World Health Organisation chief Margaret Chan said Monday that the world had been "lucky" with the swine flu pandemic after the virus failed to mutate to a more deadly form.
Chan told representatives of the WHO's 193 member states that "good news" in public health terms was normally driven by political commitment, resources and the ability to deliver care.
"Sometimes, though, we are just plain lucky. This has been the case with the H1N1 influenza pandemic," she said at the opening of the WHO's annual assembly.
"The virus did not mutate to a more lethal form," while the vaccine worked and it showed little resistance to anti-viral drugs, she added.
Chan acknowledged that "few health systems were overwhelmed and the effects were usually short-lived," with less disruption to everyday lives or travel than feared.
"Had things gone wrong in any of these areas, we would have a very different agenda before us today," she added.
Some health experts and politicians have criticised the WHO-led global effort against 2009 H1N1 flu after the new virus was uncovered in Mexico and the United States in April 2009, claiming there was an over-reaction.
Swine flu was declared a pandemic as it rapidly spread around the world, prompting massive spending on specially developed but largely unused swine flu vaccine and emergency precautions.
Pandemic flu has left 18,030 people dead since the virus was uncovered, according to latest WHO data.
A year ago, the WHO chief was warning about the threat of a growth in severe cases of the "subtle, sneaky" new swine flu virus, especially in developing nations, just weeks before she followed scientific advice and declared a pandemic.
An independent committee of experts set up by the WHO is investigating the international response to the swine flu pandemic. It is due to deliver its findings by the beginning of next year.
Chan reiterated on Tuesday that she welcomed the review and its examination of criticisms that have been levelled at the WHO.
"We want to know what went wrong and, ideally, why. We want to know what can be done better and, ideally, how," she added.
The WHO assembly runs until Friday, and is due to examine an array of global health issues.
Explore further: GlaxoSmithKline taking pandemic vaccine orders