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.
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.
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 %.
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.

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