The reduction in risk was modest and ranged from up to 9% for vitamin D; up to 14% for probiotics; up to 13% for multivitamins and up to 12% for omega 3.
1.4 million users of the ZOE COVID Symptom Study app in the UK, US and Sweden answered questions about their supplement use during the early part of the pandemic. More than 445,000 of these users reported a positive COVID-19 PCR or serology test and more than 126,000 were predicted to have COVID-19 based on their symptoms (loss of smell, the strongest predictor of the disease).
The data revealed that multivitamins, vitamin D, omega-3, and probiotic supplements all had a very small but statistically significant protective effect against COVID-19, while vitamin C, zinc or garlic supplements had no detectable effect.
However, when the researchers broke down the analysis by sex, they found the protective association was strangely only present in females.
Researchers suggest this difference may be due to the differences in immune systems between males and females. Alternatively, they said it could be due to reporting bias, with one sex reporting their supplementation more accurately, or bias caused by another currently unknown factor.
The researchers also investigated whether the so-called ‘healthy bias’ could explain the findings. People who take vitamins and other supplements may also be more likely to take better care of their overall health and engage in steps to avoid catching coronavirus, such as wearing a mask and frequent hand washing. However, if the findings were only a reflection of the healthy bias effect, all supplements would be expected to have a protective effect, but this was only seen for a limited range.
Furthermore, when the researchers adjusted their data to account for other potentially confounding factors that may reflect the ‘healthy bias’, including smoking, healthcare status, diet, socioeconomic deprivation, BMI, age and underlying health conditions, the correlation remained statistically significant.
Our research is an observational study and not a clinical trial, so we can’t make strong recommendations based on the data we have. Until we have further evidence about the role of supplements from randomised controlled trials, we recommend following the NHS guidelines on vitamins usage as part of a healthy balanced diet.– Lead researcher Dr Cristina Menni from the School of Life Course Sciences
Professor Tim Spector said: “Many people think that taking vitamins and other supplements can help maintain a healthy immune system, but spending your money on supplements in the hope of trying to avoid getting COVID-19 is largely unjustified. You’re better off focusing on getting a healthy diet with diverse fresh vegetables and fruits, which should give you all the nutrients you need for a healthy immune system.
“Over the weekend, the government announced it would be providing 2.7 million vulnerable individuals in England to be offered free winter supply of Vitamin D. Based on our research, we cannot tell whether vitamin D supplements will have any real impact on these high risk groups.”