H1N1 influenza virus, imaged in the Influenza Laboratory of the U.S. Centers for Disease Control and Prevention
Public domain images courtesy of the CDC
An early look at the H1N1 flu outbreak suggests that the World Health Organization was justified in its 29 April decision to raise the global pandemic alert level to phase 5, out of a possible 6.
"The new research, posted Monday 11 May on the Science Express Web site, indicates that the H1N1 virus is more easily transmitted than seasonal flu, and appears to be transmissible at levels comparable to lower estimates for previous flu pandemics.
"Our early analysis would suggest this is going to be an outbreak comparable to that of 20th century pandemics regarding the extent of its spread—it's very difficult to quantify the
human health impact at this stage, however," said study author Neil Ferguson of Imperial College London.
The WHO's Phase 5 designation refers to sustained human-to-human transmission of a new flu strain of animal origin in one region, with cases exported from that region to other parts of the globe.
The Science authors caution that uncertainties about the outbreak remain, including how long the virus incubates in the body and how long a particular individual remains infectious. But they say transmission estimates can help lay out a scientific basis for policy decisions such as whether to close schools in individual countries.
According to the new report, the outbreak appears to have originated in mid-February in the village of La Gloria, in the Mexican state of Veracruz, where acute respiratory illness affected 61% of community residents under 15 years old.
Ferguson and colleagues estimate that between 6000 and 32,000 infections had occurred in Mexico by 30 April, a figure they arrived at by comparing international travel patterns and detected, confirmed cases worldwide. Using this estimate, the percentage of deaths from the virus ranges between 0.4 and 1.4 percent of all H1N1 cases.
The researchers also determined the virus' transmissibility or its reproductive number, which refers to the number of cases that one case generates on average over the course of the infectious period. The H1N1 virus' transmissibility appears to be similar to or lower than that shown by the viruses of the deadly flu pandemics in 1918, 1957, and 1968.
The Science authors say their transmissibility estimates are consistent with or lower than those used in the computer simulations used to study pandemic scenarios, meaning that the policy decisions made based on these simulations could be considered appropriate for the current outbreak.