a decreasing trendP < 0.001
Joinpoint regression analysis showed an increasing trend from January to May for the whole population (APC = 22.7%, 95% CI: 8.2–39.2%, P = 0.006) and a decreasing trend from May to December (APC = -19.1%, 95% CI: -23.3% ~-14.7%, P < 0.001).