Tuberculosis (TB) remains a key public health concern worldwide. The World Health Organization (WHO) reported a global TB incidence of 9.9 million in 2020 [1]. The causative agent of TB is Mycobacterium tuberculosis (MTB), a highly aerobic bacterium [2]. TB can be active or latent, corresponding to the presence or absence of clinical symptoms, respectively. The typical symptoms of active TB include productive cough with bloody sputum, night sweats, low-grade fever, and weight loss [3, 4]. Although TB can be transmitted through various routes, air transmission is the predominant mode. The process through which TB bacilli are aerosolized is strongly correlated with coughing [5]. TB incidence varies across countries; the highest incidence (≥ 300 cases per 100,000 individuals) is observed in sub-Saharan Africa, whereas low incidences (< 25 cases per 100,000 individuals) are noted in Japan and the United States [6]. Taiwan has an intermediate TB burden; the incidence rates were 72.5 and 45.7 cases per 100,000 individuals in 2005 and 2015, respectively [7, 8].

Because of their specialized cell wall, TB bacilli exhibit increasing levels of drug resistance [9]. To reduce drug resistance, active TB is treated using antibiotic cocktails [10]. However, drug-resistant TB remains a major threat to public health worldwide; the emergence of drug-resistant MTB strains may be attributed to poor drug compliance or low-quality medication use [11]. Multidrug-resistant TB (MDR-TB) is characterized by resistance to the two most effective first-line TB drugs, rifampicin and isoniazid. By contrast, extensively drug-resistant TB (XDR-TB) is characterized by resistance to three or more of the six classes of second-line TB drugs. A study conducted in 2021 defined XDR-TB as TB caused by MTB strains resistant to isoniazid, rifampicin, fluoroquinolone, and either bedaquiline or linezolid (or both) [12].

Drug resistance complicates the prevention of TB transmission. Directly observed therapy (DOT) and DOT-Plus (MDR-TB project) have substantially improved drug compliance and medical availability [13]. The WHO reported 480,000 and 201,997 global cases of MDR-TB in 2013 and 2019, respectively. In 2020, the incidence of drug-resistant TB decreased to 157,903. This decrease is consistent with a considerable reduction (18%) in the total number of newly diagnosed TB cases from 2019 to 2020 [14]. MDR-TB has become a concern in Taiwan since 2006, when the rate of drug resistance was higher than that reported by the WHO (third global TB drug resistance surveillance report) [15]. The implementation of the Taiwan Multidrug-resistant Tuberculosis Consortium (TMTC) program with DOTS-Plus effectively reduced the incidence of MDR-TB.

COVID-19, which is caused by SARS-CoV-2, has claimed millions of lives since the end of 2019 [16]. The primary mode of SARS-CoV-2 transmission is exposure to air droplets carrying the pathogen; this is similar to the mode of TB transmission. Droplets of varying sizes are produced during exhalation processes, such as quiet breathing, singing, speaking, coughing, sneezing, and exercise [17,18,19,20].

Since the onset of COVID-19, strict protocols have been adopted worldwide to prevent COVID-19 transmission. These measures include facial masking, social distancing, and contact tracing. During the peak of the pandemic, schools and other institutes were closed; gatherings, travel, and movements were restricted; and online activities were promoted [21]. Until May 2021, most cases of COVID-19 in Taiwan were imported cases; however, after this period, an outbreak of local cases in northern Taiwan was reported [22]. Taiwan’s stringent border control policies effectively prevented viral transmission from abroad, and the aforementioned measures curbed the local outbreak.

TB can be transmitted though droplets of varying sizes, from large to fine aerosol particles [23]. Masks such as surgical masks and N95 respirators prevent the entry of contaminated droplets into the respiratory tract. The pandemic facilitated studies regarding the effects of facial masking and social distancing on TB incidence. COVID-19 limited health-care capacity and medical resources. A Korean study exploring the TB notification rate during the COVID-19 pandemic revealed that the rate in 2020 (49/100,000) was the lowest since 2012 in South Korea; the TB notification rate in 2020 was 16.4% lower than that in 2019 [24]. These findings may be attributed to social distancing during the pandemic. In younger individuals, the TB notification rate increased during the pandemic [24]. The aforementioned study inspired us to investigate whether a similar trend occurred in Taiwan during the pandemic. The Global Tuberculosis Report 2021 published by the WHO [1] indicated that TB incidence and mortality had rebounded at the end of 2020. Considering the inconsistency in TB incidence during the COVID-19 pandemic, we investigated the effects of COVID-19 prevention measures (facial masking and social distancing) on TB transmission in Taiwan. In addition, we investigated whether TB mortality, which is associated with care quality, varies across regions with different incidence rates of COVID-19.