More details on H1N1, courtesy of Science

This article was originally on a blog post platform and may be missing photos, graphics or links. See About archive blog posts.

To determine H1N1’s efficiency as a killing machine, epidemiologists have to know how many people have actually been infected with the novel influenza virus. That has been tricky – particularly in Mexico, the epicenter of the outbreak – since thousands of people could have had the so-called “swine flu” without becoming sick enough to see a doctor.

A report released today by the journal Science uses data on international travel patterns to estimate that 23,000 Mexicans were infected by late April, though it could have been as low as 18,000 or as high as 32,000. Those figures imply that the flu was fatal for 0.3% to 0.6% of patients – about the same as for the 1957 flu pandemic and less than in 1918.

The research team, made up of members of the World Health Organization’s Rapid Pandemic Assessment Collaboration, zeroed in on confirmed flu cases outside of Mexico because travelers from that country were monitored more closely than Mexicans themselves. With those figures in hand, the scientists worked backward to calculate the number of Mexicans who had the H1N1 flu. Their models assumed that tourists and Mexicans were equally susceptible to the virus, and that both groups mixed freely.

In addition, the researchers compared the genetic sequences of 23 hemagglutinin genes – the H1 of “H1N1” – that have been posted online. Based on the small differences among the samples, the team estimated that all the strains could trace their ancestry back to a virus that was circulating Jan. 12 (though it could have been any time between Nov. 3, 2008 and March 2).

The team used a variety of methods to estimate that each infected person in Mexico generated an additional1.2 to 1.6 cases of H1N1 flu there, demonstrating that sustained human-to-human transmission did occur. By comparison, that “reproduction number” has been estimated to be between 1.4 and 2 during the pandemics of 1918, 1957 and 1968, according to the study.

-- Karen Kaplan