This was the first study attempt to reveal the association between meteorological factors and different types of seasonal influenza in Macau, a subtropical climate region with a moderate vaccination coverage and the densest population worldwide. In our study, it was found that mean temperature, DTR, relative humidity and sunshine were significantly associated with Flu-A and Flu-B by different time lags.
Flu-A virus was the dominant type. It might be because of a faster rate of influenza A virus mutation rates ranging from around 1×10− 3 to 8×10− 3 substitutions per site per year, which declined the effectiveness of the vaccination.(2, 23) The prevalence of Flu-A was higher in winter and epidemics occurred in summer every other year in Macau. Flu-B witnessed an increasing trend over years and has a typical seasonality with the peak at spring each year. The prevalence pattern of Flu-A in Macau was very similar to that of Flu-A in other subtropical cities in China.(13, 24, 25) Flu-B had a peak in winter in Shanghai, which is a subtropical city at mid-latitudes in China, while the peak was found in spring in Macau.(25) Macau is at low latitudes in China and has shorter winter. The potential explanation was that the latitudes and climate characteristics might have an impact on the transmission of Flu-B.(13)
Cold weather was associated with higher cumulative risks of influenza activity in our study which was consistent with previous studies performed in temperate and subtropical areas.(5, 8, 25) Low temperature was proved to keep the stability of the influenza virus particle by promoting the ordering of lipids on the viral membrane, which was critical for airborne transmission.(26) And the mucociliary clearance reduced at 5°C which allowed the virus remain on the upper respiratory mucosa.(27) Moreover, people prefer to stay indoors which increased the contact rate.(28) Moderative high temperature also favored the influenza activity in both types and the results were similar with studies conducted in subtropical regions like Shanghai and Shenzhen, China.(5, 13) One possible reason was that these cities are all in a subtropical monsoon climate, and the high temperature and rainy summer provide favorable conditions for the spread of the virus. Because in warm, humid climates, water droplets evaporate less water, and virus-carrying droplets could easily deposit on surfaces, increasing the chance of contact transmission.(29) Hypersecretion of mucin production might increase the host sensibility and research found that mucin production increased under 25°C and 40% relative humidity, but not at 37°C, 80% relative humidity.(30) When the temperature exceeded 30°C, no aerosol transmission was observed in any humidity which might explained the lowest cumulative risks of influenza activity over 30°C.(31)
In our study, small DTR was negatively associated with influenza outbreaks including Flu-A and Flu-B. The effect of low-DTR had 3 weeks to 4 weeks lag in Flu-A, while influencing Flu-B during the whole 4 weeks. In Shanghai, the Flu-A risk dramatically increased after exposure to large DTR, whereas Flu-B had a higher risk when exposed to stable temperatures.(5) The mechanism of DTR on influenza was not elucidated. Another possible reason was that large DTR occurred in transition seasons, and the transition seasons were short in Macau which reduced the large DTR effect on the occurrence of influenza.
Both the dry and humid conditions promoted the influenza transmission and the humid effect could persist for up to 27 days, which was consistent with a cross-dimensional study in eight cities in China.(32) Animal transmission studies in the guinea pig model and ferrets model showed that high RH (> 60%) and low RH (< 40%) kept influenza virus active in droplets for longer periods of time, while those at moderate RH (40–60%) ), the virus was inactivated.(29, 31) In respiratory fluid and human mucus, influenza virus had high viability when the relative humidity below 50% or near 100%.(33) Inhalation of dry air caused shedding of guinea pig airway epithelial cilia, epithelial cell shedding, and tracheal inflammation, impairing innate antiviral defenses and tissue repair.(34, 35) Disruption of airway epithelial integrity caused by inhalation of dry air might be associated with winter epidemics of certain types of respiratory viral infections.(36) In humid conditions, virus-carrying water droplets were more likely to deposit on surfaces, increasing the chance of contact transmission.(29)
Short sunshine was found to increase the risk of each type. This founding was similar with previous studies.(9, 22) One hypothesis was that the lack of sunshine reduced the synthesis of melatonin and vitamin D in the body, which decreased human immunity.(37) Melatonin could activate intracellular signalling pathways and transcription factors, thereby inhibiting inflammatory activity.(38) Vitamin D reduced the risk of infections by inducing cathelicidins and defensins and reducing concentrations of pro-inflammatory cytokines.(39) Moreover, in our study, we found the risk of being infected increase during the moderative long sunshine (5.0h-9.0h). The mechanism was unknown because of the limited research and it required further study.
There were some strengths in this study. Firstly, this was the first study that explored the complex and delayed relationship between meteorological factors and seasonal influenza in a subtropical climate region with a moderate vaccination coverage and the most dense population worldwide. Secondly, the dominant influenza types were both included in this study, which provided more comprehensive and comparable information of the epidemic characteristics as well as relations to meteorological conditions. Several limitations were noted in this study. Firstly, a previous study figured out that the association had a slight difference among different age groups.(12) To have more accurate results, further work could explore the association between meteorological factors and influenza activity among different subgroups. Secondly, air pollutants were not adjusted in this study which might result in bias. Air pollutants such as PM2.5, PM10, O3 and mean temperature were found that had a significant interaction effect on diseases.(40, 41) Thirdly, Macau is an international tourism city. Influenza cases counted in our study covered both local residents and tourists which might lead to selection bias. However, based on our previous research finding that tourist cases account for a small percentage of total cases (Flu-A: 7.44%, Flu-B: 6.83%), our results were reliable.(42)