Regional-based within-year seasonal variations in influenza-related health outcomes across mainland China: a systematic review and spatio-temporal analysis

Abstract

Background: China experiences large variations in influenza seasonal activity. We aim to update and improve the current understanding of regional-based within-year variations of influenza activity across mainland China to provide evidence for the planning and optimisation of healthcare strategies.

Methods: We conducted a systematic review and spatio-temporal meta-analysis to assess regional-based within-year variations of ILI outpatient consultation rates, influenza test positivity rates amongst both ILI outpatients and SARI inpatients, and influenza-associated excess mortality rates. We searched English and Chinese databases for articles reporting time-series data on the four influenza-related outcomes at the sub-national and sub-annual level. After synthesising the data, we reported on the mean monthly rate, epidemic onset, duration, peak and intensity.

Results: We included 247 (7.7%) eligible studies in the analysis. We found within-year influenza patterns to vary across mainland China in relation to latitude and geographic location. High-latitude provinces were characterised by having short and intense annual winter epidemics, whilst most mid-latitude and low-latitude provinces experience semi-annual epidemics or year-round activity. Subtype activity varied across the country, with A/H1N1pdm09 and influenza B occurring predominantly in the winter, whereas A/H3N2 activity exhibited a latitudinal divide with high-latitude regions experiencing a winter peak, whilst mid and low-latitude regions experienced a summer epidemic. Epidemic onsets and peaks also varied, occurring first in the north and later in the southeast. We found positive associations between all influenza health outcomes. In addition, seasonal patterns at the prefecture and county-level broadly resembled their wider province.

Conclusions: This is the first systematic review to simultaneously examine the seasonal variation of multiple influenza-related health outcomes at multiple spatial scales across mainland China. The seasonality information provided here has important implications for the planning and optimisation of immunisation programmes and healthcare provision, supporting the need for regional-based approaches to address variations in local epidemiology.

Keywords: China; Influenza; Seasonality; Spatio-temporal; Systematic review.

Fig. 1

Flow chart of study selection. Studies could be excluded on multiple grounds

Fig. 2

Post-pandemic provincial mean monthly rate (MMR) of influenza test positivity among ILI outpatients, sorted by descending province centroid latitude. A Influenza test positive rate for all common seasonal strains combined. B Influenza test positive rates disaggregated by A/H3N2, A/H1N1pdm09 and B subtypes.

Fig. 3

Post-pandemic estimated average epidemic months across mainland China. Based on the MMR of all strain influenza test positivity rates among ILI outpatients. Only displaying administrative regions with available data

Fig. 4

Province-level normalised mean epidemic intensity of influenza test positivity among ILI outpatients, sorted by descending latitude (based on province centroid). A Influenza test positivity rate for all common seasonal strains combined. B Influenza test positivity rates by A/H3N2, A/H1N1pdm09 and B subtypes. Pandemic period excluded. Centre points denote the province mean intensity; the light and darker shaded bands represent 95% and 75% confidence intervals, respectively. Dashed vertical line represents the group mean