APC model, which is usually used for chronic diseases[19–21], can be applied to infectious diseases such as hepatitis B, tuberculosis and AIDS[22–23], as these are all long-term infections. Therefore, we used APC model to analyze the characteristics of the incidence of HIV/AIDS in both males and females in the whole, local and immigrant populations in Zhejiang, and further explored the incidence trend among HIV/AIDS patients infected by different transmission routes.
Age is an important influencing factor of AIDS. Our results showed that the incidence of AIDS was significantly affected by age in both males and females between 20 and 35 years old in the whole, local and immigrant populations, suggesting a higher AIDS risk among young people. This might be attributed to unprotected sex and multiple sexual partners[24]. A large part of people in this age group were college students, and many of them were immigrants. According to data from The Chinese Center for Disease Control and Prevention (CDC), in the past few years, the number of college students newly diagnosed with AIDS increased by 30–50% annually[25]. Our study also elucidated that people with sexually transmitted infections (especially homosexual transmission) tended to be younger[26]. Homosexual transmission has become a major way of AIDS infection among college students. Unfortunately, many homosexual male students know nothing about AIDS until they receive positive results of HIV test[27]. Therefore, it is urgent to promote health education about sex and raise the awareness of AIDS prevention among college students. The second peak of HIV/AIDS in the whole and local populations appeared at the age of 60–70, which can be attributed to the ageing population in China[28]. The elderly are more susceptible to diseases and injuries[29, 30]. Some studies have shown that [31] the proportion of elderly people suffering from AIDS is much higher than before. The impact of the age effect on young immigrants of 20–35 years can also be explained by the fact that immigrants are mostly engaged in physically demanding jobs and poor housing conditions, which limit their access to health care and health information, and thus increase their risk of HIV infection [32].
The period effect showed that the incidence of HIV/AIDS in males was consistently higher than that in females during the study period, and the rate of increase in males was significantly higher than in females, which is consistent with the results of other studies[33–34]. The incidence among immigrant males was the highest, rising sharply from 2008 to 2010, then leveling off from 2010 to 2012 (7–8 per 100,000 population) and rising sharply again from 2012 to 2017 (16–17 per 100,000 population). The number of HIV/AIDS cases via homosexual and heterosexual transmission in the immigrant population continued to rise since 2008. This can be explained as follows. On one hand, due to social discrimination, a considerable proportion of men who have sex with men(MSM) leave their registered area to work and live in other areas, and thus have a high mobility[35]. The mobility of HIV-infected MSM often leads to the increase of epidemic in the destination area and even second-generation transmission. Qin et al. [36] showed that from 2008 to 2015, the number of migrant cases of HIV infection by male and male sex behavior in China increased, and the proportion of migrant, young, unmarried patients with high education level and household registration in rural areas was relatively high. On the other hand, heterosexual transmission is still a major transmission route of AIDS epidemic in Zhejiang Province. In addition, migrant workers form a high-risk group of HIV infection. Immigrants from rural to urban areas are at a high risk of HIV infection[37], but they lack the knowledge about HIV. Yang et al. [38] showed that about 40% immigrants did not know that the use of condoms can reduce the risk of HIV infection. Due to the current household registration (hukou) system in China, it is difficult for immigrants to obtain hukou in the destination city, especially in economically developed regions like Zhejiang[39]. And in the destination cities, health services for immigrants remain underutilized and unevenly distributed. The frequently changing government regulations on health services and prevention planning for migrant population have increased uncertainty in policy implementation and exposed migrant populations to the risk of HIV transmission[40]. A survey using national demographic data showed that areas with a higher HIV prevalence have higher rates of HIV-related education among migration population; compared with the central and western regions in China, in the eastern coastal areas with more medical and financial resources, the proportion of migrant population who received HIV education was relatively low[41]. Hence, health care resources need to be allocated to areas with large migrant populations so as to provide better health care for migrant workers.
Cohort effects reflect changes in early life environments and assume that people in the same birth cohort have equal exposure to the risk factors of a disease. Exposure to adverse environmental factors early in life may exert adverse effects on later life [42]. In the entire study cohort, the incidence of AIDS peaked in males born around 1990 and declined sharply in males born after 1990, while the incidence of AIDS in females was less affected by the cohort effect in the whole population as well as the local and immigrant populations. Furthermore, the incidence of HIV transmitted by homosexual and heterosexual sex was significantly influenced by cohort effects between 1985 and 1990, which is speculated to be related to a major epidemic among commercial plasma donors in the central and eastern provinces around 1990[2], but further research is needed.
The age density diagrams of HIV/AIDS onset and transmission routes in both sexes (Fig. 1) showed widespread HIV infections in males and females between the ages of 20 and 35; the same was found in immigrant population. Homosexual transmission of HIV/AIDS cases surged between 20 and 35 years old. AIDS cases by heterosexual transmission, blood transfusion, intravenous drug use, and other ways of transmission concentrated in population between 20 and 45 years old. As shown in Fig. 2, there was a relatively large number of unmarried AIDS patients aged 20–30. Therefore, more attention should be paid to the HIV infection of unmarried population of this age group and appropriate AIDS control programs need to be developed.
Our predictive results showed that in the whole and local populations, the incidence AIDS in males and females of all age groups would rise sharply in the next five years; in 2023, the AIDS incidence among the population of over 35 years old would be significantly higher than those under 35 years old. The incidence in women under 35 years of age in the whole population was expected to increase slightly in the future. The trend remained the same for women under 35 years of age in the local population. Among the immigrant population, both males and females under 35 and over 35, as well as all age groups, were expected to have significant an increased incidence in the next five years. Regardless of transmission routes, the prevalence of HIV in all three groups will rise in the next five years. We therefore recommend that public health authorities in Zhejiang focus on the control of AIDS epidemic in the above-mentioned populations. Our findings also indicate the sex differences in the incidence of AIDS and suggest measures be devised specifically for males and females to control AIDS.
In this study, we analyzed the prevalence of HIV/AIDS in the local and immigrant populations of Zhejiang, a representative of economically developed regions, and further explored the characteristics of HIV/AIDS through different transmission routes. The main limitation of our study is the rough statistics of age distribution in the study populations, which may incur small noises and errors to the estimation. Therefore, more detailed statistics and census are needed to improve the accuracy of estimation and prediction.