2.2 Epidemiological characteristics of RSV infections
From March 2015 to February 2019, we observed a yearly RSV epidemic around Winter as expected. RSV detection rate began to rise in October, peaked in December or the following January, and fell to an ordinary level in April, with about 90% of RSV infections occurring during the epidemic season (from October to the following March) (Fig. 1A, B). In different years, the RSV epidemic season may advance or be delayed by about one month. Two groups of RSV (RSV-A and RSV-B) were prevalent each year, but RSV-A remained predominant from March 2015 to February 2019. The proportion of RSV-A infection among total RSV-positive cases was 62.8%. (Fig. 1C)
RSV detection rate was the highest in the 0-year-old group (9.98%), followed by the 1-year-old group (6.18%), and the 2–4-years-old group (4.92%); it decreased significantly to a low level in 5–44 years group (5–13 yr: 1.68%; 14–17 yr: 0.61%; 18–44 yr: 0.57%), and increased in the 45–59 years of age adults (1.15%), and more obviously in the elderly persons with 60 years of age or older (1.57%) (Fig. 2A). Both RSV-A and RSV-B were detected in all age groups, with RSV-A a dominant proportion (Fig. 2A). For further analysis, the above mentioned nine age groups were incorporated into four age groups as 0–4 years (subtotal of detection rate: 6.07%), 5–17 years (subtotal of detection rate: 1.51%), 18–59 years (subtotal of detection rate: 0.76%), and 60 years or older (subtotal of detection rate: 1.57%). The proportion of RSV infections under five years old (52%) among the total RSV infections was the greatest, followed by that of the elderly adults with 60 years of age or older (25%); the top two groups accounted for 77% of RSV infections (Fig. 2B).
The RSV detection rate of pneumonia cases (NSCAP and SCAP) was higher than that of AURTI cases by age group, which showed that RSV was more prone to cause acute lower respiratory tract infection. However, the RSV detection rate of NSCAP cases was slightly higher than that of SCAP cases despite no significant difference observed. Although RSV-A and RSV-B were both detected among each class of RSV infection in each age group, RSV-A approximately dominated in each group (Fig. 3).
The most common co-existing pathogen identified from RSV-positive cases was the influenza virus (10.2%), followed by M. pneumoniae(4.1%; Fig. 4).