The World Health Organization has declared a "public emergency of international concern" following a number of confirmed cases of swine influenza A (H1N1) in Mexico and the US. There have also been reports of cases in Canada, New Zealand, Spain, France and Scotland.

View Confirmed and suspected swine influenza A (H1N1) cases in North America and Europe in a larger map

The WHO told Scrip that it will be working with authorities in each country to determine if the cases are "actual or not".

Thailand has started to use thermal screening equipment to monitor passengers entering the country from the US and Mexico for influenza-like symptoms, including fever.

Australia's quarantine authorities will assess the health of passengers arriving from Mexico and the US with symptoms of flu-like illness.

WHO director-general Dr Margaret Chan has recommended that all countries intensify surveillance for unusual outbreaks of influenza-like illness and severe pneumonia, but stopped short of recommending any trade or travel restrictions.

The WHO has set up an emergency committee, which had its first meeting on April 25th to assess the situation, a few days after the swine flu cases started to appear. It will meet again on Tuesday. The European Commission is also to hold a meeting on the situation.

Dr Keiji Fukuda, WHO assistant director general for health security and environment, told a press conference on April 26th that the situation was serious, but that more information was needed to understand the epidemiology and behaviour of the virus. "We want to understand to the extent that these viruses cause mild infections, and the extent to which these viruses can cause serious infections."

Most infections have occurred in Mexico. Gregory Hartl, spokesperson for the WHO, told Scrip that to date there have been 22 confirmed cases and three deaths in Mexico. However, the Mexican authorities have said there have been 1,614 cases so far, and 103 deaths.

The ministry of health in Mexico is using a wider net for its definition of cases, such as including suspected influenza-like illness, acute febrile illness and pneumonia, said Mr Hartl. "We are going by lab confirmed cases," he said.

In the US, where a public health emergency has also been declared, the Centers for Disease Control and Prevention has stated that there have 20 confirmed cases. The US cases have all been mild and there have been no deaths. In Canada there have been six confirmed cases, again mild.

The WHO says it does not like to comment on "suspected" cases as the "numbers are changing so often", as some cases are discarded.

vaccine development

There is no vaccine against the swine flu virus, and it is unknown whether the current human seasonal flu vaccine can provide any protection. The WHO says the CDC is working on developing the swine virus strain for a vaccine. Generally, vaccines take up to a year to prepare. "[The CDC] has taken the wild virus, manipulated it and inserted into chicken eggs to grow the virus," said Mr Hartl.

It appears that the viruses obtained from human cases of swine influenza in the US and Mexico were sensitive to the antivirals oseltamivir (Roche's Tamiflu) and zanamivir (GlaxoSmithKline's Relenza).

Only time will tell whether the outbreak becomes a pandemic. In the past 100 years there have been three major influenza pandemics, resulting in the deaths of more than 40 million people (see table below).

The Three Main Influenza Pandemics of the 20th Century

Source: Centre for Pharmacy Postgraduate Education (University of Manchester), module on Influenza (October 2008).