BioEd Online

An early analysis of the H1N1 swine-associated flu virus outbreak suggests that the virus spreads at a rate comparable to that of previous influenza pandemics.

The results, published online today by Science¹ and compiled by the World Health Organization Rapid Pandemic Assessment Collaboration, support the designation of swine flu as a pandemic but also indicate that the fatality rates thus far are lower than those seen during the 1918 flu outbreak or those anticipated from an avian influenza pandemic.

"It's a virus that almost certainly will cause a global epidemic," says study author Neil Ferguson, an epidemiologist at Imperial College London. "But it's not the catastrophic scenario people were fearing for bird flu."

Early results

The study focuses largely on data from Mexico, where the earliest known infections occurred. Such data are still preliminary, and it is too early to gain a complete picture of how the virus will behave in the population as a whole. But early modelling efforts can give officials an indication of what may lie ahead, says Ferguson.

By plugging early data into statistical models, Ferguson and his collaborators determined that 6,000–32,000 individuals had been infected in Mexico by late April. The team also used epidemiological data and information about the virus' genetic diversity to determine that the swine flu virus has a basic reproductive rate — a number that takes into account how easily the virus spreads within a population — of 1.2–1.6. Seasonal flu typically hovers around 1.2, whereas the second, more severe wave of the 1918 flu reached about 2.

"So far, I would put it at the cusp of a severe seasonal strain or a mild pandemic strain," says epidemiologist Ira Longini of the University of Washington School of Public Health in Seattle, who was not involved with the study. But Longini notes that these calculations are based on data compiled after the peak flu season. "It's hard to tell what it will be during the late fall and winter months in North America," he adds.

Children hardest hit

The World Health Organization analysis also supports observations that the swine flu strikes children more than the elderly — an unusual pattern compared with that of seasonal flu. When Ferguson and his collaborators attempted to fit their models to data collected from a community outbreak of swine flu in La Gloria, Veracruz, they found that the data fit best when susceptibility to the virus varied according to age.

One possibility, says Ferguson, is that H1N1 viruses are commonly seen in normal seasonal flu epidemics. Adults are more likely to have encountered those viruses and developed immunity to them, and it may be that in some cases this immunity is enough to provide protection against swine flu. This is still a hypothesis, however, and researchers have not yet found evidence that there is crossover protection from immunity against previous strains of H1N1.

Meanwhile, fatality rates from this season's swine flu outbreak are so far lower than those observed during the 1918 pandemic but on par with those seen in the milder 1957 influenza pandemic. The new analysis puts these rates at around 0.4%, but the data are very preliminary.

Nevertheless, healthcare providers should be on alert for the upcoming flu season, Ferguson says. In a normal flu year, officials expect about 10% of the population to become sick, whereas Ferguson estimates that 30% of the population could become sick if the swine flu returns next season.

"This means that even if the virus is as mild as normal seasonal flu — and people die from seasonal flu every year — there will be a substantially greater burden on health systems," he says.