Imagine there's a pandemic flu virus on the loose, and a vaccine has just been introduced. Then come reports of dozens of cases of Guillain-Barré syndrome (GBS), a rare type of paralysis. Did the new vaccine cause it? How would you even begin to know? One first step (though certainly not the only one) is to think about the background rate of disease:
Inappropriate assessment of vaccine safety data could severely undermine the eff ectiveness of mass campaigns against pandemic H1N1 2009 influenza. Guillain-Barré syndrome is a good example to consider. Since the 1976–77 swine influenza vaccination campaign was associated with an increased number of cases of Guillain-Barré syndrome, assessment of such cases after vaccination will be a high priority. Therefore, it is important to know the background rates of this syndrome and how this rate might vary with regard to population demographics. The background rate of the syndrome in the USA is about 1–2 cases per 1 million person-months of observation. During a pandemic H1N1 vaccine campaign in the USA, 100 million individuals could be vaccinated. For a 6-week follow-up period for each dose, this corresponds to 150 million person-months of observation time during which a predicted 200 or more new cases of Guillain-Barré syndrome would occur as background coincident cases. The reporting of even a fraction of such a large number of cases as adverse events after immunisation, with attendant media coverage, would probably give rise to intense public concern, even though the occurrence of such cases was completely predictable and would have happened in the absence of a mass campaign.
That's from a paper by Steven Black et al. in 2009, "Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 infl uenza vaccines". They also calculate background rates for spontaneous abortion, preterm delivery, and spontaneous death among other things.