Self-reported influenza and influenza-like symptoms in U.S. adults age 18–64 between September 1, 2019 and April 15, 2020

Background: The Influenza-like Illness Surveillance Network (ILINet) can indicate the presence of novel, widespread community pathogens. Comparing week-to-week reported influenza-like illness percentages may identify the time of year a novel pathogen is introduced. However, changes in health-seeking behavior during the COVID-19 pandemic call in to question the reliability of 2019−2020 ILINet data as a comparison to prior years, potentially rendering this system less reliable as a novel pathogen surveillance tool. Corroboration of trends seen in the 2019−2020 ILINet data lends confidence to the validity of those trends. This study compares predicted versus reported influenza and influenza-like illnesses in vaccinated adults as a surrogate measure of novel pathogen surveillance. Methods: An online survey was used to ask US adults their influenza vaccination status, whether they were diagnosed with influenza after vaccination, and whether they experienced an influenza-like illness other than flu. Results: Prevalence of self-reported flu diagnosis in adults age 18–64 who received the flu vaccine between September 1, 2019 and April 15, 2020 (n = 3,225) was 5.8 %, while self-reported flu or flu-like illness (without a flu diagnosis) was 17.9 %. Conclusion: Flu and flu-like illness in this sample of flu-vaccinated U.S. adults is significantly higher than predicted, consistent with substantially higher ILI's in 2019−20 compared to ILI's from 2018−19, suggesting that the ILI values reported during the COVID-19 pandemic may be appropriate for comparison to prior years.