For example, in the early days, they ignored advice to close the Mexican border and pre-emptively shut school systems. They released part of the national Tamiflu stockpile, but did not give it to millions of healthy people prophylactically, as Britain did. They ordered vaccine made with a 50-year-old egg technology rather than experimental methods. They bought adjuvants  chemical “boosters”  that could have stretched the first 25 million vaccine doses into 100 million, but did not use them for fear of triggering a backlash among Americans made nervous by the messages of the antivaccine movement.

To alert the public without alarming it, a stream of officials  from doctors in the navy blue and scrambled-eggs gold of the Public Health Service to a somber President Obama in the White House  offered updates, at least twice a week for months.

It is now clear that this is the least lethal modern pandemic. The flu appears to kill about one of every 2,000 people who get it, American researchers say. (British researchers found half that death rate.) By contrast, the Spanish flu of 1918 killed about 50 of every 2,000, and the 1957 and 1968 pandemics killed about 4 of every 2,000.

The flu has reached more than 200 countries and is still peaking in places like Eastern Europe and Russia. Even though there was no vaccine yet, it killed fewer than expected during the Southern Hemisphere’s winter, June through August.

Officials in the United States conceded that some mistakes were made.

For example, they could have spotted the new virus earlier if there had been better cooperation with Mexico. In late April, the United States isolated it in samples from Texas and California just as Canadian officials were testing Mexican ones. The outbreak probably began in rural Mexico in January, but was spotted only when thousands fell ill in late March or early April in Mexico City.

The C.D.C. tests viruses in Southeast Asia, where new flus are usually born. “This time,” said Dr. Thomas R. Frieden, the C.D.C. director, “one happened to emerge in a place where we don’t have a surveillance system.”