Emerging studies have suggested a link between vitamin D deficiency and COVID-19 symptoms. A review has shown that vitamin D deficiency is not associated with a higher chance of having COVID-19 but is associated with a greater severity of the disease, including 80% increases in the rates of hospitalization and mortality. Schizophrenia: Vitamin D deficiency is associated with the development of schizophrenia. People with schizophrenia generally have lower levels of vitamin D. The environmental risk factors of seasonality of birth, latitude, and migration linked to schizophrenia all implicate vitamin D deficiency, as do other health conditions such as maternal obesity. Vitamin D is essential for the normal development of the nervous system. Maternal vitamin D deficiency can cause prenatal neurodevelopmental defects, which influence neurotransmission, altering brain rhythms and the metabolism of dopamine. Vitamin D receptors, CYP27B1, and CYP24A1 are found in various regions of the brain, showing that vitamin D is a neuroactive, neurosteroid hormone essential for the development of the brain and normal function. Inflammation as a causative factor in schizophrenia is normally suppressed by vitamin D.

Risk factors
Those most likely to be affected by vitamin D deficiency are people with little exposure to sunlight.
Severe deficiency: <12 ng/mL = <30 nmol/L
Deficiency: <20 ng/mL = <50 nmol/L
Insufficient: 20–29 ng/mL = 50–75 nmol/L
Normal: 30–50 ng/mL = 75–125 nmol/LVitamin D levels falling within this normal range prevent clinical manifestations of vitamin D insufficiency as well as vitamin D toxicity. Signs and symptoms
In most cases, vitamin D deficiency is almost asymptomatic. It may only be detected on blood tests but is the cause of some bone diseases and is associated with other conditions:

Complications
Rickets, a childhood disease characterized by impeded growth and deformity of the long bones. The earliest sign of vitamin D deficiency is craniotabes, abnormal softening or thinning of the skull. Osteomalacia, a bone-thinning disorder that occurs exclusively in adults and is characterized by proximal muscle weakness and bone fragility. Women with vitamin D deficiency who have been through multiple pregnancies are at elevated risk of osteomalacia. Osteoporosis, a condition characterized by reduced bone mineral density and increased bone fragility
Increased risk of fracture
Muscle aches, weakness, and twitching (fasciculations), due to reduced blood calcium (hypocalcemia)
Periodontitis, local inflammatory bone loss that can result in tooth loss. Pre-eclampsia:  There has been an association of vitamin D deficiency and women who develop pre-eclampsia in pregnancy. The exact relationship of these conditions is not well understood. Maternal vitamin D deficiency may affect the baby, causing overt bone disease from before birth and impairment of bone quality after birth. Respiratory infections and COVID-19:  Vitamin D deficiency may increase the risk of severe acute respiratory infections and COPD.