By Carol A. Kemper, MD, FACP

Clinical Associate Professor of Medicine, Stanford University, Division of Infectious Diseases, Santa Clara Valley Medical Center

SOURCE: Bénet T, Amour S, Valette M, et al. Incidence of asymptomatic and symptomatic influenza among healthcare workers: A multicenter prospective cohort study. Clin Infect Dis 2020; Aug 4. doi:10.1093/cid/ciaa1109.

It happens every year: Patients in the hospital for other reasons suddenly develop a fever and test positive for influenza (or respiratory syncytial virus or other viral illness). Despite all precautions, influenza vaccination, handwashing campaigns, and messaging to staff not to come to work with respiratory symptoms, healthcare workers (HCWs) are an important source of nosocomial influenza and respiratory infection. Now, it is happening with COVID-19 — employees with sniffles come to work, thinking they have a cold, only to test positive for SARS-CoV-2.

Bénet et al demonstrated just how common subclinical influenza really is in HCWs. The authors enrolled 278 HCWs providing active care at five French hospitals during the 2016-2017 winter season. Participants maintained a daily diary of symptoms and were seen for physical examination the first time in October through December before the beginning of the flu season, again in January during peak flu season, and then approximately three weeks following their second visit. Researchers obtained nasopharyngeal swabs for influenza PCR (Virocult) and serologies by hemagglutination inhibition (IHA) for influenza A H3N2- and B Victoria lineage B/Brisbane/60/2008-specific antibodies. In the event of symptoms, participants returned for an additional visit with these tests. The median age of participants was 36 years and 84% were female. Vaccine coverage was 42% for 2015-2016 and 49.6% for 2016-2017. Pauci-symptomatic infection was defined as the presence of one or more signs or symptoms for more than one day, with no fever (< 37.8°C), or the absence of cough and sore throat, whereas symptomatic influenza was defined as fever ≥ 37.8°C with either cough or sore throat.

Sixty-two participants developed influenza infection during the five-month study, with a cumulative incidence of 22.3%. Impressively, 46.8% and 41.9% of these were asymptomatic and pauci-symptomatic, respectively, while only 11.3% developed more classic symptoms. Fever occurred in less than 10% of cases. At the second evaluation in January, people with confirmed influenza reported runny nose (68%), cough (64%), and headache (56%). At the third evaluation, those with confirmed influenza reported runny nose (55%), cough (45%), and headache (36%). The cumulative incidence of influenza infection did not appear to differ between those who received the 2016-2017 influenza vaccination and those who did not (20.3% vs. 24.3%; P = 0.38), although receipt of the 2015-2016 influenza vaccination was protective (16% vs. 27%). Working in a nursing capacity increased the risk of pauci-symptomatic or symptomatic influenza. However, work in the ICU or the presence of three or more adults in the home was associated with an increased risk of asymptomatic infection.

Nearly nine of 10 hospital staff with confirmed influenza had subclinical infection (half were entirely asymptomatic). Symptoms in 42% were atypical, with at most minor sniffles, headache, or sore throat. Attempts to keep such minimally symptomatic HCWs out of the hospital has been challenging. As our emergency room medical director said the other day, physicians still will come to work with a runny nose; they are just too important, and we do not have sufficient staff to keep everyone home with a cold. Although little can be done about asymptomatic infection, we must find a way to rapidly screen minimally symptomatic employees for subclinical serious infections, such as the flu or COVID-19.