'Very eerie': This winter had virtually no flu

Is it possible for a pandemic to save lives?

The short answer is that it just did. Since swine flu first burst onto the scene one year ago this month, fewer Americans appear to have died of influenza-related causes than in any recent flu season.

The pandemic flu kept at bay seasonal strains that normally kill thousands of elderly people. And it did so, somehow, while not sticking around itself. The result: a winter flu season with virtually no seasonal flu, no pandemic flu, no flu of any kind, at least not yet.

"It is very eerie," said Gregory Storch, director of the infectious diseases division at St. Louis Children's Hospital.

But before you get all giddy with pandemic appreciation, consider the following:

No one knows why swine flu didn't return this winter. (Handwashing and vaccination probably helped but don't fully explain what happened.) Flus of pandemics past have always come back; it was just a question of when and how severe. And it could well be that as more months pass without any strains of flu, fewer people are developing immunity against them - and could get sicker next season.

As for the lower death toll, raw numbers don't tell the whole story. Yes, very few people died this winter, suppressing the yearlong tally. But the victims, mainly last spring and fall, were nearly all younger people with much of their lives ahead of them. In actuarial terms, this pandemic may turn out to have been more harmful than it seems.

Estimating deaths from any influenza is an inexact science. Most people succumb to underlying respiratory or circulatory conditions that are worsened by the flu, a word that may not even appear on the death certificate.

The standard estimate of 36,000 annual seasonal flu deaths is based on 10 seasons during which complex mathematical models found large variations from year to year.

Estimating swine flu deaths on short notice - without even death certificates to go by - is trickier. The latest approximation, through mid-February, is 12,000.

The numbers, both from the U.S. Centers for Disease Control and Prevention, were calculated using different methodologies, and they represent the midpoints of wide ranges that overlap slightly. Still, the disparity - one-third the usual number of deaths this year - is striking.

Drill down deeper, however, and the pandemic count shows "three to five times the number of pediatric deaths reported than normal," said David L. Swerdlow, a member of the CDC team that determined the statistics. Nearly 90 percent of pandemic-flu related deaths were people younger than 65. With seasonal flu, 90 percent are 65 and older.

In a paper published last month in the Public Library of Science, epidemiologist Cécile Viboud and colleagues attempted to account for age differences by calculating what are called "preventable deaths." Since everyone eventually dies of something, this technique - counting years of potential life lost, based on life expectancy - is offered as a better measure of disease burden than raw death counts.

It still determined that the pandemic has likely done less harm than an average seasonal flu.

Although counterintuitive, the finding is not unprecedented. A handful of regular flu seasons in the last century have been worse than the mildest pandemics.

"Pandemic" means widespread, not necessarily severe. The World Health Organization declared a pandemic in June because so much of the population had so little immunity to swine flu that there was a potential for disaster.

That same infectiousness explains the virtual disappearance of seasonal flu. Dozens of strains circulate every year, but only one - the most successful at causing the infections needed for the virus to reproduce - generally dominates in a given region and time of year.

In a normal season, the losers bide their time, often coming back another year in a slightly changed form that allows them to infect more humans and dominate other flu viruses. Seasonal vaccine is updated annually in response to these small changes.

Occasionally, bigger changes make a virus dramatically more successful at infecting humans, causing a pandemic and giving it more strength against the viral competition.

"The seasonal virus is slowly driven to extinction," explained Viboud, who studies flu transmission patterns at the National Institutes of Health. As the population develops immunity over several years, the winning virus falls into a seasonal pattern, and the vulnerable elderly again become its easiest target.

The new flu was already so dominant this winter that the virology laboratory at Children's Hospital of Philadelphia detected a seasonal strain in only two samples since September. The most recent hit, in March, was the only positive sample reported to the CDC that week nationwide.

New flus drove out the old in the pandemics of 1918, 1957, and 1968, too. Like this one, those pandemic flus, with their great power to infect, first appeared at times of the year that are not well-suited to influenza transmission.

Each of them also returned the following winter. Why didn't this one?

And, more mysteriously, how did this pandemic flu manage to suppress all other strains in absentia?

A season with almost no flu of any kind is not unheard of. But few scientists think it is a coincidence.

Public campaigns to encourage washing hands and coughing into sleeves may have helped. The CDC estimates that 59 million people were infected with swine flu through mid-February, nearly all of them in the spring and fall waves, and so were largely immune. And an estimated 72 million were vaccinated, although many of the same people would have been counted in both tallies.

Nearly 37 percent of children in Pennsylvania and nationally got the vaccine; in New Jersey, 33 percent did. "The kids are the reservoirs," said Richard L. Hodinka, director of Children's Hospital's virology lab, "and they have the worst personal hygiene."

Perhaps the pandemic's timing was slightly off, and then snow days intervened. Several regional increases last month, mainly in the South, prompted the CDC to urge continued vaccination.

"Never in my life has there been so little flu," said Annika Linde, Sweden's state epidemiologist, who has been responsible for tracking the disease since 1993.

Linde is one of a handful of scientists, mainly in Europe, who are floating a novel explanation. In addition to all the above, she said, it could be that the various microbes that cause respiratory-tract infections - rhinoviruses, respiratory syncytial viruses, influenza type A and type B viruses, among others - normally peak at different times of the year because they occupy niches that prevent other viruses from butting in.

Then the pandemic upended the pattern.

This interference idea was first suggested by a researcher in neighboring Norway.

Like many people, viruses "usually live in peaceful coexistence with each other," each with its own success strategy, said Gabriel Ånestad, now a senior scientist at the Norwegian Institute of Public Health. "If a foreign invader" such as pandemic swine flu appears, he said, the competition can create "a war situation."

Ånestad's hypothesis, largely ignored when he first published it 30 years ago, is still on the fringe. And it doesn't help him predict what swine flu will do next.

No one else can predict with any certainty either.

For example, in each of the three pandemics of the 20th century, most recently in 1968-69, the new virus outcompeted and then replaced the single dominant strain of seasonal flu.

For the last three decades, however, two type A strains have jockeyed for dominance. The H3N2 virus usually came out on top, and it caused the most widespread as well as the most severe illness.

The seasonal H1N1 strain, when it won, was usually mild, particularly for older people.

Most scientists believe that the pandemic H1N1 will replace the seasonal H1N1 because they are most similar. If that happens, then the two flus in future seasons theoretically could, on average, be worse than the experience of recent decades.

Is it possible for the pandemic virus to replace both seasonal strains?

"It is," said Viboud, the NIH scientist.

Would that be a good thing?

No one knows, she said. "We don't have any situation in the past that we can use as a template for what will happen."