Dr. Nancy Cox of the Centers for Disease Control and Prevention calls it "quite surprising" that more genetic variation has not emerged given the virus' quick spread - sickening more than an estimated million people in the U.S. alone since April.
People under 25 get sick most often, although most of the 262 deaths recorded in the U.S. so far have been in 25- to 49-year-olds. People older than 65 have far lower risk of getting sick - perhaps because of years of exposure to viruses in the same influenza family.
Meanwhile, federal health officials say the U.S. expects to have 160 million doses of H1N1 flu vaccine available sometime in October - if all goes well.
That's despite continued bad production from manufacturers around the world. The chief ingredient for vaccine is grown in chicken eggs, and companies are getting far fewer doses per egg than is usual with regular winter flu.
Another complication: That 160 million estimate assumes a low dose will work. Studies will begin in a few weeks to see if this new vaccine will protect with a low dose or if people will need a higher one. Scientists also expect people will need two separate inoculations, about a month apart, for protection.
On Wednesday, the government called for several thousand volunteers to start rolling up their sleeves for the first H1N1 flu shots, in a race to test whether a new vaccine really will protect against the virus before its expected rebound later in the year.
The first shots should go into volunteers' arms by the second week of August. A network of medical centers around the country is enrolling for the series of studies directed by the National Institutes of Health.
First, doctors will test different doses of the swine flu vaccine in healthy adults, including the elderly - two shots, given 21 days apart. If there are no immediate safety concerns, such as allergic reactions, the same testing quickly will begin in babies and children, said Dr. Anthony Fauci, head of NIH's National Institute of Allergy and Infectious Diseases.
"The questions are clear. One is: is it safe? And if so, what is the right dosage and number of doses? We need to ask that question both in healthy young adults, we need to ask the question in elderly individuals, and we need to ask the question in children," Fauci told CBS News medical correspondent Dr. Jennifer Ashton.