A series of diagnostic contamination events from seasonal influenza vaccines

BACKGROUND: Polymerase chain reaction (PCR) is preferred for diagnosing influenza, allowing differentiation between influenza A and B viruses and their subtypes/lineages. However, PCR's high sensitivity can lead to false positives from contamination. During the 2022-2023 influenza season in Norway, a pattern of diagnostic samples testing positive for both influenza A and B or the presumably extinct B/Yamagata lineage suggested external contamination. METHODS: We conducted PCR testing for these samples, environmental samples from vaccination sites, and vaccines, alongside a retrospective review of test data. RESULTS: This review revealed an increased frequency of unusual detection patterns, particularly during weeks 42-46 of 2022, aligning with the peak of the vaccination campaign. During this period, suspected vaccine-contaminated samples comprised 3.45 % of all influenza-positive samples. Environmental sampling at vaccination sites confirmed the presence of influenza A and B RNA, supporting suspicion of vaccine-derived contamination. Detection in suspect diagnostic specimens of multiple influenza types/subtypes/lineages, including the extinct B/Yamagata lineage, corroborated the seasonal influenza vaccine's contamination source. CONCLUSION: These findings highlight the risk of diagnostic sample contamination with RNA from influenza virion-derived vaccines, impacting diagnostic accuracy and public health surveillance.