It’s a reassuring thing when science works the way it’s supposed to and the pieces mesh together. A paper by McLeish and colleagues just came out looking at sero-prevalence of pandemic H1N1 from 2009 to winter 2010 in Scotland. This comes from measuring antibodies specific to pandemic H1N1 in a large sample of the population. McLeish et al. find that through March 2010, approximately 35% of people in Scotland were infected with pandemic H1N1 influenza. This number apparently came as a surprise to the media and the story has been getting a lot of play.

However, what’s cool is that this 35% number is exactly what we expected from our epidemiological models. The basic reproductive number R₀ measures the number of secondary infections expected to result from a given infection in a naive host population (a population with no previous exposure). This number was quite small for the H1N1 pandemic, estimated at around 1.2 to 1.3 from the early upswing of the pandemic in 2009. The original paper detailing the SIR model (Kermack and McKendrick 1927) gives the formula: $Z = 1 - e^{-R_0 Z}$, where Z represents the final size of an epidemic (in terms of proportion of the population infected). Numerically solving this for these values of R₀ gives an expected final epidemic size of 31% to 42%. This is amazingly on target.