3.1. Overall Trends of HIV/AIDS Age-standardized Incidence in China and the U.S.
Figure 1 presented time sequence changes of the age-standardized incidence rate (ASIR) of HIV/AIDS in male and female of China and the U.S. from 1994 to 2019. As we can see, male invariably higher than female in China and U.S. ASIR of both male and female in the U.S. rose first and hit a peak in 1997, after plummeted and went through a smooth period, it turned to gradually went up and attain to a high level in 2019. While in male of China the ASIR of HIV/AIDS from 1.76/100,000 in 1994 to 6.34/100,000 in 2005, which represented an upward trend, then it dropped from 6.19/100,000 in 2006 to 3.30/100,000 in 2019, curve of female remained in a bottom and followed male’s trend, from 0.98/100,000 in 1994 to 1.07/100,000 in 2019, the range of changes did not exceed 1.5/100,000.
3.2 Gender Disparities Between Male and Female in Both Countries
Gender gaps between male and female can be seen clearly in Figure 1, it expanded gradually before 2006 in China, raising from 0.78/100,000 in 1994 to 4.07/100,000 in 2006, but had shrunk from 3.99/100,000 in 2007 to 2.23/100,000 in 2019. A huge gap exists in U.S. and expanded from 8.38/100,000 in 1994 to 17.08/100,000 in 2019. Albeit gap decreased a little from 1997 to 2001, this promising trend did not keep hold. It still maintained an exaggerated gap and had no sign of narrowing. We classified 13 age groups into three stage of life: young age(15-34), middle age(35-59) and old age(60-79). In Figure 2, gender difference in HIV/AIDS incidence increased gradually with age and was readily apparent in China. Ratio in young age was modest in three stage further demonstrated that young people are most vulnerable to infection whatever male and female. At the same time, with the change of time, almost all age groups showed an increase in the sex ratio. However, Figure 2 can’t indicate gender disparities in which country was much severer, it only reflected trends of gender disparities with time change or with age change, this can be attributed to incidence rate of HIV/ADIS in U.S. was considerably higher than China. Rate ratio cover the gender disparities in some degree, for example, the incidence rate was 26/100,000 in male and 21/100,000 in female in U.S. while 2/100,000 in male and 0.6/100,000 in female in China, the former rate ratio was 1.24 but latter was 3.33.
The RR of a particular age, period, or birth cohort was calculated based on these coefficients of age, period, and cohort effects estimated by using the age-period-cohort model (Table A1 and Table A2).
3.3.1. Age effect
Relative risks (RRs) of HIV/AIDS incidence in China and the U.S. were depicted in Figure3,4,5. Age effect could be seen in Figure 3. In groups younger than 50 years old, we found the fluctuations of RRs, which rose from 15-19 age group in a quick speed and reached peak in 25-29 age group then had a steep slide in middle-aged groups (30-34 to 45-49 age groups), were resemblant in both countries and genders. In the peak of young age, risks of HIV/AIDS in the U.S. were 1.5 and 1.4 times larger than China for male and female respectively. Although risks of both male and female in the U.S. had a steep glide and hit a rock-bottom position in 75-79 age group, trend of old groups in China was in opposite. Curve of RRs exhibited an uninterrupted escalating trend from 45-49 to 75-79 age groups in male of China. For female, down trend among 60-64 to 65-69 age groups mitigated previous uptrend, risks of both gender in China surpassed the first peak and reached to a new high in 75-79 age group. In this group, risks were 9.5 and 8.4 times larger than male and female in the U.S. respectively. This finding indicate that younger groups exposed to high risks of HIV/AIDS incidence, but for China, groups over 70 years old climbed in an exaggerate speed and bear higher risks than younger groups ultimately.
3.3.2. Period effect
We observed RRs of HIV/AIDS incidence in the U.S. moderately fluctuated at low level with advancing time in Figure 4. However, it showed an increasing trend in past decade and RRs of incidence increased by 1.6 times in men and 1.2 times in women. Risks in China plotted an oscillating trend. Curves did not display a sign of decline until it reached the summit in 2004, at this point risks were 1.87 times and 1.9 times larger than male and female in the U.S., from then on, upward momentum of risks was curbed and RRs of incidence glided down to the level of 1994 finally in China.
3.3.3. Birth cohort effect
Birth cohort effect on HIV/AIDS incidence presented in Figure 5. RRs of incidence almost same in male and female. Compare to the U.S., RRs of incidence had more complex fluctuations in China.
Curves showed an upside-down V trend which indicated that RRs of incidence rose up first then drop down from 1919-1923 to 1979-1983 birth groups, risks began augment from 1989-1993 birth groups then acquired its highest value in 2004-2009 birth group. In the U.S. risks of incidence hiked remarkably to the first peak then experienced a transitory dip from 1919-1923 to 1934-1938 birth cohort, after a succession of steady growth, a new height appeared in 1984-1989 cohort group. From 1989-1993 to 2004-2009 birth groups, RRs increased by 227.6% in male and 329.0% in female in China; in the U.S., it increased by 238.2% in male and 290.1% in female. In term of birth cohort, later birth groups exposed relatively higher risks of HIV/AIDS incidence than earlier birth groups.