Influenza A (H1N1) vaccine example

In response to the pandemic influenza A H1N1 strain, many countries had engaged in mass immunization against flu in 2009. Awareness of the expected background rates of possible adverse events was estimated crucial to the assessment of possible vaccine adverse reactions.34

Highly visible health conditions, such as Guillain-BarrĂ© syndrome, spontaneous abortion and death, can occur in close proximity to vaccination in substantial numbers when large populations are vaccinated. 

For example, for every 10 million individuals vaccinated in the United Kingdom, 21.5 cases of Guillain-Barré syndrome and 5.75 sudden deaths were expected to occur as unrelated coincidental events within 6 weeks of vaccination.34

Careful interpretation of vaccine safety signals was crucial to detect real reactions to vaccine and to ensure that coincidental events were not caused by vaccination and did not affect public confidence in the vaccine. Experts compared background incidence rates of the condition with the rate following a vaccination programme to be able to monitor potential increases of events.