Study area
Changsha (27°51ʹ~28°41ʹ N, 111°53ʹ~114°15ʹ E), a large city with 7.04 million people in central south China, is the capital city of Hunan province. It includes 6 districts and 3 counties. There are 4586 health departments including hospitals, clinics, and public health departments all over the city.
The modified influenza surveillance system
The ISS, based on two sentinel hospitals which locates in the south and the north of the city respectively, was set up in Changsha in September 2005. In 2006 and 2008, the system became a branch of Hunan and national influenza surveillance network respectively. In the 6th week in 2012, the HIS was adopted for the surveillance in one of the sentinel hospitals (hospital A), and was modified after the 25th week in 2013. Therefore, the ISS in Changsha underwent five stages (stage 1: week 39, 2005 to week 52, 2005; stage 2: week 1, 2006 to week 52, 2007; stage 3: week 1, 2008 to week 5, 2012; stage 4: week 6, 2012 to week 24, 2013; stage 5: week 25, 2013 to week 41, 2016.). During stage 1 to 3, two sentinel hospitals registered ILI cases manually in five outpatient departments which were outpatient and emergency departments of respiratory medicine, outpatient and emergency departments of pediatrics, fever clinic. During stage 4 to 5, hospital B remained the manual surveillance in the same outpatient departments. ILI case was defined as “fever (axillary temperature ≥ 38°C) + cough or sore throat”[18, 19].
Differently, in hospital A, HIS was adopted into the ISS during stage 4 to 5 which was also named as “HIS (stage 1)” and “HIS (stage 2)” respectively. During stage 4, all outpatient departments of the hospital were enrolled into the ISS, and the computer would emerge a popup window by HIS with the question that “ILI or not” if physicians diagnosed one of the 108 influenza-associated diseases coded by International Classification of Diseases 10th Revision (ICD-10). The physician should answer the question to continue the later part to deal with the patients. But we found that some ILI cases would still be missed because of some reasons like the misremembering of the definition of ILI or the sense of the responsibility of the physician, especially if the physician was in the department which was not in the ISS system during stage 1 to 3. Therefore, during stage 5, the question was changed as three options: a) fever (axillary temperature ≥ 38°C), b) cough, c) sore throat. The procedure of HIS would count the ILI automatically by computing the number of “a) + b)”, “a) + c)”, and “a) + b) + c)”.
Therefore, patients visiting the outpatient departments of the two hospitals, among whom ILI cases were monitored, were calculated every week. At least 5-20 throat swab samples of ILI cases per hospital per week were collected for testing the influenza virus by reverse transcription polymerase chain reaction (RT-PCR) and / or cell culture in the laboratory of Changsha CDC. This system may monitor influenza and emerging avian influenza cases with mild symptoms or at the early stage of the infection (Figure 1). Data of the system from week 39, 2005 to week 41, 2016 were collected in our study.
The new pneumonia surveillance system
The PSS was built in Changsha in March, 2009. Pneumonia related inpatient departments of all 49 hospitals (excluding the primary health care centers and private clinics) in Changsha were enrolled into the system. This system monitors pneumonia cases among inpatient population. The public health staffs in the surveillance hospitals would count the total number of monitored inpatient people, pneumonia cases, severe or death pneumonia cases diagnosed by physicians and report the data to local CDC monthly. Throat swab or lower respiratory tract samples of suspicious patients were collected for testing avian influenza virus by RT-PCR in the laboratory of Changsha CDC. The PSS system may monitor influenza and emerging avian influenza cases with severe symptoms or death (Figure 1). In this study, we collected the data of the system from March, 2009 to September, 2016.
Statistical methods
The sentinel hospitals A and B are in a same city, although they locate in the south and north respectively, we assumed that the outpatients came from a same age structure population. Therefore, three indicators (d1, d2 and d3) were used to compare the difference between the two hospitals among the five stages. They were the differences of weekly number of monitored outpatients, ILI, and ILI% of the two hospitals, and were expressed as follows:
d1 = xA – xB
d2 = yA – yB
d3 = zA – zB
xA, xB, yA, yB, zA and zB refer to weekly number of monitored outpatients of hospital A, weekly number of monitored outpatients of hospital B, weekly ILI of hospital A, weekly ILI of hospital B, weekly ILI% of hospital A, and weekly ILI% of hospital B.
Analysis of variance (ANOVA) was employed to show the d1, d2 and d3 among the five surveillance stages of the two sentinel hospitals. If there is statistical significance, the Least Significant Difference (LSD) method will be adopted to conduct the multiple comparisons between any two stages. The difference is significant at the 0.05 level.