People with severe vitamin D deficiency are twice as likely to experience severe complications – including death – if they fall ill with COVID-19.

That's the headline finding of a study conducted at Northwestern University in the US, which undertook a statistical analysis of data from hospitals and clinics across China, France, Germany, Italy, Iran, South Korea, Spain, Switzerland, the UK and the US.

After studying the global data relating to the COVID-19 pandemic, the team – led by professor of biomedical engineering, Vadim Backman – discovered a strong correlation between severe vitamin D deficiency and mortality rates.

Patients from countries with high COVID-19 mortality rates, such as Italy, Spain and the UK, had lower levels of vitamin D, compared to patients in countries that were not as severely affected.

By analysing the publicly available patient data, the study discovered a strong correlation between vitamin D levels and cytokine storm — a hyperinflammatory condition caused by an overactive immune system — as well as a correlation between vitamin D deficiency and mortality.

Cytokine storm can severely damage lungs and lead to acute respiratory distress syndrome and death in patients – and seems to be what is killing a majority of COVID-19 patients.

This is exactly where Backman believes vitamin D plays a major role. Not only does vitamin D strengthen our immune systems, but it also prevents them from becoming dangerously overactive.

This means having healthy levels of vitamin D could protect patients against severe complications, including death, from COVID-19.

“Our analysis shows that the impact might be as high as cutting the mortality rate in half,” Backman said.

“It will not prevent a patient from contracting the virus, but it may reduce complications and prevent death in those who are infected.”

Backman added that the correlation might help explain the many mysteries surrounding COVID-19, such as why children are less likely to die.

Children do not yet have a fully developed "acquired" immune system, which is the immune system’s second line of defence and more likely to overreact.

“Children primarily rely on their innate immune system,” Backman said. “This may explain why their mortality rate is lower.”

Backman and his team were motivated to examine vitamin D levels after noticing unexplained differences in COVID-19 mortality rates from country to country.

Among the initial hypotheses was that differences in healthcare quality, age distributions in population, testing rates or different strains of the Coronavirus might be responsible.

Backman, however, remained sceptical and wanted to research the data – and is now confident that he has found the real reason.

“None of the factors (that people thought were causing the differences) appear to play a significant role,” Backman said of the hypotheses.

“The healthcare system in northern Italy is one of the best in the world and differences in mortality exist even if one looks across the same age group. And, while the restrictions on testing do indeed vary, the disparities in mortality still exist even when we looked at countries or populations for which similar testing rates apply.

“Instead, we saw a significant correlation with vitamin D deficiency."

Backman is careful to note, however, that people should not take excessive doses of vitamin D, which might come with negative side effects.

The body creates vitamin D from direct sunlight on the skin when outdoors. During the summer months, most people should be able to get all the vitamin D they need from sunlight, however, the lockdown in many countries has led to people being indoors for many weeks, only to emerge with lower vitamin D levels while COVID-19 is still circulating in the population.

This fact points to the importance of encouraging outdoor exercise.

Vitamin D is also found in a small number of foods, including oily fish – such as salmon, sardines, herring and mackerel – red meat, liver, egg yolks and fortified foods.

He said the subject needs much more research to know how vitamin D could be used most effectively to protect against COVID-19 complications.

• To access and read the full study, titled The Possible Role of Vitamin D in Suppressing Cytokine Storm and Associated Mortality in COVID-19 Patients click here for the health science journal Medrxiv.