The World Health Organisation yesterday declared a global influenza pandemic, the first in more than 40 years, after weeks of delay for fear of causing unnecessary public alarm.

“The scientific criteria for an influenza pandemic have been met. I have therefore decided to raise the level of influenza pandemic alert from phase 5 to phase 6,” said Margaret Chan, WHO director-general.

“The world is now at the start of the 2009 influenza pandemic.”

Phase 6, the maximum on the WHO's pandemic alert scale, means the flu strain is spreading globally. It is not a measure of severity.

Dr Chan said that on the basis of evidence so far the pandemic would be of “moderate severity”. The WHO would maintain its recommendations against travel bans and other restrictions.

The organisation has also decided not to recommend an immediate switch in vaccine production from seasonal influenza to the pandemic strain. Seasonal flu vaccine production will shortly be complete, Dr Chan said, allowing full capacity to be devoted to a pandemic vaccine.

The agency says anti-viral medicines such as Tamiflu have so far proved effective for the minority of patients needing treatment.

The decision to move to phase 6 follows a meeting of the WHO's panel of flu experts. The trigger is likely to have been confirmation by Australia of a generalised flu outbreak in the state of Victoria.

By the agency's own criteria, the conditions to trigger phase 6 had been in place for a number of days: the existence of a new flu virus that was spreading widely between people in two different parts of the world. But officials have been scrambling to manage reactions and to counter panic or over-reaction by both the public, for whom a “pandemic” implies a highly lethal infection, and governments, many of which had been preparing responses to a more severe form of the virus.

Authorities will need to grapple with local assessments of how to change their approaches to treatment, whether to impose quarantine measures such as school closures; and how to cope with the impact of the virus and the policies adopted by neighbouring countries, which may vary sharply.

There is no doubt that the “swine flu” H1N1 virus first identified early this year in Mexico is a new infection that has since spread widely in the Americas and now in Australia. To date, some 30,000 cases have been officially confirmed in at least 74 countries, with many more likely to have gone undetected.

That uncertainty makes it difficult to judge the severity of the infection. Just over 140 confirmed deaths have resulted so far, but without knowing more clearly how many total infections have occurred, it is impossible to be clear on fatality rates.

The provisional conclusion is that this latest pandemic is “moderate”: less severe than the 1918 Spanish flu, more in line with that of 1957, and perhaps up to four times as dangerous as a typical seasonal flu virus, which each year kills an estimated 500,000 people around the world.