Updated: Dec. 10, 2009
Since early November, cases of H1N1 have continued to decline nationwide, and scientists keeping track of the numbers say that as pandemics go, 2009 H1N1 may turn out to be a mild one at least for the time being.
The question now on health officials' minds is: Will there be a second wave of cases in the new year? The answer depends on whom you ask. "We took an informal poll of about a dozen of some of the world's leading experts in influenza," Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention (CDC), told reporters recently. "About half of them said, Yes, we think it's likely that we'll have another surge in cases. About half said, No, we think it's not likely. And one said, Flip a coin."
It is an accurate reflection of how unpredictable the influenza virus can be. Although flu activity has been waning for the third week in a row, health officials warn that there are still four to five months left in the official influenza season, plenty of time for the virus to make its rounds and find new hosts. "The story of pandemics, and the story of H1N1 in general, is the story of persistent uncertainty where we never quite know what we are going to get or when," says Dr. Irwin Redlener, director of the National Center for Disaster Preparedness at the Columbia University Mailman School of Public Health.
How severe the current H1N1 pandemic seems depends on what you use as a measuring stick. Compared with previous pandemics, like the 1918 Spanish flu, which killed 20 million people and infected up to 40% of the world's population, or even the far less deadly 1957 and 1968 bouts with a strain of H1N1 influenza similar to the 2009 strain, things don't seem as bad this time around. Fewer people are getting severely ill when infected, and fewer have died or required hospitalization from the flu than in previous pandemics.
Marc Lipsitch, an epidemiologist at the Harvard School of Public Health, and his colleagues studied the course of the 2009 H1N1 pandemic last spring in two cities New York and Minneapolis and determined that 0.048% of people who developed symptoms of H1N1 died, and 1.44% required hospitalization. Based on that data, published in PLoS Medicine, Lipsitch anticipates far fewer deaths from 2009 H1N1 than was initially believed. By the end of the flu season in the spring of 2010, Lipsitch predicts, anywhere from 6,000 to 45,000 people will have died from H1N1 in the U.S., with the number most likely to end up between 10,000 and 15,000. Those estimates are far below the death toll of the 1957 flu, which killed 69,800 people in the U.S., according to government figures, and smaller also than the early predictions for the 2009 H1N1 flu deaths, which ranged from 30,000 to 90,000.
It is not clear, however, that past pandemics are an appropriate gauge for evaluating the current flu or that the new projections are based on complete data. The eventual death toll of 2009 H1N1 may be less grim than the outcomes of previous pandemics, but it should be noted that 90 years ago, and even 40 years ago, health officials lacked the antiviral therapies and nationwide vaccination capabilities that are available today. That may have contributed to pandemics having a more devastating effect on the health of past populations.
The new estimates are also less alarming than those provided also by Lipsitch to the President's Council of Advisers on Science and Technology last summer near the start of the pandemic. At the time, researchers had only patchy data on the number of people infected by, and seeking treatment for, the new flu. The initially bleak prediction of the impact of H1N1 with up to 50% of the U.S. population becoming infected in the fall and winter of 2009, resulting in as many as 90,000 deaths was based on modeling of previous pandemics.
Fortunately, the worst case scenario did not come to pass. "The worst case consistent with the data we have now is a lot milder than the worst case consistent with the data we had in the summer or spring," Lipsitch says.
Still, Lipsitch and other health officials acknowledge that the 2009 H1N1 pandemic is not over. What worries health officials most is that as both seasonal and H1N1 flu viruses circulate among the population, the two strains could recombine into a more virulent and aggressive version that could cause more widespread illness and even death. How viruses behave once they nestle into a host is completely unpredictable, but scientists know that in a lab dish, seasonal and H1N1 flu strains mix and match readily. "I'm thinking we may have dodged a bullet here if in fact we don't get a more severe wave coming on the heels of the current wave," says Redlener. "But we'll see what happens."
A second wave could still prove more deadly than the seasonal flu, especially for young children. To date, 189 children have died of influenza in the U.S., the majority of them related to H1N1 infection, and that number is already higher than the total number of pediatric deaths attributed to flu in 2008. Lipsitch says that if current trends hold, H1N1 may end up causing as many influenza deaths, if not more, than the seasonal flu, which kills about 36,000 Americans each year. Instead of hitting the elderly the hardest, though, most of the deaths may be among young children and infants.
Update: According to the most recent CDC estimates, released by health officials Thursday, about 10,000 Americans have died from H1N1 and 50 million or one-sixth of the population have been infected. The CDC says also that about 200,000 people have been sick enough to require hospitalization, and that most of them were adults under age 65. The new figures cover the first seven months of the pandemic, from April through mid-November, and represent a large increase over the previous estimates, which included data through mid-October and said H1N1 had killed 4,000 and infected 22 million. But experts said the jump was not surprising, given that the current wave of disease was peaking in early November.
The looming concern over another winter wave of flu is all the more reason, says Lipsitch, to continue aggressive antiflu efforts, from washing your hands to covering your cough and getting vaccinated. In some states, including New York, there is now enough vaccine to vaccinate everyone over six months old, and not just those in priority groups. "We would expect that prior exposure to a similar strain in the form of a vaccine will provide some priming for future exposures, even if the virus changes a bit," says Lipsitch. In other words, the more people who are vaccinated this year, the less likely the H1N1 virus, which will probably still be around next year, will take hold and spread.