SAN FRANCISCO — During the 2017-2018 influenza season, amid reduced vaccine effectiveness and a preponderance of H3N2 cases, the CDC reported rates of influenza-related illness not seen since the 2009 pandemic and the highest rate of influenza-related hospitalization ever recorded.
According to surveillance numbers presented at IDWeek, among the season’s unique characteristics, researchers found that hospitalization rates among older adults peaked several weeks earlier than children compared with previous influenza seasons.
“Hospitalization rates during 2017-2018 were the highest we’ve seen since all-age surveillance began in 2005,” Shikha Garg, MD, MPH, of the CDC’s Influenza Division, told Infectious Disease News. “We conducted this study to better understand what factors may have contributed to the high rates.”
Garg and colleagues included all patients in the Influenza Hospitalization Surveillance Network (FluSurv-NET) who were hospitalized with laboratory-confirmed influenza during the 2017-2018 season, calculating influenza rates by type/subtype per 100,000 population and comparing results to the four most recent influenza seasons.
The study results showed that the overall unadjusted hospitalization rates per 100,000 population varied from 31.5 during 2015–2016 to 105.1 during 2017-2018. Last season also had the highest hospitalization rate observed for H3N2 (62.8 per 100,000 people) and influenza B (28.5) and the third-highest rate of H1N1 (13.5)
During the 2017-2018 season, hospitalization rates in adults aged 65 years and older peaked 3 weeks before they peaked in children aged 0 to 4 years. According to Garg and colleagues, during the four prior seasons, rates in adults peaked the same week or 1 week after they peaked in children. Additionally, during the 2017-2018 season, the distribution of influenza types or subtypes varied significantly by age group. For example, the authors said, the proportion of patients with H1N1 ranged from 19% to 29% in those younger than age 65 years to only 7% in those aged 65 years and older.
“We observed that even though it was an H3N2-predominant season, there was substantial co-circulation of H1N1 and B in addition to H3N2,” Garg said. “Interestingly, rates of H1N1 were much higher than we typically see during H3N2-predominant seasons. In addition, the number of weeks of peak activity overall, and by type and subtype, were high when compared to prior seasons.”
Garg noted that the total number of patients hospitalized with influenza during the 2017-2018 season broke records, driven by those age 56 years and older, and said it is imperative that patients eligible for vaccination receive it annually.
“Influenza vaccination is the best tool we have to prevent serious complications from influenza, including hospitalization. Clinicians should encourage all of their patients older than 6 months to receive annual influenza vaccination to prevent flu and its complications,” Garg said. – by Caitlyn Stulpin
Reference: Garg, S, et al. Abstract LB17. Presented at: ID Week; Oct. 3-7. 2018; San Francisco.
Disclosures: Garg reports no relevant financial disclosures. Please see the abstract for all authors’ relevant financial disclosures.