London ? Earlier this year, bird flu panic was in full swing: The French feared for their foie gras, the Swiss locked their chickens indoors and Americans enlisted prison inmates in Alaska to help spot infected wild birds.

The H5N1 virus – previously confined to Southeast Asia – was striking birds in places as diverse as Germany, Egypt and Nigeria, and a flu pandemic seemed inevitable.

Then the virus went quiet. Except for a steady stream of human cases in Indonesia, the current flu epicenter, the past year’s worries about a catastrophic global outbreak largely disappeared.

What happened?

Part of the explanation may be seasonal. Bird flu tends to be most active in the colder months, as the virus survives longer at low temperatures.

“Many of us are holding our breath to see what happens in the winter,” said Malik Peiris, a microbiology professor at Hong Kong University. “H5N1 spread very rapidly last year. So the question is, was that a one-off incident?”

Some experts suspect poultry vaccination has, paradoxically, complicated detection. Vaccination reduces the amount of virus circulating, but low levels of the virus may still be causing outbreaks – without the obvious signs of dying birds.

“It’s now harder to spot what’s happening with the flu in animals and humans,” said Dr. Angus Nicoll, influenza director at the European Centres for Disease Control and Prevention.

While the pandemic has not materialized, experts say it’s too early to relax.

“We have a visible risk in front of us,” said Dr. Keiji Fukuda, coordinator of the World Health Organization’s global influenza program. But although the virus could mutate into a pandemic strain, Fukuda points out that it might go the other direction instead, becoming less dangerous for humans.

H5N1 has primarily stalked eastern and southern Asia. This year, however, it spread and infected people in Turkey, Iraq, Egypt, Djibouti and Azerbaijan.

But despite the deaths of 154 people, and hundreds of millions of birds worldwide dying or being slaughtered, the virus still has not learned how to infect humans easily.

Flu viruses constantly evolve, so the mere appearance of mutations is not enough to raise alarm. The key is to identify which mutations are the most worrisome.

“We don’t really know how many changes this virus has got to make to adapt to humans, if it can at all,” said Dr. Richard Webby, a bird flu expert at St. Jude Children’s Research Hospital in Tennessee.

The most obvious sign that a pandemic may be under way will almost certainly come from the field: a sudden spike in cases suggesting human-to-human transmission. The last pandemic struck in 1968 – when bird flu combined with a human strain and went on to kill 1 million people worldwide.

In May, on Sumatra island in Indonesia, a cluster of eight cases was identified, six of whom died. The World Health Organization immediately dispatched a team to investigate.

Luckily, the Sumatra cluster was confined to a single family. Though human-to-human transmission occurred – as it has in a handful of other cases – the virus did not adapt enough to become easily infectious.