Review of vitamin D for COVID-19, concluding that the available evidence is very suggestive of protective and preventive effect of vitamin D.
Authors note that strict lockdown (longer time indoors and home quarantine) may increase the risk of developing vitamin D deficiency. They suggest it is the duty of governments to strengthen recommendations regarding nutritional supplementation (especially vitamin D and C), particularly under quarantine and lockdown times.
Basha et al., 10/21/2020, peer-reviewed, 4 authors.