Seasonal influenza vaccine effectiveness study in older adults during three influenza seasons (2015-2018) in Lithuania

Background: Due to lack of knowledge about seasonal influenza vaccine effectiveness (SIVE) against laboratory-confirmed influenza in patients aged ≥65 years a study to measure SIVE in patients admitted to hospital due to severe acute respiratory infection (SARI) in Lithuania during the 2015-2018 was conducted.

Materials/methods: A test-negative case-control study was performed between 2015-2016 and 2017-2018 influenza seasons. Subjects with multiplex RT-PCR confirmed influenza were defined as cases, and subjects negative for influenza were selected as controls. SIVE and its 95% confidence intervals (95%CI) were calculated as (1-OR)*100%.

Results: Overall 407 patients were recruited, 46(11.3%) of whom were vaccinated against influenza at least two weeks before the onset of SARI symptoms. Influenza was confirmed to 188(46.2%) subjects, while 219(53.8%) were influenza negative. Influenza A(H1N1)pdm09 was predominant in 2015-2016(96.2%), influenza A(H3N2) dominated in 2016-2017(93.3%), while influenza B/Yamagata was detected most often in 2017-2018(93.1%).

The unadjusted SIVE against any influenza was 41% (95%CI:-110% to 69%). The unadjusted SIVE in 2015-2016 influenza season was 41% (95%CI: -134% to 85%), and 63% (95%CI:-213% to 96%) against any influenza and influenza A(H1N1)pdm09 respectively. The unadjusted SIVE in 2016-2017 influenza season was 30% (95%CI:-74% to 72%), and 18% (95%CI:-103% to 67%) against any influenza and influenza A(H3N2) respectively. The unadjusted SIVE in 2017-2018 influenza season was 51% (95%CI:-76% to 86%), and 54% (95%CI:-79% to 88%) against any influenza and influenza B/Yamagata respectively.

Conclusions: Nearly half of the hospitalized SARI cases were confirmed with influenza, which shows high influenza disease burden in Lithuania for the older people. Although SIVE estimates confidence intervals are broad and results can serve only as indicatory, the point estimates suggest moderate SIVE in 2015-2016 and 2017-2018, and low SIVE in 2016-2017.