The Centers for Disease Control and Prevention (CDC) yesterday released new fatality estimates for the H1N1 influenza virus, also known as swine flu. Since the start of the 2009 H1N1 flu outbreak in April, there have been some 3,900 deaths, more than the 1,200 previously estimated, but significantly less than the 36,000 deaths each year attributed to seasonal flu. In the six months from April to October 17, 2009, the CDC puts the total number of swine flu cases at 22 million and the number of hospitalizations at 98,000. The Wall Street Journal Health blog ponders what these numbers might mean. For starters, it observes that thereÃ¢â‚¬â„¢s a lot of uncertainty around the figures because so many cases of swine flu go unreported. Indeed, the CDC itself says that estimating the number of individual flu cases in the U.S. is very challenging because many people with flu donÃ¢â‚¬â„¢t seek medical care and only a small number of those that do seek care are tested. More people who are hospitalized or die of flu-related causes are tested and reported, but under-reporting of hospitalizations and deaths occurs as well. The WSJ also points out that the CDC presents a wide range of numbers around their estimates. For example, the possible number of total cases ranges from 14 million to 34 million and the estimated total number of deaths ranges from 2,500 to 6,100. There are also several big caveats to comparisons between H1N1 and seasonal flu estimates. Demographic difference is one. While about 90 percent of seasonal flu-related deaths occur in people over 65 years old, the proportion of younger people being impacted by H1N1 is much greater. Another caveat, according to the WSJ, is that the six months of data the CDC is looking at are typically fluÃ¢â‚¬â„¢s off-season. How H1N1 and seasonal flu data will compare when we hit the traditional big flu months this winter is something to watch for. I.I.I. facts and statsÃ‚ on public health risks show that flu and pneumonia are among the leading causes of death each year in the U.S. However, pandemic influenza viruses have the potential to be far more deadly.