The Risk of Severe Influenza Among Infants Younger Than 6 Months

Infants younger than 6 months are at risk of severe influenza, according to a recent study that evaluated risk factors and disease severity in infants in Australia for whom influenza vaccination is not currently recommended.

The study, published in the Journal of Paediatrics and Child Health, included 680 infants hospitalized with laboratory-confirmed influenza at multiple locations in Australia from 2011 to 2019. Cases were identified by Paediatric Active Enhanced Disease Surveillance network (PAEDS) and The Influenza Complications Alert Network (FluCAN).

The median age of infants included in the study was 2.6 months, 57.9% were male, 14.5% were indigenous Australians, 130 (19.2%) were born premature, 129 (19%) had at least one comorbidity other than prematurity, such as cardiac disease, respiratory disease, genetic condition and neurologic disease. Some 530 (77.9%) of infants had influenza A, 100 (14.7%) were admitted to the ICU, 127 (18.8%) received oseltamivir and 53 (7.8%) cases were nosocomial.

“This data underpins the need for preventative strategies such as maternal immunization and continued investigation into the possibility of safe and efficacious vaccination prior to 6 months of age,” wrote the study authors, led by Jocelynee McRae, MHSc, MPH, of the National Centre for Immunization Research and Surveillance, Kids Research, Sydney Children’s Hospitals Network.

The authors noted that influenza vaccination isn’t recommended for infants younger than 6 months and that maternal vaccination has been shown to provide some protection from illness and reduce hospitalization related to influenza among infants by 45% to 92%.

Among those study participants for whom maternal vaccination status was known, 66 of 223 (29.6%) of mothers had a current influenza vaccine during pregnancy.

“Despite recommendations to prioritize influenza vaccination of pregnant women globally, uptake is suboptimal in many countries and others do not have influenza vaccination programs for pregnant women,” the authors wrote. “The risk of severe influenza disease in infants is under recognized, and messaging to immunization providers and pregnant women may be inadequate.”

Risk factors for ICU admission included age younger than 1 month (adjusted odds ratio 3.95, 95% confidence interval: 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), with neonates having the highest rate of ICU admission and the longest hospital stay.

The study authors noted disparities, including indigenous Australian infants had longer hospital stays and appeared less likely to have received oseltamivir in a primary analysis. Influenza vaccination uptake was higher among indigenous Austalians, reaching 64%.

“Without influenza vaccination in this age group, efforts to increase infant protection via improving maternal vaccine uptake is needed, both in Australia and elsewhere,” the study authors wrote. “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.”

Limitations of the study include a lack of data on respiratory virus co-infections, possibility of under-ascertainment of cases, exclusion of cases because of changes in reporting criteria and small numbers in some subgroups.