Surveillance for Influenza among Healthcare Workers in a Tertiary Children's Hospital

Background: Healthcare workers (HCWs) are a common source of healthcare-associated influenza, and transmission via subclinical infection has been suspected. We conducted prospective surveillance for influenza in a cohort of HCWs during the 2009-10 influenza season to measure the incidence of asymptomatic and mildly symptomatic infection.  

Objective: The primary aim was to determine the incidence of laboratory-confirmed influenza among HCWs. Secondary aims were to identify any differences in the amount or duration of viral shedding between vaccinated and unvaccinated HCWs. 

Methods: From September – October 2009, HCWs were recruited from 2 inpatient units at the Monroe Carell Jr. Children’s Hospital at Vanderbilt, a 238-bed freestanding children’s tertiary care hospital. Nasal swabs were collected every 2 weeks over 20 weeks, from November 16, 2009 through April 16, 2010, and tested for influenza by polymerase chain reaction. Subjects reporting new onset of illness underwent collection of an additional (“sick”) nasal swab. Serum samples were collected pre- and post-surveillance for influenza hemagluttinin inhibition assay (HAI) testing. Volunteers completed symptom questionnaires with each collection, and were surveyed before and after the surveillance period on the presence of risk factors for influenza infection, self-report of influenza vaccination, recall of influenza-like illness (ILI), and missed work due to ILI. Vaccination was confirmed by record review. Study staff were blinded to laboratory results and vaccination status until study completion. 

Results: The study enrolled 170 HCWs. Median age was 31 (IQR 16), and 82% of the cohort was female. Nurses (n=64; 38%) and physicians (n=39; 23%) comprised the majority of the study population. A total of 1394 nasal swabs (median 10/subject), including 121 sick swabs, were collected. No influenza virus was detected by PCR. Of 125 volunteers with paired blood samples, 1 volunteer demonstrated a ≥4-fold rise in Novel 2009 H1N1 (nH1N1) titer (reciprocal geometric mean 5.7); however, the timing of the first blood draw in relation to vaccination suggested a vaccine effect. Of the 151 volunteers who reported their vaccination status, 112 (74.2%) received nH1N1 vaccine. 

Conclusions: Influenza virus was not detected by PCR or paired serology in our cohort. Several factors may have impacted the detection of influenza in this study:  1) Influenza circulation peaked prior to the onset of surveillance, with limited community circulation during the surveillance period, and 2) a large majority of the study population received influenza vaccine. Previous studies have indicated a substantial frequency of influenza shedding in asymptomatic and mildly symptomatic persons; the above work merits repeating during a typical influenza season to assess influenza shedding in this important population.