Objective: To study the epidemiological characteristics of respiratory virus infection and its relations to climatic factors in Suzhou.

Methods: From 2006 to 2009, viral etiology surveillance was conducted among 6655 children hospitalized with acute respiratory tract infections (ARIs). Direct immunofluorescence method was used to test respiratory secretion samples for respiratory syncytial virus (RSV), influenza viruses A and B (Inf-A, Inf-B), parainfluenza virus types I, II, and III (Pinf-I, Pinf-II, Pinf-III) and adenovirus. Samples were tested for human metapneumovirus (hMPV) with reverse transcription polymerase chain reaction (RT-PCR). Samples from Jan 2006 to Dec 2009 were also tested for human bocavirus (HBoV). Climatic factors, including mean temperature, relative humidity, rainfall amount, sum of sunshine and mean wind velocity were collected monthly. The relationship between activity of each virus and climatic factors were analyzed by linear regression and stepwise regression analysis.

Results: From 2006 to 2009, in the total virus detection rate was 32.2% (2142/6655) in Suzhou. RSV was the most common virus and the average detection rate was 15.7% (1048/6655), followed by hMPV 8.9% (596/6655), HBoV 7.8% (148/1883), Pinf-III 2.7% (183/6655), Inf-A 2.4% (161/6655), ADV 1.3% (89/6655), Pinf-I 0.4% (29/6655), Inf-B 0.37% (25/6655) and Pinf-II 0.16% (11/6655). The positive rates of RSV, hMPV and ADV were significantly different in four years (χ(2) = 17.71, 33.23, 8.42, all P values < 0.05). Different virus has different epidemiological characteristics and distinct seasonality. The detection rate of RSV, hMPV, Inf-A were higher in Winter as 37.2%, 13.2%, 4.4%, respectively. ADV and Pinf-III were higher in summer as 2.3% and 4.6% respectively. The peak of HBoV existed in Autumn as 3.3%. The total virus detection rate showed significant inverse correlation with month average temperature (r = -0.732, P < 0.001) and a weak inverse correlation with average wind velocity was also found (r = -0.36, 0.01 < P < 0.05). The highest month total virus detection rate was from 47.6% to 84.4% when average temperature was from 3.2°C to 9.4°C and mean wind velocity was from 1.2 - 1.9 m/s. The associations of average temperature, sum of sunshine and wind velocity with RSV activity were statistical significant (r = -0.88, P < 0.001; r = -0.43, P < 0.01; r = -0.47, P < 0.01). The highest rate was from 24.3% to 58.2%, when mean temperature was from 5.3°C to 19.9°C, mean wind velocity was from 1.3 - 2.4 m/s and sum of sunshine was 61.0 to 153.4 hours. hMPV detection rate was inversely correlated with mean temperature and rain account (r = -0.43, P < 0.01; r = -0.29, P < 0.05). The rate was highest from 11.7% to 31.6% when mean temperature was from 5.3°C to 21.9°C and rain account was from 27.5 millimeter to 150.9 millimeter. Only mean temperature was positively correlated with Pinf-III (r = 0.53, P < 0.001). The rate was from 2.8% to 7.2% when mean temperature was between 11.9°C and 30.4°C. ADV detection rate was positively correlated with mean temperature and sum of sunshine, but negatively correlated with wind velocity (r = 0.35, P < 0.05; r = 0.30, P < 0.05; r = -0.32, P < 0.05). The rate was from 2.2% to 6.6% when mean temperature was between 15.9°C and 30.4°C, and sum of sunshine between 93 hours to 240.7 hours and mean wind velocity was from 1.1 - 2.8 m/s. Average temperature and relative humidity showed interactions on the detection rate of ADV (r = 0.36, P = 0.0093; r = -0.34, P = 0.016), but temperature showed higher effect on ADV detection rate. ADV detection rate was high at higher temperature (15.9 - 30.4°C) and low humidity (56% - 71%).

Conclusion: RSV was one of the most common viruses among hospitalized children in Suzhou, and hMPV and HBoV also played an important role in respiratory tract infection of children. Different virus has different cycle and seasonality. Climatic factors, especially mean temperature, was the main factor affecting the virus prevalence.