The level of flu activity across the nation has dropped for the fourth week in a row, federal health officials reported Monday, indicating that the second wave of the swine flu pandemic in the United States has peaked. While officials warned that the number of people getting infected with the H1N1 virus remains high, and cases could surge again, the extended period of falling activity suggests that the intensity of the outbreak has reached a high. 'We're certainly on the downward slope of the curve,' said Thomas Skinner, spokesman for the Centers for Disease Control and Prevention in Atlanta. The CDC noted, however, that 27 deaths from swine flu among people younger than 18 had been confirmed in the most recent week -- the largest one-week tally of pediatric deaths since the pandemic started -- bringing the total to at least 198. About 40 or 50 children usually die from the flu during an entire season. Skinner said officials were hesitant to use the term 'peak' because of concerns about creating the impression that the outbreak was over. 'We're far from being out of the woods,' Skinner said. 'There's still a lot of flu out there. And we wouldn't be surprised to see another uptick in activity as we approach the end of December and beginning of January, when kids come back from Christmas break.' But other experts said they thought the wave had peaked. 'Most of the time with the flu you have a four-week or sometimes six-week rise to a peak and then a four- to six-week fall. I think we're past the peak. We're probably on the way down,' said Richard Wenzel, an infectious disease specialist at Virginia Commonwealth University. 'I think the worst is over.' Wenzel also predicted that if there is another surge, it will be relatively mild because millions of people have either been exposed to the virus or will have been vaccinated. 'After a while the virus runs out of susceptible people,' Wenzel said. 'This virus is running out of hits.' Michael Osterholm, an infectious disease expert at the University of Minnesota, agreed that the current wave has peaked. 'Just about everywhere now the numbers are coming down and pretty much in all age groups,' he said. But Osterholm worried that the drop would prompt people to stop trying to get vaccinated, which he said could be disastrous if another wave hits. In its weekly update on flu activity, the CDC reported that the number of states experiencing widespread flu cases during the week that ended Nov. 21 had fallen to 32, down from 43 states the week before and a high of 48 states in late October. Influenza-like illnesses accounted for 4.3 percent of all visits to doctor's offices during the week, down from nearly double that proportion in October. At the same time, the number of U.S. school closings due to swine flu has been steadily falling. The Department of Education reported Monday that no new closings had occurred in the past week. Since the virus emerged in April, the swine flu has sickened at least 22 million Americans, hospitalized 98,000 and killed at least 4,000, the CDC estimates. While the seasonal flu typically hits the elderly hardest, the H1N1 virus has affected children and young adults much more because most have little or no immunity against it. Experts suspect another surge in cases could occur this month or the beginning of 2010, similar to what happened during the 1957 flu pandemic, which most resembles the H1N1 pandemic. 'We certainly won't be surprised if we see another uptick of activity late this year or early next year if we look at the 1957 pandemic as a gauge,' Skinner said. The federal government has spent more than $2 billion to buy vaccine. But production problems slowed the campaign, causing widespread frustration and intense criticism, including from members of Congress. More than 60 million doses of vaccine have become available and officials are predicting millions more will be shipped to states in the coming weeks. 