Quotes of the Day

Few global health decisions have created quite as much commotion as that on April 29, when the World Health Organization (WHO), responding to the escalating spread of the H1N1 flu, raised its pandemic alert level for the first time to phase 5, meaning that a full pandemic was considered imminent. As of May 11, the WHO has reported more than 4,600 cases in 30 countries — including 2,600 cases in nearly every state in the U.S., according to the Centers for Disease Control and Prevention (CDC) — and the threat level remains at phase 5.

But over the past two weeks, fears over H1N1 have cooled considerably, as the virus has failed to spread easily outside North America and the number of deaths from the disease has remained low, leaving the WHO fending off critics who questioned whether the international agency overreacted. "We know that we are seeing things change on an almost daily basis," said Keiji Fukuda, the WHO's interim director for health, safety and the environment. "It is still a confusing situation." (See pictures as the swine flu hit Mexico.)

Now new research suggests that the WHO acted wisely in raising the pandemic alarm — and that the threat of H1N1 may not have passed. In a study released May 11 in the journal Science, researchers from Imperial College London, along with WHO staff and Mexican scientists, conclude that H1N1 is transmitted considerably easier than the regular seasonal flu and is about as deadly as the 1957 Asian flu, which killed about 2 million people worldwide. A World Bank study last year found that a pandemic of similar severity today might kill 14.2 million people around the world, and cut 2% from the global economy.

The authors of the Science paper based their findings on early data from the H1N1 outbreak, estimating that about 23,000 people had been infected in Mexico by late April, with a fatality rate of about 0.4%. Those numbers come with a wide margin for error on either side, and there are still holes in the epidemiology that need to be filled, but the consensus is that the WHO's handling of H1N1 was reasonable. "Our research indicates that the WHO was justified in its actions in the early days," says Christophe Fraser, an epidemiologist at Imperial College and the lead author of the paper. "There was no overreaction there."

The Science study also indicates that young people — those who are generally the most resistant to the seasonal flu — are especially vulnerable to H1N1. It's still not clear why that is. It could be that older people are more likely to have contracted a virus similar to H1N1 in the past, which might give them some immunity. Or it may be that young people simply encountered H1N1 more often in the early days of the outbreak. The only thing that's clear is that young people are contracting it and they are getting sick. In the small Mexican town of La Gloria, where the virus appears to have originated in early February, H1N1 infected over 61% of the under-15-year-olds in the community. That's worrying because the 1918 Spanish flu, which killed up to 50 million people, was particularly virulent in the young.

As epidemiologists begin to crunch the data on H1N1, we should have a better idea of how it spreads — and how dangerous it might be. New studies published recently in the New England Journal of Medicine indicate that older people in the U.S. also appear to have escaped the virus — just 5% of U.S. patients with confirmed cases of H1N1 are 51 or older. Still, since health officials have so far focused mostly on outbreaks in schools, it's possible they are simply missing older cases. "This is an evolving outbreak and we're still learning how this virus works," said Fatima Dawood, a CDC epidemiologist.

One major question is what impact the virus will have in the next flu season, when the conditions could be ripe for the disease to spread rapidly. Another concern is what will happen in developing countries that haven't yet had to deal with H1N1. Rich countries like the U.S. can afford to spend millions on antivirals like Tamiflu, but in poorer nations, especially in those parts of sub-Saharan Africa where rampant HIV makes the population more vulnerable to secondary infections like flu, H1N1 will likely take a far greater toll. Indeed, health officials said last week that early evidence suggests underlying conditions — including asthma, heart disease, diabetes and tuberculosis — could make H1N1 patients more likely to land in the hospital or die. "That's the big uncertainty," says Fraser.

It's those uncertainties that keep the WHO on edge, even after most of the world has moved on from H1N1. The agency is looking to develop a new pandemic alert system that would reflect the potential severity of a new virus — as opposed to the current system, which registers only the transmissibility of a new virus, not how deadly it might be. That would be a good idea, although in the early days of a potential pandemic, there may not be time to wait and see how virulent a new pathogen is before alerting the world that it needs to respond.

"If countries had not been thinking about what to do in this kind of situation, the fact is we would have had much more confusion," said the WHO's Fukuda. "The key to being better off is to be as prepared as possible."

(See pictures of soccer in the time of swine flu.)

• Bryan Walsh
• A new study in Science says that while data is preliminary and conclusions still premature, global health officials accurately represented the ongoing pandemic threat