Under the status quo, the estimated number of COVID-19 infections and deaths from February 29 to June 7 were 195,617 (95% CIs 151,580–239,653) and 17,322 (13,144–21,501), which were well consistent with reported data, as shown in Fig. 2. In the absence of face mask use, the estimated number of infections and deaths would be 295,134 (224,459–365,809) and 25,301 (18,845–31,756), respectively (Table 1), indicating 99,517 (72,723–126,312) infections and 7,978 (5,692–10,265) deaths would have been averted through face mask use.
If the executive order was implemented 1 week earlier on April 10, the estimated number of infections and deaths were 183,659 (142,710–224,608) and 16,284 (12,389–20,178), respectively (Table 1), suggesting that 111,475 (81,593–141,356) infections and 9017 (6446–11,589) deaths would be averted compared with no face mask use. If the executive order was implemented 2 weeks earlier on April 3, the same day as the CDC started recommending face mask use, 128,598 (94,373–162,824) infections and 10,515 (7540–13,489) deaths would have been averted.
The sensitivity analysis (Fig. 3) showed that when applied to the status quo, if the effectiveness of a face mask is 50%, the executive order would have averted 73,757 (53,663–93,850) infections and 5852 (4157–7547) deaths. Even if the effectiveness of face mask was as low as 20%, the executive order would still avert 37,206 (26,944–47,468) infections and 2916 (2062–3771) deaths. If the effectiveness of a face mask is 50% but the proportion of face mask use decreased to 75%, the executive order would have averted 60,678 (44,071–77,285) infections and 4792 (3398–6186) deaths. If the proportion of face mask use further decreased to 50%, the executive order would have averted 44,632 (32,346–56,917) infections and 3506 (2481–4531) deaths. This suggests that even with reduced effectiveness and use rate, the executive order on face mask use would prevent a large number of COVID-19 infections and deaths.