The Yangtze River Delta is one of the top five Chinese regions affected by COVID-19, as it is adjacent to Hubei Province, where COVID-19 first emerged. We investigated the impact of COVID-19 non-pharmaceutical interventions (NPIs) on changes in respiratory infectious diseases (RIDs) incidence and air quality in the Yangtze River Delta by constructing two proportional tests and fitting ARIMA and linear regression models. Compared with the pre-COVID-19 period, the average monthly incidence of seven RIDs decreased by 37.80% (p < 0.001) and 37.11% (p < 0.001) during the COVID-19 period and the post-vaccination period, respectively, in Shanghai, and decreased by 20.39% (p < 0.001) and 22.86% (p < 0.001), respectively, in Zhejiang. Similarly, compared with the pre-COVID-19 period, the monthly overall concentrations of six air pollutants decreased by 12.7% (p = 0.003) and 18.79% (p < 0.001) during the COVID-19 period and the post-vaccination period, respectively, in Shanghai, and decreased by 12.85% (p = 0.008) and 15.26% (p = 0.001), respectively, in Zhejiang. Interestingly, no significant difference in overall incidence of RIDs and concentrations of air quality was shown between the COVID-19 period and the post-vaccination period in either Shanghai or Zhejiang. This study provides additional evidence that the NPIs measures taken to control COVID-19 were effective in improving air quality and reducing the spread of RIDs. However, a direct causal relationship has not been established.

To investigate the changes of the respiratory infectious diseases (RID) and air quality during the COVID-19 outbreak over Yangtze River Delta Region, China. We investigate the impact of COVID-19 control measures on changes of RID and air quality by constructing two proportional test and fitting ARIMA and piecewise regression models. A total of 81,345 and 1,048,511 cases of RID were identified in Shanghai and Zhejiang Province, respectively. The incidence of seven RID and influenza decreased by 37.80% and 49.57% in 2020 in Shanghai and decreased by 20.39% and 43.40% in Zhejiang Province, respectively. The monthly concentrations of overall air pollutants decreased by 12.7% and 12.85% in 2020 Shanghai and Zhejiang compared to the 2017–2019 period; the most rapid decrease was observed in SO2 concentrations (32.39% and 33.37% in Shanghai Province and Zhejiang Province, respectively). A moderate correlation was seen between influenza incidence and monthly SO2 concentrations in Shanghai (r = 0.59). A 10 μg/m3 decrease of SO2 was significantly associated with the reduction of influenza incidence(2907.76 per 100,000). This study provided the additional evidences that the measures taken for COVID-19 were effective in improving the air quality and reducing spread of other common respiratory diseases, but direct causality is not established.

The incidence of scarlet fever and pertussis has increased significantly in China in recent years. During the COVID-19 pandemic, stringent non-pharmaceutical intervention measures were widely adopted to contain the spread of the virus, which may also have essential collateral impacts on other infectious diseases, such as scarlet fever and pertussis. We compared the incidence data of scarlet fever and pertussis in Mainland China and Hong Kong from 2004 to 2021 before and after the COVID-19 pandemic. The results show that the incidence of both diseases decreased significantly in 2020–2021 compared to the after-re-emergence stage in these two locations. Specifically, in 2020, scarlet fever decreased by 73.13% and pertussis by 76.63% in Mainland China, and 83.70% and 76.10%, respectively, in Hong Kong. In the absence of COVID-19, the predicted incidence of both diseases was much higher than the actual incidence in Mainland China and Hong Kong in 2020–2021. This study demonstrates that non-pharmaceutical measures implemented during the COVID-19 pandemic can partially reduce scarlet fever and pertussis re-emergence in Mainland China and Hong Kong.