Epidemiological situation of influenza in Fuzhou

Between 2013 and 2022, the total number of cumulative reported influenza cases in Fuzhou was 19,135, with 2 cumulative reported deaths, with a case fatality rate of 0.01%. In the same period, the average annual reported incidence rate of influenza was 24.42 per 100,000 population, with the highest incidence rate observed in 2022 at 53.52 per 100,000 population. During the seasonal influenza period from 2013 to 2019, the cumulative number of reported influenza cases in Fuzhou was 11,427, accounting for 59.66% of the total number of reported cases during the study period. Notably, the number of reported cases demonstrated an increasing trend as a whole during this period. As a result of COVID-19 pandemic (2020–2022), the number of reported influenza cases in Fuzhou experienced a rapid decline, with 1,945 and 1,242 cases reported in 2020 and 2021, respectively. However, there was an increase in reported influenza cases in 2022, reaching 4,521 cases. The number of reported cases of influenza for the remaining years is presented in Fig. 2 and Table S1.

During COVID-19 pandemic period, the average annual reported incidence rate of influenza was 30.55 per 100,000 population, which was 1.50 times than the annual average reported incidence rate during the seasonal influenza period (20.36 per 100,000 population).

Number and incidence of reported influenza cases in Fuzhou, China, 2013–2022.

Epidemiological characteristics

Characteristics of the distribution of the population

Among the 19,135 reported influenza cases from 2013 to 2022, 10,843 (56.67%) were male cases, while 8,292 (43.33%) were female cases, resulting in a male-to-female ratio of 1.31:1. Moreover, statistical analysis indicated a significant difference in the gender composition of reported cases over the 10-year study period, as illustrated in Fig. 3 and Table S1.

Gender composition of reported influenza cases in Fuzhou, China 2013–2022.

For the age distribution among reported influenza cases in Fuzhou from 2013 to 2022 reveals that the age range of reported cases spanned from 1 year old to 101 years old, with an average age of 18.68 years and a median age of 7 years. Within each year, the age group with the highest composition ratio consistently belonged to the 1–9 years old category. Furthermore, statistical analysis demonstrated a significant difference in the age group composition of reported cases over the 10-year study period, as depicted in Fig. 4 and Table S1.

Age composition of reported influenza cases in Fuzhou, China 2013–2022.

An analysis of the occupational distribution among influenza cases reveals that out of the total reported cases, the top five occupations in terms of composition ratio were diaspora children (6,570 cases, 34.33%), students (4,402 cases, 23.00%), preschool children (2,595 cases, 13.56%), houseworkers and unemployment (1,145 cases, 5.98%), and retired workers (802 cases, 4.19%). Moreover, statistical analysis confirmed a significant difference in the occupational composition of reported cases over the 10-year study period, as presented in Fig. 5 and Table S1.

Occupational composition of reported influenza cases in Fuzhou, China, 2013–2022.

Time distribution characteristics

Based on a dataset of 19,135 reported influenza cases in Fuzhou between 2013 and 2022, the temporal distribution of reported cases was analyzed and plotted. As depicted in Fig. 6, the peak incidence of influenza in Fuzhou was observed primarily between December and February (5,246 cases, 45.91%) during the seasonal influenza period, showing an obvious trend of high incidence in winter. However, during COVID-19 pandemic, the incidence of influenza in Fuzhou exhibited two distinct peaks, showing a high incidence in summer and winter. Statistical analysis confirmed a significant difference in the monthly distribution of seasonal influenza cases compared to the incidence in 2022, as presented in Fig. 6 and Table S2.

Monthly distribution of reported influenza cases in Fuzhou during the seasonal influenza pandemic and COVID-19 pandemic.

Spatial distribution characteristics

Throughout the period from 2013 to 2022, influenza cases were reported across all areas in Fuzhou. However, the distribution of influenza cases exhibited regional disparities. The top three areas with the highest number of reported influenza cases were Gulou District (4,656 cases, 24.33%), Cangshan District (2,960 cases, 15.47%) and Jin’an District (2,441 cases, 12.76%). Notably, the majority of reported cases were concentrated in urban areas, accounting for 12,936 cases (67.60%), which represented over 50% of the total cases.Additionally, the average annual reported incidence rates of influenza were calculated for each county, city, and district during the study period. The top three regions with the highest average annual reported incidence rates of influenza were Mawei District (65.70/100,000), Gulou District (65.34/100,000) and Minqing County (52.56/100,000). In contrast, Pingtan County had the lowest average annual reported incidence rate, with only 5.55 cases per 100,000 population (Table S3).

