Prior season vaccination and risk of influenza during the 2014-2015 season in the U.S.
Manjusha Gaglani M.D.
Chung, J. R., B. Flannery, R. K. Zimmerman, M. P. Nowalk, M. L. Jackson, L. A. Jackson, J. G. Petrie, E. T. Martin, A. S. Monto, H. Q. McLean, E. A. Belongia, M. Gaglani and A. M. Fry (2017). “Prior season vaccination and risk of influenza during the 2014-2015 season in the U.S.” J Infect Dis: 2017 Jun [Epub ahead of print].
The US Flu VE Network conducts annual studies of VE using the test-negative study design that is also used in Canada. In the Canadian study, current and prior-season vaccination status is based on a combination of patient self-report and sentinel practitioner documentation. In the US Flu VE Network, current season vaccination status is also based on a combination of patient self-report and electronic immunization records; however, prior-season vaccination is based on immunization records only. Misclassification of vaccine history may result from inaccurate self-report or incomplete immunization records. One study that compared self-reported influenza vaccination to an immunization registry found that patients overreported vaccination by approximately 10% ; recall of prior seasons’ vaccination may be less accurate. To minimize misclassification of vaccination history in 2 prior seasons, we considered documented doses only among patients aged ≥9 years with medical records available for at least 2 years prior to enrollment, and excluded patients who reported 2014–2015 influenza vaccination that was not documented. After adjusting for age and other potential confounding variables, we found no statistically significant association between vaccination in 3 consecutive seasons and A(H3N2)-related illness during 2014–2015 (Table 1). However, we observed the highest point estimate among persons vaccinated in 2014–2015 only. A sensitivity analysis restricted to the main genetic group (clade 3C.2a) of antigenically drifted A(H3N2) and influenza negatives resulted in similar estimates (data not shown). Although the higher point estimate for vaccination only in 2014–2015 is consistent with potential negative interference from prior vaccination , our results do not support evidence of increased likelihood of influenza due to A(H3N2) viruses among repeatedly vaccinated individuals compared to those unvaccinated in 3 consecutive seasons.