Estimating the reproduction number, sometimes shown or referred to as "R", is very important because it tells us how well a particular influenza virus will spread. Reproduction number refers to the average number of secondary influenza cases caused by one primary case. In other words, if we have an influenza virus with a reproductive number of 3, then each person infected will likely spread the virus to three other people, on average.
Of course, the true reproduction number is difficult to know since we can't be certain of the total number of influenza cases, etc. However, scientists have pretty good methods for estimating it.
A study just out from New Zealand puts the reproduction number at 1.96. The study notes that estimates from Mexico were ranged from 1.4–1.6 and in Japan, for the current wave, 2.0–2.6.
What does this mean? Seasonal flu has a reproduction number of 1.3 - 1.5, so if estimates in New Zealand and Japan are correct, then it would mean that H1N1 is transmitting more efficiently (and will spread more quickly) than seasonal flu. However, these estimates contradict some preliminary studies of H1N1 in ferrets. We'll have to keep an eye on the evidence as it accumulates.
To read the study, visit: http://www.nzma.org.nz/journal/122-1299/3722/
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