Historically, influenza pandemics have encircled the globe in two, sometimes three, waves. During the previous century, the 1918 pandemic, the most deadly of them all, began in a mild wave and then returned in a far more deadly one. In fact, the first wave was so mild that its significance as a warning signal was missed.
As we are seeing, the world today is much more alert to such warning signals and much better prepared to respond.
Influenza pandemics are caused by a virus that is either entirely new or not known to have circulated among humans in recent decades. This means, in effect, that nearly everyone in the world is susceptible to infection. It is this almost universal vulnerability to infection that makes influenza pandemics so disruptive.
Large numbers of people falling ill can be highly disruptive to economies and to the functioning of routine medical services.
But let me be frank. Global manufacturing capacity, though greatly increased, is still not sufficient to produce enough antiviral medication and pandemic vaccines to protect the entire world population in time.
This is the reality. But we can acquire the data that guides the wise and targeted use of these interventions, conserves supplies, and, in the case of antiviral medicines, reduces the risk of drug resistance.
An influenza pandemic is a global event that calls for global solidarity. As the chief technical and administrative officer of WHO, it is my job to do whatever is possible to ensure that developing countries are not left without protection.

H1N1 influenza situation
Dr Margaret Chan 2009
Director-General of the World Health Organization http://www.who.int/dg/speeches/2009/influenza_a_h1n1_situation_20090504/en/ The overall severity of a pandemic is further influenced by the tendency of pandemics to encircle the globe in at least two, sometimes three, waves. For many reasons, the severity of subsequent waves can differ dramatically in some or even most countries.
A distinctive feature of influenza viruses is that mutations occur frequently and unpredictably in the eight gene segments, and especially in the haemagglutinin gene. The emergence of an inherently more virulent virus during the course of a pandemic can never be ruled out.

Different patterns of spread can also influence the severity of subsequent waves. For example, if schoolchildren are mainly affected in the first wave, the elderly can bear the brunt of illness during the second wave, with higher mortality seen because of the greater vulnerability of elderly people.
During the previous century, the 1918 pandemic began mild and returned, within six months, in a much more lethal form. The pandemic that began in 1957 started mild, and returned in a somewhat more severe form, though significantly less devastating than seen in 1918. The 1968 pandemic began relatively mild, with sporadic cases prior to the first wave, and remained mild in its second wave in most, but not all, countries.
Assessing the severity of an influenza pandemic
11 May 2009
Global Alert and Response (GAR) http://www.who.int/csr/disease/swineflu/assess/disease_swineflu_assess_20090511/en/ Introduction
The repetitive occurrence of yearly, seasonal influenza epidemics is due to the fact that influenza viruses are continuously changing antigenically. This was well illustrated by the emergence of the influenza A H3N2 'Fujian' strain, which appeared in July 2003 in the southern hemisphere then spread to the northern hemisphere a few months later to become a dominant strain [53] . To face this continuous change, virus strains to be included in the vaccine are updated annually so as to match the circulating virus strains. An alternative would be to develop "universal" influenza vaccines which would cover all possible influenzavirus strains (see below). Meanwhile, the recent emergence of highly pathogenic avian influenza H5N1 in poultry farms and markets in several countries in Asia, from where it spread