Marc Lipsitch, a Harvard School of Public Health epidemiologist, speculated that the official count of swine flu deaths - it stands below 1,600, as of the middle of last week - “is certainly an underestimate of the number of deaths. It may be a bad underestimate or it may be a modest underestimate, but it is certainly an underestimate.’’

For example, research from New York City, particularly hard hit by the virus in the spring, showed that almost 20 percent of the people killed by swine flu hadn’t been hospitalized, Lipsitch said, making it less likely such deaths would be linked to the disease. Multiplied across the country, that phenomenon could contribute to making the flu strain appear less lethal than it is.

Assessing swine flu’s impact was further muddied by a decision in the past month to change what counts as an H1N1 death.

Until the end of August, only cases confirmed by sophisticated labs were included in the national tally of deaths, which, by then, had climbed to 593. But state health departments, overwhelmed by the number of people infected, have mostly stopped doing the sophisticated and expensive tests to confirm swine flu.

So federal disease specialists expanded the definition to include deaths that doctors attribute to flu and pneumonia based on symptoms, and both the swine type and the seasonal strain count. Since that change was made, the count has grown by an additional 936.

Flu stands in stark contrast to other diseases that are far easier to monitor because they cause fewer cases with more distinctive symptoms. “With flu, we can have 60 million people potentially infected,’’ said Dr. Lyn Finelli, flu surveillance chief at the US Centers for Disease Control and Prevention. “We could never count all those cases, and we could never count all those deaths.’’

Flu viruses are capable of making subtle alterations in their genetic machinery that transform them into more - or less - fearsome pathogens. Timing is critical, too, and that’s especially true with swine flu: A vaccine isn’t expected to be widely available until mid-October, and the germ is already circulating among a population with little natural protection against it.

... A recent study found that people under 30 have little existing protection against the swine flu virus, while older adults harbor disease-fighting cells against the germ. It’s believed the older adults’ immune systems carry the memory of encounters with ancestors of the virus.