The 2009 H1N1 influenza outbreak was officially elevated to pandemic status on June 11, 2009.
Earlier this month, on August 10, 2010, the World Health Organization (WHO) declared it officially over.
What does this mean?
To many people, both the start and end dates seem somewhat arbitrary. Indeed, there are undoubtedly some who think the outbreak never should have been declared a pandemic at all. Certainly, there are many local communities seemingly untouched by 2009 H1N1.
As with much in public health, to understand these determinations, it is useful to consider the Big Picture.
Pandemics have three basic defining characteristics:
- They must involve the emergence of a novel, infectious microbe to which people have had no previous exposure and therefore little to no natural immunity. In this case, “novel” means that there is not a documented history of the disease having infected people or the disease hasn’t infected people in such a long time that it unlikely anyone will have immunity.
- They must be pathogenic; that is, they must have potential to make people sick, and often significantly so.
- Last, but not least, pandemics must involve sustained human-to-human transmission over a large geographic area. (The official WHO criteria for a Phase 6 pandemic—the highest level—requires sustained community level outbreaks in at least two countries in one WHO region, plus at least one other country in another region.)
The 2009 H1N1 virus handily meets all three criteria. It was clearly a unique influenza subtype with its association with swine-origin viruses. It had significant morbidity and mortality: almost 1.5 million cases worldwide, including roughly 45,000 laboratory-confirmed US cases; and just over 25,000 deaths worldwide, including 10,837 US deaths. And by the end of the pandemic this month, there had been sustained human-to-human transmission in dozens of countries all over the world. In fact, during the pandemic, 2009 H1N1 crowded out other influenza viruses to become the dominant strain, infecting 20-40% of populations in some areas.
In this post-pandemic period, a significant chunk of the world population has now been exposed to the virus—either directly or through vaccination—and thus has some immunity. Rates of sustained transmission are generally leveling off or falling off. Many countries are reporting a mix of circulating flu viruses….. In other words, 2009 H1N1 is behaving more and more like a seasonal flu virus. In fact, now that it is no longer novel—meaning that it now has a well documented history of having infected people—it should never again qualify as a pandemic virus. But that doesn’t mean it has gone away.
Given the justified concern about a pandemic involving avian influenza, with its 50-60% mortality rate, 2009 H1N1 may, in retrospect, seem mild to some. This is good news, partly a result of effective public health intervention and, as stated by the WHO director-general, partly “pure good luck.”
Pete Shult, PhD, head of the Communicable Disease Division at the Wisconsin State Laboratory of Hygiene and an influenza expert, said that back in April 2009, health authorities had “no clue what was going to happen here.”
He said, “Did the government overreact? No. Absolutely not. I think we should take our just credit. We were able to diagnose [2009 H1N1] quickly. We were able to get vaccine out there. We were able to help the health community respond effectively. I’m not sure in the absence of those efforts this wouldn’t have been a worse pandemic. . . .You always have to plan for the worst case.”
And now that it’s over, what can we expect from 2009 H1N1 in the future?
Said Shult, “We’re in uncharted territory. We’ll see what happens.”