Vitamin D not only is well-known as a sunshine vitamin for bone health but also is essential for non-bone cells including immune cells, as shown in Fig.1 (7,12) . Besides, Vitamin D can regulate anti-inflammatory responses  by inhibiting pro-inflammatory cytokines. Cytokine storm is considered as a deadly disaster in response to SACR-CoV-2 infection and/or secondary infection because they can damage multiple organs and lead deaths (15). Vitamin D has also been well-reported as part of the potential process of fighting many infections including Tuberculosis, HIV, hepatitis C virus, bacterial vaginosis and colds and flu. Vitamin D receptor on the plasma membrane of immune cells regulates the signalling pathways of vitamin D in the immune response. There are six identified major VDR gene polymorphisms including Cdx, A1012G, FokI, BsmI, ApaI and TaqI (8). Some ongoing studies also explored the susceptibility to coronavirus of an individual is possibly depended by Vitamin D receptor genes relating to our immune responses against the virus differ between individuals. For example, immune cells called macrophages play an essential role in immune responses to viral infections. In particular, activated macrophages may intracellularly produce cathelicidin against viruses. Evidence shows Vitamin D is like the key that fits in the lock as the VD receptor on the plasma membrane of macrophages and turns it to open the door and activate these macrophages (9). Once activated, they increase the production of cathelicidin to fight the virus. A review article by William Grant and colleagues supposed that vitamin D probably reduces the risk of infectious diseases, including COVID-19, and minimize relevant deaths (10). An optimal Vitamin D level for an individual in a reduction of the infection remains unclear. The answer is probably determined by the VDR gene polymorphism. This might reflect the individual difference in the immune response as well as the effectiveness of vitamin D consumption. In a recent article published by Hoang Anh Nguyen, VDR gene polymorphisms particularly Fokl should be taken into consideration for COVID-19 and vitamin D’s antimicrobial strategy (11). A study by Marina Laplana and colleagues demonstrated that the Fokl is significantly associated with the susceptibility to enveloped virus infection (8). Notably, FokI is also related to obesity(7).

Despite the promising effects of vitamin D, few ongoing studies have examined the preventative and therapeutic effect of vitamin D on COVID-19. An ongoing trial by Cédric Annweiler and colleagues (2020) is investigating the effect of high dosages of Vitamin D supplementation of 400,000 IU in a single oral dose on the elderly aged 70 and above in France, compared with standard dosages of 50,000 IU (13). In another trial by Manuel J Castillo and co-workers (2020), the COVID-19 patients with mild symptoms in Spain will receive a standard medical treatment incorporating Vitamin D supplementation of 25,000 IU(14). To our knowledge, no studies explore the precise dosages of vitamin D on COVID-19. Understanding genetic susceptibility to SAR-CoV-2 is challenging, but it has the potential to fill the gap of knowledge between the stratification of patients and personalised treatment. Thus the governments, researchers and consumer genomics labs across the globe should work on further vitamin D and COVID-19 study to figure out a bigger picture on precision strategy. It is a possibility that the VDR gene polymorphisms, probably FokI, may support a personalised treatment for COVID-19. Ethical issues and confidential relating to genetic data sharing should be carefully taken into account.