Vaccine Case–Population: A New Method for Vaccine Safety Surveillance

Introduction The case–population approach compares exposure among cases to that of their source population. By using aggregated data to estimate the denominator, this approach can provide a real-time estimate of an association that could be particularly valuable to explore urgent vaccine safety concerns and to generate signals during a vaccine campaign. Objective Our objective was to present the vaccine case–population method, a method derived from the case–population approach and adapted for vaccine safety surveillance, and to test it using several published examples. Methods For the vaccine case–population method, exposure in the population is estimated from the sum of at-risk periods using the number of vaccinated individuals, or data of vaccine sales, and the at-risk period considered for the vaccine–event pair. The vaccine case–population method was applied to data from published case–control studies retrieved from the MEDLINE database and having quantified risks associated with vaccines. Odds ratios derived from the vaccine case–population method were compared with those from published case–control studies. Results A total of 20 vaccine–event pairs were retrieved in which the vaccine case–population method could be applied. For all identified vaccine–event pairs, when a significant association was found using the vaccine case–population method, a significant association was also found in the corresponding case–control study. Conversely, when no association was found by the vaccine case–population method, no association was found in the corresponding case–control study. Conclusion These results suggest that the vaccine case–population method can produce coherent conclusions and may be used in the future for prospective investigation of urgent vaccine safety concerns or for the prospective generation of vaccine safety signals. This method could also be used to identify selection bias from cases excluded from the case–control study.