Understanding the complexity of influenza subtype seasonality is critical to promoting a suitable vaccination program. The aim of this study was to identify and compare the seasonality and epidemiological features of seasonal influenza subtypes after the 2009 A/H1N1 pandemic and to lay a foundation for further investigation into the social and environmental factors affecting seasonal influenza virus transmission. Influenza-like illness (ILI) case surveillance was conducted in two sentinel hospitals in Pudong New Area, Shanghai between 2012 and 2018. Weekly data on ILI cases were analyzed. A time-series seasonal decomposition analysis was used to reveal the seasonality of influenza and epidemiological features among different subtypes. In total, 10977 ILI patients were enrolled of whom 2385 (21.7%) had laboratory-confirmed influenza. Compared to influenza A (16.3%), influenza B (5.4%) was less frequently detected among the ILI patients (p<0.001). Semiannual epidemic peaks were identified in four of the years during the 6-year study period, while only one annual epidemic peak was found in the other two years. An epidemic peak occurred in each winter season, and a secondary peak also occasionally occurred in summer or spring. A/H3N2 predominated in both summer and winter, while A/H1N1, B/Yamagata, and B/Victoria circulated almost exclusively in winter or spring. Two lineages of influenza B seemed to predominate in alternating years. This study highlights the complexity of seasonal influenza virus activity in a subtropical region of China, presenting both semiannual and annual epidemic peaks in different years. The results of this study may provide further insight into possible improvements in the timing of influenza vaccination in Shanghai, China.

Background: Older individuals are at high risk for morbidity and mortality due to influenza, and the most effective way to prevent influenza is yearly vaccination. In China, the influenza vaccine is not covered by the national Expanded Program on Immunization, and more evidence is needed about influenza vaccine usage among older individuals. Objective: To determine the influenza vaccination coverage and its influencing factors, and understand barriers to older adults receiving influenza vaccinations in Shanghai, China. Methods: A cross-sectional survey was conducted with residents aged 60 and older. Vaccination status in the 2016–17 influenza season and reasons for or against vaccination were surveyed. The vaccination coverage rates were adjusted by gender, age and community distribution, and potential factors influencing vaccination uptake were determined by bivariate logistic regression. Results: In total, 253 of the 4417 respondents received an influenza vaccine during the 2016–17 season, yielding an adjusted coverage rate of 5.2% (95% CI, 4.5-5.8). The frequency of receiving the vaccine was higher for older individuals living with family/friends than that for those living alone (p < 0.05), and lower among individuals with chronic respiratory diseases (p < 0.05). Among unvaccinated respondents, lack of influenza vaccine awareness was the most common reason for being unvaccinated (48.3%, 2012/4164). Conclusion: Influenza vaccination coverage is extremely low among older people in Shanghai, and lack of awareness of the influenza vaccine might be a potential barrier to vaccination. Our study highlights the need for an appropriate influenza vaccination strategy and program targeting the older population.

Background: Seasonal influenza vaccination coverage remains low in most areas of China. Its influencing factors and barriers in various populations receiving influenza vaccinations need to be well understood to promote vaccination. Methods: A cross-sectional survey was conducted with residents in 48 communities. Vaccination status in the 2018–2019 influenza season and reasons for or against vaccination were surveyed. The potential factors influencing vaccination uptake were determined using bivariate logistic regression. Results: In total, 1301 of the 11053 respondents received an influenza vaccine during the 2018–2019 season with a coverage rate of 11.8% (95% CI, 11.2–12.4). The vaccine coverage was highest among children (26.6%, 95%CI: 24.8–28.5), followed by adults (8.2%, 95%CI: 7.4–9.0) and elderly people (7.3%, 95%CI: 6.5–8.1) (p < .001). Those with chronic underlying conditions all had higher vaccine coverage than did those without for different groups (p < .001). Among the three groups, the most common reason for being unvaccinated was worrying about the side effects (45.0%), believing they were healthy and did not need to get vaccinated (42.2%), and lack of influenza vaccine awareness (48.3%). Low education level and lack of awareness were identified as predictors of low coverage rate. Conclusion: Influenza vaccination coverage is low among different populations in Shanghai. Our study highlights the need for appropriate influenza vaccination strategies and programmes targeting different populations.

The vaccination rate for seasonal influenza remains low in most regions of China. It is essential to understand the factors that associated with the low influenza vaccination rate in various populations after the COVID-19 pandemic. A cross-sectional survey was conducted with residents in Pudong New Area, Shanghai, China. Respondents' vaccination condition during the 2021-2022 flu season and the reasons for receiving or not receiving influenza vaccine were investigated. Binary logistic regression was conducted to explore potential factors influencing vaccination uptake. 2,476 of 14,001 respondents received an influenza vaccine, with a total coverage of 17.68% (95% CI: 17.05%, 18.32%). Children had the highest vaccination coverage (35.68%; 95% CI: 34.02, 37.33), followed by adults (12.75%; 95% CI: 11.91%, 13.58%) and elderly individuals (11.70%, 95% CI: 10.78%, 12.62%). For children, lower household income was an significant promoting factor. For adults, factors significantly associated with vaccination were household income, sex, and education level. For elderly, factors significantly associated with vaccination were household income, education level, living state, and having underlying diseases. (P < .05)The main reason for vaccine hesitancy among children was worried about side effects (21.49%), for adults and elderly was self-rated good health (adults: 37.14%, elderly people: 30.66%). The overall influenza vaccination coverage rate in Shanghai, especially among elderly individuals, is lower than many developed countries. Appropriate strategies and programs targeting different populations need to be implemented to enhance influenza vaccine coverage.