Flu Near You (FNY) is a web-based participatory syndromic surveillance system where participants in the United States report their health using a weekly survey. Guest participants may also submit ad hoc reports. Participants are asked if they or household members feel healthy or sick. If sick, participants are prompted with a populated list of symptoms to select (Multimedia Appendix 1). Seasonal influenza vaccination status is also collected. Self-reported influenza diagnosis data were captured starting February 2020.

Data from 2016-2021 were included, with influenza seasons defined as Morbidity and Mortality Weekly Report weeks 40 through 20 (Multimedia Appendix 2, Table S1). Data were included for the entire FNY week that captured the influenza season start and end dates. Weekly reports by both registered and guest participants were included. Registered participants had a single identifier, while guest participants were assigned one per report. FNY used token validation between the front and back ends of the website to reduce the risk of spam reporting. Reports by registered participants were deduplicated by week if multiple reports were captured, with the most recent report included. For demographic analysis, each user was counted once per flu season, and those with missing data were excluded. No further restrictions were applied [17].

This study of FNY data received approval from the Boston Children’s Hospital Institutional Review Board (IRB-P00023700) and received a waiver of informed consent. FNY participants may report anonymously. Those who opted to provide contact information were deidentified. No compensation was provided to participants.

ILI was defined as fever in addition to sore throat, cough, or both [19]. Influenza diagnosis was classified as self-reported influenza diagnosis (hereafter called self-reported diagnosis). Influenza vaccination rate was defined as the proportion of unique reporters who indicated being vaccinated any time during the influenza season. ILI rates were determined as the number of reports meeting the ILI definition over the total reports during that time. Season and monthly ILI rates were assessed with October and May rates truncated to align with the defined season. CIs were calculated via a 1-sample test of proportions.

Total ILI cases and unique reports from the 2016-2017, 2017-2018, and 2018-2019 influenza seasons were averaged by month and are presented as “baseline seasons.” Demographics across the baseline seasons were aggregated to retain their specific proportions, accounting for participants reporting across seasons. Analyses were conducted with R (version 4.0.4; R Foundation for Statistical Computing).

During the baseline influenza seasons, 237,309 unique individuals reported on FNY, while 68,800 individuals reported for the 2019-2020 season and 25,996 in 2020-2021. There were more female respondents to FNY across seasons (Table 1). Higher proportions of older individuals reported across seasons (Table 1). In 2019-2020 and 2020-2021, respondents aged 65 years and older comprised the largest proportions (n=9812, 32.6% and n=8236, 39.9%, respectively) among those who reported age (n=30,139, 43.8% and n=20,650, 79.4%). Those younger than 18 years were the smallest proportion across influenza seasons (Table 1).

ILI variability was observed across the baseline seasons (Table 2). The characteristics of ILI captured on FNY in the described seasons followed patterns observed in traditional surveillance [9]. The 2016-2017, 2018-2019, and 2019-2020 influenza seasons were classified as moderate, and the 2017-2018 season was severe [9]. On FNY, the observed rate of ILI was 1.51% in 2016-2017, 4.98% in 2017-2018, and 4% in 2018-2019. Across the baseline seasons, the rate of ILI was 3.8%. Both the 2019-2020 and 2020-2021 seasons had lower ILI rates compared to the baseline seasons (Table 2). An ILI rate of 2.55% was observed in 2019-2020 and a rate of only 0.27% was captured in 2020-2021.

Monthly ILI rates displayed expected fluctuations within influenza seasons. The baseline seasons had ILI rates between 2.1% (95% CI 1.89%-2.34%) and 5.79% (95% CI 5.56%-6.02%), with peak activity seen from December to February (Figure 1). ILI activity peaked in January 2020 (4.5%) before moderately declining in February (3.29%) and further declining over the remainder of the season (Figure 1). For the 2020-2021 season, there was no evident peak in ILI (Figure 1).

Reductions in self-reported influenza diagnosis were detected in FNY (Table 3). Starting at the point of collection (February 2020), the rates of self-reported influenza diagnosis continually decreased (Table 3). February 2020 saw the highest rate of self-reported diagnosed influenza (0.97%) and the highest positivity rate (11.72%) among those tested. The lowest rates of influenza (0.01%) were detected in May 2020 and February and March 2021. From February to May of the 2019-2020 influenza season, the overall influenza rate was 0.21%, and the positivity rate was 7.04%. For the entire 2020-2021 influenza season, there was a lower rate of influenza (0.03%) and a lower positivity rate (1.1%). Reduced reports of influenza-positive ILI were also observed (Table 3).

Influenza vaccination rates varied seasonally, with 58.6% (160,610/274,235) of participants being vaccinated in the baseline seasons. Both the 2016-2017 and 2018-2019 seasons had over 80% vaccinated participants (23,632/27,024, 87.4% and 68,588/84,185, 81.5%, respectively). In the 2017-2018 season, which had more guest participants, 42% (68,390/163,026) of participants reported influenza vaccination. The 2019-2020 season had a high vaccination rate of 84% (57,823/68,800). The 2020-2021 season had a lower vaccination rate (66.5%) (17,299/25,996) compared to the previous season.