Severe Influenza Experienced By Infants Highlights Need For Preventative Strategies


Data show the proportion of infants with influenza aged <6 months ranged from 1.0 – 2.6% among all infant hospitalizations in the 2019 influenza season.

Although infants aged <6 months are vulnerable to severe influenza, vaccinations are not currently recommended for this age group due to potentially inadequate immunologic response.

As a result, investigators led by Jocelynne E. McRae, National Centre for Immunization Research and Surveillance, Kids Research, Sydney Children’s Hospitals Network aimed to describe the clinical epidemiology and risk factors associated with severity outcomes in infants <6 months hospitalized with influenza in Australia.

Their findings showed these patient populations experienced severe influenza, highlighting the need for preventative strategies including maternal immunization and a continued investigation into safe and efficacious vaccination.


All infants aged <6 years hospitalized with influenza were captured through the Pediatric Active Enhanced Diseases Surveillance (PAEDS) network and The Influenza Complications Alert Network (FluCAN) across 9 consecutive influenza seasons from 2011 - 2019.

During 2011 - 2013, infants who were laboratory test positive for flu by either immunofluorescence assay (IF) or rapid antigen test at 2 sites were eligible to be included. Additionally, a patient was eligible if they exhibited acute respiratory illness (ARI) symptoms with or without fever.

Then, from 2017 on, patients with non-ARI symptoms compatible with influenza disease who had laboratory-confirmed influenza were also included.

Data on risk factors including age, indigenous status, presence of underlying medical risk factors, premature birth, maternal vaccination, and household smoking behavior were analyzed. In addition, the proportion of infant influenza hospitalizations and nosocomial cases among all hospitalizations were reported.


During the study period, a total of 680 hospitalized infants aged <6 months were identified. Of all infants, most were diagnosed with influenza A (n = 530, 77.9%) and male (57.9%), while 14.5 identified as Indigenous Australian.

Additionally, the median age was 2.6 months (IQR 1.4 - 4.2 months) with 130 (19.2%) born premature. A total of 129 infants (19.0%) had ≥1 comorbid conditions, with the most common being cardiac disease (48.8%) and investigators found 30.9% (n = 210) had a preexisting risk factor for severe influenza.

Moreover, maternal vaccination data from 2016 - 2019 found 66 of 220 mothers (29.6%) had received a current season influenza vaccine. There were 53 nosocomial cases (7.8%) reported.

McRae and colleagues observed ICU admission occurred in 14.7% of patients and oseltamivir was prescribed for 18.8%. They found factors associated with ICU admission included age ≤1 month (adjusted odds ratio, 3.95; 95% CI, 1.47 - 10.60), comorbidity (aOR 7.69; 95% CI, 4.04 - 14.64), and prematurity (aOR 2.60; 95% CI, 1.40 - 4.81).

Those associated with oseltamivir use included ICU admission (aOR 3.09; 95% CI, 3.39 - 10.95) and prematurity (aOR, 2.30; 95% CI, 1.34 - 3.95). Through the primary analysis, investigators observed an Indigenous Australian background was negatively associated with oseltamivir use (aOR 0.43; 95% CI, 0.19 - 0.96).

Lastly, in the 2019 influenza season, the proportion of infants with influenza among all infant hospitalizations was considered to be 1.5% (1.0 - 2.6%), at a rate of approximately 1 out of every 67 admissions.


Overall, investigators noted that without influenza vaccinations, efforts to increase infant protection via improved maternal vaccination uptake is needed throughout the world.

“Increasing other preventative measures including physical distancing from unwell contacts and good hygiene practices should also be encouraged, given their observed effectiveness in the COVID-19 pandemic response,” investigators wrote.

The study, “Risk factors and disease severity in Australian infants aged under 6 months hospitalised with influenza 2011–2019,” was published in the Journal of Paediatrics and Child Health.

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