Swine flu may be milder than feared, study suggests

Swine flu may be less severe than originally feared, a new analysis says, sending people to the hospital and causing death at rates similar to seasonal flu. The research, based on projections derived from H1N1's first appearance in the spring and early summer, also confirms that children and young adults are more susceptible to swine flu than typical seasonal strains.

The researchers warn that swine flu, which has waned in recent weeks in Massachusetts and many other parts of the country, could return in full force in January after a temporary lull.

An international team including senior author Marc Lipsitch of the Harvard School of Public Health released a preliminary report in September suggesting a milder course for the pandemic than the first frightening case reports from Mexico indicated. In March and April, it looked like about 4 percent of people infected with swine flu, also known as H1N1, were dying, but that estimate was culled from the most serious cases rather than infections among the wider population. For the new analysis, published online today in PLoS Medicine, the authors examined data from Milwaukee and New York City from April through July. The two cities were chosen because of the consistent way they reported their data.

To gauge the flu's impact, they looked at people who had flu-like symptoms and then calculated the proportion who sought medical care, were hospitalized, got care in intensive care units, or died. They also considered people who answered a telephone survey asking if anyone in the household had been ill with the flu. Based on people hospitalized in the two cities, they estimated that about 1 in 2,000 people with H1N1 symptoms died, about 1 in 400 needed to be treated in intensive care units, and about 1 in 70 needed to be admitted to the hospital.

When information from the New York telephone survey was included, the estimates of people dying or needing hospital care were 7 to 9 times lower. For example, 1 in 600 people with flu symptoms might end up in the hospital, and a fraction of those will die under that approach.

In a typical winter, about 36,000 people in the United States die of seasonal flu, most of them over age 65. Swine flu so far has caused more deaths among children and adults under 65 than seasonal flu. The proportion of people infected is harder to know, according to Al Ozonoff, a biostatistician at the Boston University School of Public Health. He was not involved in Lipsitch's study.

"It's very, very hard to get a good estimate on even very basic [numbers] like how many people are sick and how many people have died, so this work is really an important step," he said. "Without good quantitative estimates, we are really making decisions based on potentially faulty numbers."

In the spring, public health officials were contemplating a death rate of 2 percent, which triggered pandemic planning. Lipsitch and his colleagues now put the rate at under 1 percent.

"A lot of plans have to be made by governments and individuals and health-care systems before anyone really knows what the numbers are going to be," he said in an interview. "People have started asking me, 'Did we overreact to this?' My feeling is that when you have a problem and you don't know how big the problem is, you have to plan for something that's anywhere in the range you think it might be. And that range includes very, very mild and very, very severe."

Cases across the state and around the country have been dropping since mid-November, but Lipsitch isn't taking any chances. He brought his children to a swine flu vaccine clinic over the weekend, which was not as well attended as the Boston Public Health Commission anticipated.

"There have been resurgences of pandemics around January," he said. "We don't know if that will happen this year, but it could. The fact it's gone down doesn't mean it's gone for good. If enough people get vaccinated between now and then, it becomes less likely there will be a rebound of flu in January."