For the third consecutive week, flu-like illnesses in Salt Lake County have continued to decline.

But flu pandemics are unpredictable, and public officials are urging the public to continue to take preventive measures and get vaccinated.

"We do not know how the virus will progress in the coming weeks and months," Salt Lake County Mayor Peter Corroon said at a press conference Monday. "What we do know is pandemics come in waves, and we're currently experiencing the second wave of the H1N1 virus."

Dagmar Vitek, the Salt Lake Valley Health Department's medical officer, said the first wave of H1N1 hit in May, peaked in June and was over by July; the second wave started in September, peaked in mid-October and has been declining ever since. All told, 419 Salt Lake County residents have been hospitalized; nine have died.

The Centers for Disease Control and Prevention reported last week that 22 million Americans have gotten sick with the H1N1 flu; about 100,000 have been hospitalized and nearly 4,000 have died.

It's hard to know whether a third wave is imminent, Vitek said. Historical data shows some flu pandemics were over after two waves, while others petered out after more.

"It really is very unpredictable if we're going to have a third wave, and when that happens, and how severe it is going to be," she said.

While the virus has been detected in some animals, including pigs, cats and ferrets, there is no indication

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it has mutated and become more virulent. "The virus is stable, which is great news," Vitek said.

As of last week, 100,000 doses of H1N1 vaccine had been administered to people in the priority groups, said Gary Edwards, executive director of the Salt Lake Valley Health Department. That's 20 percent of the 500,000 doses the county expects to receive through the end of December.

An additional 28,000 doses will be divvied out this week in clinics, private practices, hospitals and pharmacies. For now, Edwards said, the vaccination plan remains the same: Vaccinate as many people as possible, as quickly as possible, with an emphasis on people in the priority groups.

Seasonal flu, meanwhile -- which doesn't typically peak until February -- hasn't yet come into play. "But it will," Corroon said. "We know that it will."

lrosetta@sltrib.com