Coronavirus disease 2019 (COVID-19) is a severe acute respiratory syndrome caused by Coronavirus 2 (SARS-CoV-2). In the light of its rapid global spreading, on 11 March 2020, the World Health Organization has declared it a pandemic. Interestingly, the global spreading of the disease is not uniform, but has so far left some countries relatively less affected. The reason(s) for this anomalous behavior are not fully understood, but distinct hypotheses have been proposed. Here we discuss the plausibility of two of them: the universal vaccination with Bacillus Calmette–Guerin (BCG) and the widespread use of the antimalarial drug chloroquine (CQ). Both have been amply discussed in the recent literature with positive and negative conclusions: we felt that a comprehensive presentation of the data available on them would be useful. The analysis of data for countries with over 1000 reported COVID-19 cases has shown that the incidence and mortality were higher in countries in which BCG vaccination is either absent or has been discontinued, as compared with the countries with universal vaccination. We have performed a similar analysis of the data available for CQ, a widely used drug in the African continent and in other countries in which malaria is endemic; we discuss it here because CQ has been used as the drug to treat COVID-19 patients. Several African countries no longer recommend it officially for the fight against malaria, due to the development of resistance to Plasmodium, but its use across the continent is still diffuse. Taken together, the data in the literature have led to the suggestion of a possible inverse correlation between BCG immunization and COVID-19 disease incidence and severity.

冠状病毒病2019（COVID-19）是由冠状病毒2（SARS-CoV-2）引起的严重急性呼吸系统综合症。鉴于其在全球的迅速传播，世界卫生组织于2020年3月11日宣布其为大流行病。有趣的是，该疾病在全球的传播并不均匀，但是到目前为止，使一些国家受到的影响相对较小。尚未完全了解这种异常行为的原因，但提出了不同的假设。在这里，我们讨论其中两个的合理性：芽孢杆菌的普遍疫苗接种Calmette-Guerin（BCG）和抗疟药氯喹（CQ）的广泛使用。在最近的文献中，对这两种方法都进行了充分的讨论，并得出了肯定和否定的结论：我们认为，全面介绍有关它们的可用数据将很有用。对报告的COVID-19病例超过1000的国家进行的数据分析表明，与没有普遍接种疫苗的国家相比，没有或已经停止接种卡介苗的国家的发病率和死亡率更高。我们对可用于CQ的数据进行了类似的分析，CQ是在非洲大陆和疟疾流行的其他国家中广泛使用的药物；我们在此进行讨论，因为CQ已被用作治疗COVID-19患者的药物。疟原虫，但其在整个大陆上的使用仍很广泛。综上所述，文献中的数据导致了对BCG免疫与COVID-19疾病发生率和严重程度之间可能存在反相关的暗示。