Annual influenza immunization is widely recommended for healthcare workers (HCWs) to reduce the burden of healthcare-acquired influenza (HAI).^{1,Reference Grohskopf, Alyanak, Broder, Walter, Fry and Jernigan2} Aside from the benefits to HCWs in reducing their own risk of illness, HCW immunization is associated with a decrease in morbidity and mortality and the risk of influenza-like outbreaks, particularly in residents in long-term care settings.^{Reference Carman, Elder and Wallace3–Reference Shugarman, Hales, Setodji, Bardenheier and Lynn6}

In acute-care hospitals, an inverse relationship between HCW influenza immunization rates and healthcare-acquired influenza (HAI) has been reported, but data are lacking regarding the minimum immunization rate that needs to be achieved to prevent HAI in this setting.^{Reference Salgado, Giannetta, Hayden and Farr7–Reference Frenzel, Chemaly and Ariza-Heredia9} Our goal was to assess the association between HCW influenza immunization and the incidence of HAI across 2 acute-care facilities to inform our HCW immunization programs.

We performed a multicenter prospective cohort study at 2 acute-care hospitals including 1 tertiary-care, university-affiliated, teaching hospital (638 beds) and 1 community-based academic hospital (431 beds). HCW influenza immunization rates and the incidence of HAI among patients were collected for the 2013–2014 to 2018–2019 influenza seasons from all inpatient acute-care wards including maternity, pediatrics, and psychiatry wards but excluding long-term care and rehabilitation wards as well as the emergency department and outpatient areas. The 2017–2018 season was excluded due to significant vaccine mismatch. The annual HCW influenza immunization rate was defined as the proportion of HCWs immunized prior to December 15 (the date for submission to public health authorities); it included all nurses as well as allied and support staff assigned to an inpatient ward but excluded physicians, students, and volunteers because they are not assigned to specific units. HCW influenza immunization at both facilities was voluntary and did not require a signed declination. During the 2014–2015, 2015–2016, and 2016–2017 seasons, there was a vaccine or mask policy. A case of HAI was defined as laboratory-confirmed influenza A or B, detected from upper-respiratory-tract swabs tested via multiplex polymerase chain reaction (PCR), with symptom onset >72 hours after admission. All patients were assessed for new onset of fever and/or new or worsening respiratory symptoms (eg, cough, shortness of breath, sore throat, and runny nose/congestion) on a daily basis. Those patients meeting these criteria were tested for Influenza. If a test had not already been ordered, the infection prevention and control professional recommended that one be obtained. The incidence of HAI was calculated per 1,000 patient days and the association between inpatient ward HCW influenza immunization rate and the incidence of HAI were compared using a Poisson regression analysis adjusting for hospital and influenza season.

Over 5 influenza seasons, HCW influenza immunization rates and cases of HAI were collected on 36 inpatient acute-care wards: 20 wards at facility A and 16 wards at facility B. The median HCW influenza immunization rate was 58.9% (IQR, 46.6%–70.3%) overall; 57.3% (IQR, 42.5%–66.4%) at facility A and 66.6% (IQR, 50.6%–76.8%) at facility B. In total, 144 cases of HAI were identified, for an overall incidence of 0.23 per 1,000 patient days, including 0.24 and 0.22 per 1,000 patient days at facilities A and B, respectively. In the regression analysis, HCW influenza immunization rates of ≥65% and 70% were not associated with a decrease in HAI incidence. In contrast, a HCW influenza immunization rate of ≥75% was associated with a trend toward reduced HAI (IRR, 0.65; 95% CI, 0.39–1.08; P = .096), whereas inpatient wards >80% immunization had significantly lower risk of HAI (IRR, 0.28; 95% CI, 0.089–0.89; P = .03) (Table 1). During the 2017–2018 mismatch season, the HAI was highest compared to other seasons at 0.46 per 1,000 patient days. When the regression model was repeated including 2017–2018, the overall association was no longer significant.

Table 1. Multivariate Poisson Regression Analysis of the Association Between Healthcare Worker Influenza Immunization Rates and Hospital Acquired Influenza in Patients

Note. HCW, healthcare worker; IRR, incidence rate ratio; CI, confidence interval.

Previous modeling studies have demonstrated that increasing HCW influenza immunization rates correspond to protection of patients against HAI with no maximum beyond which there is no effect^{Reference van den Dool, Bonten, Hak and Wallinga8}; however, the minimum target is not well established. Our multicenter study indicates that acute-care hospitals with HCW immunization rates as high as 70% may offer no significant protection of patients from HAI. Rates of at least 75% and ideally 80% are required to see a significant impact on prevention of HAI. Our results are similar to a study in oncology patients where increased HCW influenza immunization rates from 56% to 94% resulted in decreased HAI incidence.^{Reference Frenzel, Chemaly and Ariza-Heredia9}

The minimum target for hospital influenza immunization has significant implications for the type of interventions required to prevent HAI. Most hospital HCW immunization programs rely on education and improved access to vaccine, but these are unlikely to achieve the minimum 75% threshold.^{Reference Salgado, Giannetta, Hayden and Farr7} Achieving a rate >80% requires higher effectiveness systems of care such as institutional policy requiring influenza immunization or opt-out programs that require education for HCW declining vaccination.^{Reference van den Dool, Bonten, Hak and Wallinga8,Reference Pitts, Maruthur, Millar, Perl and Segal10}

Our study has several limitations. It included only 2 acute-care hospitals, and we excluded the mismatch season during which no association between immunization rate and HAI was detected. The HCW influenza immunization rates also excluded physicians; therefore, we may have underestimated the benefit of influenza immunization. We cannot exclude confounding of the incidence of HAI by the use of masks during the mask or vaccine policy; however, this would have biased toward a lack of difference from immunization. Cases of HAI in patients with onset of symptoms after discharge were not included unless they were readmitted to the study facility, which may have underestimated true HAI incidence.

Our study suggests that acute-care hospitals must target influenza immunization rates >75% to see appreciable reductions in HAI. In the context of a COVID-19 pandemic, it will be even more important for hospitals to augment their immunization programs beyond educational initiatives to achieve this minimum threshold.