The United States will this week reach a landmark in the fight against swine flu as 100 million doses of vaccine become available to the public, Health Secretary Kathleen Sebelius said Thursday.

"By the end of this week, we will have 100 million doses of H1N1 vaccine available to the people. In fact, we're at 99.5 million doses today," Sebelius told reporters at a briefing on the swine flu pandemic.

Sebelius, flanked by half a dozen top US health officials, urged Americans to "take advantage of the increased supply and get vaccinated," even though the second wave of swine flu was ebbing.

"We have a chance to lessen the impact or even prevent a big third wave when flu season really hits, and need to seize that opportunity now," she said.

Flu season in the United States usually begins in October and lasts until May, but the (A)H1N1 virus erupted off-calendar, in April, and has so far infected people year-round.

Although most people who catch it have comparatively mild symptoms, Sebelius warned that the virus "is a serious flu that targets people who don't normally get seriously sick from the flu."

Seasonal flu usually hits the elderly hard, but swine flu has disproportionately impacted children and young adults.

"The number of children and young adults killed through mid-November was five times more than in an average flu season," Doctor Thomas Frieden, head of the Centers for Disease Control and Prevention (CDC) told the briefing.

Health officials estimated last week that around 1,100 children and 7,500 young adults died of swine flu in the United States from April to November.

In the same seven-month period, Frieden said there were an estimated 47 million cases of swine flu, 213,000 hospitalizations and 10,000 deaths in the United States.