WASHINGTON — Health and Human Services Secretary Kathleen Sebelius said today a massive school closing wouldn't stop the spread of the swine flu virus, saying vaccinations must be the defense against a menace one report said could infect up to half of the population.

"What we know is that we have the virus right now traveling around the United States," Sebelius said in a nationally broadcast interview. "And having children in a learning situation is beneficial ... What we learned last spring is that shutting a school down sort of preemptively doesn't stop the virus from spreading."

Sebelius appeared on NBC's "Today" show one day after a special presidential advisory panel presented a grim report to the Obama White House, saying among other things that a "plausible scenario" for the United States later this year is wide-scale infections, possibly 30,000 to 90,000 deaths, mostly among young children and young adults, and perhaps as many as 300,000 sick enough to require intensive care unit treatment at hospitals.

Asked in the interview what people should do while awaiting the arrival of a vaccine, with first supplies likely by October but most not until the Thanksgiving season, Sebelius said: "I think it's important that people begin to anticipate that we will have a vaccine. We think it's likely that we're going to need two shots for the vaccine."

She said people should plan ahead for this, particularly those with pre-existing medical conditions, pregnant women and health care industry workers. Sebelius said federal health authorities also are recommending people should immediately get their regular "seasonal" flu vaccine to bolster their health for the scenario yet to play out later this year regarding the swine flu virus.

"Seasonal flu vaccine is ready at the beginning of September," she said. "We want the population that is most at risk to begin their seasonal flu vaccine now."