

The UC-Irvine study was published online in the open-access journal PLOS ONE, where it can be read in full. The New England Journal of Medicine study is also available in full on that journal’s website.

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