SCENARIOS: What will be the impact of a flu pandemic?

LONDON (Reuters) - The World Health Organization has declared the first flu pandemic in more than 40 years, after calling an emergency meeting of experts on Thursday to discuss the spreading H1N1 outbreak.

There have been 27,737 cases reported in 74 countries to date, including 141 deaths, according to the WHO's latest tally.

Here are some ways the situation could develop and the impact the disease might have on a global economy already in recession:

MILD PANDEMIC

The new flu has been largely mild so far, suggesting it may be similar to the 1968 pandemic, when the H3N2 strain killed an estimated 1 million people. Experts think the death toll from a pandemic of similar virulence today would be lower, since doctors now have access to antiviral medicines.

By comparison, the ordinary seasonal flu kills between 250,000 and 500,000 people each year.

A mild pandemic could still have an economic impact if a lot of people stay off work for a week or so with heavy cold-like symptoms, but it would not cause wholesale economic disruption.

SEVERE PANDEMIC

The worst-case scenario is that the H1N1 flu strain returns in a far more virulent form in the northern hemisphere winter, causing widespread sickness and death, mass work absences, and major disruption to travel and trade flows.

The 1918 pandemic killed at least 40 million people in 18 months -- but this was in a time before antibiotics, which today save the lives of many flu patients with secondary infections.

Even so, experts predict a 1918-like flu could keep 40 percent of the workforce out at any time, with people either sick, caring for sick relatives or children out of school, or simply lying low.

The World Bank estimated last year that a serious pandemic could cost $3 trillion and lop nearly 5 percent of global gross domestic product.

REGIONAL VARIATION

Severity is likely to vary substantially from country to country and even within populations. Experts have been surprised, for example, by the speed at which H1N1 has spread in Australia and among Canadian Inuits.

People in their late 50s and older appear largely immune to the disease because of previous exposure to similar flu strains.

DRUGS & VACCINES

The world is better prepared than ever before for a pandemic, with stockpiles of antiviral drugs like Roche's Tamiflu and GlaxoSmithKline's Relenza.

Work is also already under way on a vaccine, expected to be available from October from major manufacturers such as Sanofi-Aventis, Glaxo and Novartis.