An analysis of the regional distribution of influenza epidemics during different periods, including the seasonal influenza and COVID-19 pandemic, revealed distinct patterns. In terms of the seasonal influenza period, three counties and districts reported an average annual incidence rate of more than 40.00/100,000. These regions included Mawei District (61.04/100,000), Minqing County (55.00/100,000), and Gulou District (41.56/100,000). Conversely, during COVID-19 pandemic period, four counties and districts exhibited an average annual reported incidence rate exceeding 40.00/100,000. These regions were Gulou District (120.82/100,000), Mawei District (76.58/100,000), Minqing County (52.56/100,000), and Jin’an District (67.10/100,000). Detailed year-by-year information on the regional distribution of reported influenza cases during the study period can be found in Fig. 7.

Spatial distribution of reported incidence of influenza in Fuzhou, China, 2013–2022.

Characteristics of Spatial and Temporal aggregation of influenza cases

Characterization of the evolution of global Spatial trends

In this study, we utilized trend-surface analysis to generate three-dimensional perspective maps, where the average annual reported influenza incidence rate was represented as the height of the spatial dataset. We then analyzed the global trend of the reported influenza incidence rate in Fuzhou from different perspectives. During the seasonal influenza period, the spatial distribution of reported influenza incidence in Fuzhou maintains a consistent pattern, characterized by higher incidence rates in the western and central regions and lower rates in the eastern and peripheral areas. This spatial trend indicates a directional prevalence of influenza, with the central region emerging as a hotspot for the disease. Amidst the COVID-19 pandemic, the reported incidence of influenza in Fuzhou exhibited a distinctive spatial pattern resembling a flattened inverted “U” curve in both the east-west and north-south directions. This pattern suggests a concentration of influenza cases in the central urban area, accompanied by higher incidence rates, while the surrounding suburban areas experienced fewer cases and lower incidence rates, indicating localized clustering of influenza during this period. Further details can be found in Fig. 8.

Global trend analysis of the reported incidence of influenza in Fuzhou, China, 2013–2022. Notes: The green line denotes the east-west direction, while the blue line signifies the north-south direction.

Characterization of the evolution of the global Spatial structure

The findings revealed that the reported influenza incidence rate in Fuzhou exhibited a distinct spatial pattern. The central urban area showed a high-density distribution of reported cases, while the surrounding suburban areas displayed a scattered distribution. During the seasonal influenza period, the spatial structure of reported influenza incidence in Fuzhou demonstrated a “three-core” distribution. The first urban high-aggregation nucleus was observed in the urban area with Gulou District as its core, followed by the second urban high-aggregation nucleus with Mawei District as its core. Additionally, the suburban counties exhibited the first suburban high-aggregation nucleus, with Minqing County serving as its core. In contrast, during COVID-19 pandemic, the spatial structure of reported influenza incidence in Fuzhou exhibited a “double nucleus” distribution. This distribution featured high aggregation nuclei in the urban area, specifically in Jin’an District, Gulou District, and Mawei District. Overall, the high incidence of influenza during different periods was predominantly concentrated in the urban areas of Fuzhou City. For more detailed information, please refer to Fig. 9.

Kernel density of the reported incidence of influenza in Fuzhou, China, 2013–2022.

Spatiotemporal aggregation features

The space-time scanning analysis revealed two aggregation zones, categorized as Class I and Class II. In the Class I aggregation area, Gulou District served as the center with an aggregation radius of 2.40 km. This area covering a total of 2 counties (cities) encompassed Gulou District and Jin’an District, and the aggregation of influenza-reported cases occurred from June to July 2022. The RR for this area was 47.990 with a LLR of 6917.944 (P < 0.01). The Class II aggregation area had Fuqing City as its center with an aggregation radius of 37.92 km. This area covered Fuqing City, Changle City, Mawei District, Taijiang District, and Cangshan District. The aggregation of influenza-reported cases in this area took place from December 2018 to February 2020. The RR for this area was 2.600 with an LLR of 868.602 (P < 0.01). These results obtained from the statistical analysis of space-time scanning indicate a clear spatial and temporal distribution pattern of reported influenza cases in Fuzhou from 2013 to 2022, characterized by significant aggregation. For detailed information, please refer to Table 1; Fig. 10.

Spatiotemporal scan clustering plot of influenza cases in Fuzhou, China, 2013–2022.