Published online 27 April 2009 | Nature | doi:10.1038/news.2009.405
Updated online: 27 April 2009
Updated online: 27 April 2009

News: Briefing

Swine flu jumps continents

Influenza virus spreads around the world.

pigsHow likely is a swine flu pandemic?Ingram Publishing / Alamy

Swine flu has been confirmed in Mexico, the United States and Europe — and may have spread as far afield as New Zealand. With more than 80 people suspected to have died of the disease in Mexico, where this outbreak seems to have originated, the director general of the World Health Organization, Margaret Chan, said the outbreaks "constitute a public-health emergency of international concern". Nature News examines whether the outbreak could become a pandemic.

Where are the known cases of swine flu?

The Mexican government has reported three separate outbreaks. Cases began to emerge in mid-March and over 850 people with pneumonia have been reported in the capital, Mexico City. Other cases have been reported in San Luis Potosi, in central Mexico and near the US border in Mexicali.

In the United States, 40 cases have been confirmed across the country. A case has been confirmed in Spain and suspected cases are under investigation in the United Kingdom, Brazil, Israel, Australia and New Zealand.

How is the flu transmitted?

Influenza viruses are reasonably common in pigs. Traditionally, most human cases of swine flu are contracted through contact with infected animals, rather than from human-to-human transmission but this flu does seem to be spreading between people.

It spreads in the same way as normal flu, with most of those infected likely to have contracted it through airborne viruses. Infection by touching contaminated surfaces is also possible. Symptoms are also likely to resemble those of normal flu.

"With this sort of virus we would expect a mild, influenza-like illness, with fatalities in the minority of cases," says Wendy Barclay, chair in influenza virology at Imperial College London. "Over the next few days, as the numbers in other countries increase, we'll have a much better idea what the true case fatality is. Looking genetically at the virus there are no hallmarks of high pathogenicity."

What sort of flu is this?

This is an influenza A virus. These viruses are classified on the basis of two surface proteins: haemagglutinin and neuraminidase. There are 15 known forms of haemagglutinin and 9 of neuraminidase. The haemagglutinin type gives the H number and the neuraminidase type the N number.

Up to now, 18 of the Mexican cases have been laboratory confirmed as the H1N1 strain, says the WHO, and 12 of those are "genetically identical" to the California outbreak. The strain of avian flu that jumped from birds to humans in 2004 was H5N1.

The Mexican flu appears to be a new virus. Some of the samples have been sequenced and seem to have recombined from several different viruses.

"We know so far that the viruses contain genetic pieces from four different virus sources," Anne Schuchat, deputy director of the science and public health program at the US Centers for Disease Control and Prevention (CDC), told reporters last week. "This is unusual." Those genetic pieces are from North American swine influenza viruses, North American avian influenza viruses, human influenza viruses and swine influenza viruses found in Asia and Europe.

Barclay adds that this means these segments originated in humans or birds, not that all four types of the virus have recently recombined in this case.

H1N1 was also responsible for the deadly 1918 pandemic but in that case was a human influenza virus that mutated, not a virus generally found in pigs that transferred to humans.

A previous outbreak of H1N1 swine flu occurred in 1976 and claimed the life of a soldier at Fort Dix in the United States, but did not spread further. A number of people also developed Guillain-Barré syndrome from a rapidly implemented nation-wide vaccination programme.

Is this a pandemic?

The WHO currently classes the outbreaks as pandemic stage 3, where an animal disease is causing infections in humans but "has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks".

Phase 4 requires human-to-human driven community-level outbreaks and Phase 5 requires these in two countries in one WHO region. Phase 6 is a pandemic, when community-level outbreaks occur in two WHO regions.

The WHO's Emergency Committee is meeting today and may decide to raise the level to phase 4, depending on the latest information.

"At this stage it looks very like this virus has good human-to-human transmission, and therefore, although today nobody would want to commit to this, within a week I think we'll have a very good idea whether or not this virus is going to go global," says Barclay. "In a sense we already know that it is in three different continents, with all the evidence that it transmits between people. That's all the hallmarks of a new pandemic."

What can we do about swine flu?

Some people might have some level of resistance to this flu virus, but it is likely that most people do not. Currently, there is no vaccine against the new virus and standard seasonal flu vaccines are unlikely to offer protection.

Researchers are preparing the virus as a seed strain for vaccine manufacture, a process that can take weeks. If thought necessary, a vaccine could be produced around three to six months after the seed strain is ready.

The antiviral drugs that some countries have been stockpiling in case of a bird-flu pandemic might be effective. The CDC says H1N1 is susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza) and is recommending these be used to treat and prevent infection.

For now, the WHO suggests avoiding contact with people who may be unwell, covering the nose and mouth if you have to deal with ill people, and maintaining high standards of hygiene.

For Nature 's ongoing coverage of the H1N1 outbreak, see 


The World Health Organization has now raised its pandemic threat level from phase 3 to phase 4.


UK cases of swine flu have been confirmed.

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