• Medical News: Swine Flu
2009 Swine Flu More Transmissible Than Seasonal Flu
LITTLE FALLS, N.J., May 11 -- The new A H1N1 (swine) flu virus appears to be as clinically severe as the A H2N2 virus responsible for the 1957 Asian flu pandemic, preliminary data showed.
In addition, epidemiological studies and a genetic analysis found that the 2009 virus seems to be more transmissible than seasonal flu, Neil Ferguson, D.Phil., of Imperial College London, and colleagues reported online in Science.
The study authors are all members of the World Health Organization (WHO) Rapid Pandemic Assessment Collaboration.
The team made their estimates on the basis of information from an outbreak in a small Mexican community, early data on the international spread of the virus, and genetic sequencing data.
"While these estimates are informative, it should be emphasized that some uncertainties remain regarding the denominator population," they said. "Household data would be particularly useful in reducing remaining uncertainty."
Using epidemiological data collected since the beginning of the outbreak, the researchers estimated that 23,000 individuals (range 6,000 to 32,000) had been infected with the H1N1 virus in Mexico by the end of April.
That put the estimated case fatality rate at 0.4% (range 0.3% to 1.5%).
"Thus while substantial uncertainty remains," the team said, "clinical severity appears less than that seen in 1918 but comparable with that seen in 1957."
The 1957 Asian flu pandemic, caused by an A H2N2 strain, killed about 70,000 people in the U.S., according the National Vaccine Program Office of the Department of Health and Human Services.
These estimates "formed part of the evidence base used by WHO to move to phase 5" on the influenza pandemic alert scale, according to the researchers. Phase 6 is the full pandemic phase.
Three different epidemiological analyses were used to estimate the virus's basic reproduction number: the number of secondary cases that would be generated by each infected individual in a completely susceptible population.
The estimates ranged from 1.4 to 1.6, similar to or lower than those for previous pandemics. The numbers for the 1918, 1957, and 1968 flu pandemics ranged from 1.4 to 2.0.
The average basic reproduction number for seasonal flu is 1.3, according to a perspective written by Mark Miller, M.D., of the NIH in Bethesda, Md., and colleagues last week in the New England Journal of Medicine.
"Transmissibility is therefore substantially higher than seasonal flu," the WHO scientists said this week.
Dr. Ferguson and colleagues stressed the preliminary nature of their study.
"There are uncertainties about all aspects of this outbreak, including the virulence, transmissibility, and origin of the virus, and this, in turn, results in uncertainty in judging the pandemic potential of the virus and when reactive public health responses, such as recommendations to stay at home or to close schools, should be implemented in individual countries," they said.
Specifically, they cited uncertainties involving incubation and infectious periods, the possibility of cross-immunity from previous exposure to other influenza subtypes, the future evolution of the transmissibility, antigenicity, virulence, and antiviral resistance profile, and whether the new virus will displace existing influenza A subtypes in the human population.
In addition, they said, "much remains to be done to estimate clinical severity of infection, and to understand regional variations seen so far (or indeed whether they exist)."
Finally, they said, "The extent to which seasonal damping of transmission in North America and Europe is responsible for the moderate transmissibility seen to date is uncertain; the progress of transmission in the Southern Hemisphere (which is just entering its influenza season) needs to be carefully monitored in the next few months."
Primary source: Science
Source reference:
Fraser C, et al "Pandemic potential of a novel strain of influenza A (H1N1): early findings" Science 2009; DOI: 10.1126/science.1176062.
