"Officials described the move as similar to a declaration ahead of a hurricane making landfall."
CDC Director Dr. Thomas Frieden told reporters Friday. "To be basically in the peak of flu season in October is extremely unusual," he said."We expect that influenza will occur in waves and we can't predict how high, how far or how long the wave will go or when the next will come," he added.
"Many millions" of Americans have had swine flu so far, according to an estimate he gave at a Friday press conference. The government doesn't test everyone to confirm swine flu so it doesn't have an exact count.
Also Friday, the World Health Organization reported more than 414,000 laboratory confirmed cases of H1N1 worldwide, with nearly 5,000 deaths.
But the Geneva-based health agency noted that the figures were only the tip of the iceberg.
"As many countries have stopped counting individual cases, particularly of milder illness, the case count is significantly lower than the actual number of cases that have occurred," WHO said.