Spinal corticosteroid injections are not associated with increased influenza risk.

Background Context: Spinal corticosteroid injections (CSI) are often used to treat radicular and axial pain arising from the spine. Systemic corticosteroids are well known to cause immunosuppression, and locally injected spinal CSI are known to have some systemic absorption. However, it is unknown whether spinal CSI increases the risk of systemic viral infections, such as influenza.

Purpose: To determine whether spinal CSI causes an increased risk for influenza infection and whether they reduce the protective effect of vaccination STUDY DESIGN/SETTING: A retrospective cohort study was performed at Kaiser Permanente Northern California, a large healthcare system with a diverse population.

Patient Sample: Adults (n=60,880) who received a spinal CSI during influenza seasons from 2016 to 2019. A comparison was made with 121,760 case-matched individuals who did not receive a spinal CSI.

Outcome Measures: The primary outcome was odds of influenza diagnosis following spinal CSI compared with case-matched controls. Secondary analysis examined odds of influenza diagnosis based on vaccination status, multiple same-day injections, and epidural versus non-epidural route of injection.

Methods: The electronic health record and associated research databases were analyzed to identify patients who received a spinal CSI during three consecutive flu seasons, 2016 through 2019. Injections were stratified into epidural versus non-epidural CSI and single injections versus multiple same-day injections. Additionally, the rate of influenza in vaccinated versus non-vaccinated individuals was examined. Inpatient flu diagnosis was used as a proxy for severe disease. After case matching was completed, odds ratios for flu diagnosis were calculated using a logistical regression model.

Results: The odds of flu diagnosis following spinal CSI were not increased compared with controls (OR 0.93 [0.87-1.01, 95% Wald CL]). For epidural CSI the OR was 0.91 (0.83-1.00, 95% Wald CL), and non-epidural it was 1.00 (0.89-1.13, 95% Wald CL). There were similar findings for multiple same-day injections and when looking at inpatient flu diagnosis. For vaccinated individuals, the OR for flu following spinal CSI was 0.86 (0.80-0.92, 95% Wald CL), which indicates a protective effect in these patients.

Conclusions: Spinal CSI did not increase the odds of subsequently receiving a diagnosis of influenza, regardless of vaccination status, location of injection, single versus multiple same-day injection, or co-morbidity. Vaccination had a protective effect against influenza, and this was not adversely affected by receiving spinal CSI during the flu season.