After weeks of edging its way toward the declaration, on Thursday the World Health Organization (WHO) raised its swine flu alert level to the highest phase. A pandemic is under way.

What that means is that the flu virus designated as influenza A (H1N1) has proved itself capable of global spread by proliferating not just in the Americas, where it was first identified in April, but also in far-off Australia, where cases mushroomed to well over 1,200 this week. But the announcement of a full-blown pandemic doesn’t mean that this flu will be any more deadly than that all-too-familiar affliction, the seasonal flu.

In fact, public health authorities like the WHO are wrestling with a challenge they didn’t exactly anticipate in the stepped-up pandemic planning of the last several years: how to communicate with the public about a scary-sounding pandemic that, in terms of its ability to take lives, appears to be relatively mild. So far.

Tens of Thousands of U.S. Cases Expected

Days before the declaration, as reporters in a briefing pressed WHO Assistant Director General Keiji Fukuda, M.D., to explain why the organization had taken so long to declare the pandemic, given clear evidence of global spread, the official cited concern about “adverse effects,” including undue panic and ill-advised restrictions on travel and trade.

Cases of the swine flu, which combines genetic elements of pig, bird and human viruses, have mounted to some 28,000 worldwide, with more than 140 deaths attributable to the virus, most of them in the United States and Mexico. David Weber, M.D., a specialist in infectious disease at the University of North Carolina, Chapel Hill, says he suspects the United States will see "tens of thousands of cases” before the bug winds down for the season in late June or early July.

How Deadly is the Swine Flu?

It’s much trickier to define the risk of serious illness or death from this flu. To arrive at a proportion, you need an accurate denominator—in this case, the total number of people infected—and with testing now confined in many places to the sickest patients, and many more recovering at home without medical attention, that number is elusive at best.

In New York City, an epicenter of the disease and site of widespread school closures in May, some 530 people were hospitalized and 12 died from H1N1 between late April and mid-June. But a survey by the city health department found that 7 percent of all New Yorkers—some half a million people—had flu-like symptoms in May, suggesting that H1N1 infection may have been rampant during that time, with severe consequences for only a tiny fraction of sufferers.

Certainly the death rate from swine flu is a far cry from the estimated 60 percent who succumb from identified cases of the avian flu, which has stalked Asia (but which is not easily spread among humans). Eighty-three American children—more than half the number of people worldwide who have died from swine flu thus far—died from regular seasonal flu during the 2007-2008 season alone.

A Pandemic Just Beginning

But public health experts caution that the world debut of H1N1 is far from over. “I think everyone feels that you can’t count it out yet,” says Jeffrey Levi, executive director of Trust for America’s Health, a nonprofit public health organization that recently issued a report on lessons from the swine flu epidemic. “We need to see what’s going to happen in the Southern Hemisphere over the winter and how it returns in the fall,” says Levi. “I don’t think anyone believes we’re out of the woods yet.”

The emergence of new cases in such places as Australia and Chile suggests that the virus could continue to spread in the Southern Hemisphere during its winter flu season, which is only now beginning. That means it could return to the United States and other northern climes next fall. And there’s always a chance that it could return in a more virulent form, having evolved or picked up new genetic elements from other circulating viruses.

So far, however, the virus has not displayed troubling changes. Peter Palese, chair of microbiology and an expert in viral infectious disease at Mount Sinai School of Medicine, in New York, says H1N1 lacks an important protein that was present in other lethal pandemics. “If you have a car that’s missing a tire, you won’t be able to drive very far,” he says.

Earlier Pandemics

Still, the worry is not without precedent. There were three influenza pandemics in the 20th century—in 1918, 1957 and 1968—and each involved successive waves of flu spread over a period of two to five years. It was a follow-up wave of the 1918 flu that decimated enormous swaths of the population. Another concern is that while developed countries are well girded against such a terrible outcome—with effective antiviral medications, plenty of basic medical supplies like antibiotics to treat secondary infections, the capability to produce an effective vaccine and a highly developed communications apparatus—poor countries are much more vulnerable.

Young Most Vulnerable

The H1N1 flu is also markedly different from regular seasonal flu—and more like historical pandemic flus—in its disproportionate attack on teenagers and young adults rather than older people. According to WHO, the median age of those affected ranges from 16 to 25. While public health agencies have tended to emphasize that severe illness has occurred mostly in people with underlying health problems like asthma, diabetes and heart disease, or risk factors like pregnancy, other experts have not. The WHO’s Fukuda conceded this week that roughly half of H1N1 deaths have been in previously healthy people, a fact that “continues to give us a great deal of concern.”

Older People May Have Immunity

People over age 50, considered the prime targets of seasonal flu, make up only about 5 percent of H1N1 cases. Those over age 65 make up only 1 percent of those falling ill. This may be partly because older people have different patterns of movement that have delayed their contracting swine flu. But there’s strong evidence that they may actually enjoy some immunity to it. A study by the U.S. Centers for Disease Control and Prevention (CDC) of stored blood samples found that a third of samples from people over age 60 produced antibodies to the new virus. This is the benefit of “immunological experience” with long-ago viruses more similar to H1N1 than those circulating today, says Palese. “In essence we have been exposed to descendants of the 1918 [pandemic flu] virus for 85, 90 years now.”

This immunity may mean fewer infections, and perhaps even milder cases, among older people, but it doesn’t prevent infection and may not apply to everyone in that age group. “A healthy elderly person is probably more protected,” says Palese. “But that is not true for someone who, for example, is an asthmatic or a diabetic.”

How the virus is spreading and perhaps changing, how dangerous it is and who is most vulnerable are all questions the CDC and other health authorities will try to work out in the coming months.

Vaccine in the Works

Getting a vaccine developed, tested and ready to administer in the fall is a particular challenge. The federal government has already committed $1 billion for a vaccine to be used in clinical trials this summer, as well as for bulk ingredients necessary for larger-scale commercial production. The actual development and production of such a vaccine is relatively straightforward, says Levi, but capacity is such that the vaccine will only become available on a rolling basis. The government will have to figure out who gets the vaccine first, and whether they should get the regular seasonal vaccine at the same time.

Implementing a complex vaccination program on a tight schedule is a steep hill to climb in a world where many high-risk groups regularly fail to get their flu shots. The CDC estimates that in the 2006-2007 season, only two-thirds of people over 65 got their regular flu shots, and little more than a third of those 50 to 64 were immunized against flu.

Can Emergency Rooms Cope?

The capacity of emergency rooms and doctors’ offices to handle a bigger surge in flu cases is also a question, according to the report by Trust for America’s Health. This spring’s “low-level ‘stress test’ ” flooded some emergency departments with the worried well and people with mild symptoms, while government laboratories were hard-pressed to keep up with the deluge of samples they received during the epidemic’s early weeks.

Levi’s group recommends a plan for alternative triage centers where people could go to get screened.

On the whole, though, expert observers give the CDC and local health departments high marks for devising a lucid, flexible communications system that’s left most Americans disinclined to point fingers or panic over the new flu. Yale University medical historian Naomi Rogers points out that despite the association of the word pandemic with a “scary, global killing disease,” the CDC and other sources of reliable information have been effective at tamping down stigmatizing misnomers like “Mexican flu” and in stemming public fear.

As Anne Schuchat, M.D., director of the CDC’s National Center for Immunization and Respiratory Diseases, remarked earlier this month, “We really aren’t even looking at a three-month experience yet with this particular virus. We need to remain humble and learn as we go.”

Katharine Greider writes about medical and health issues.