A study of data from healthcare workers in six countries has suggested people who follow vegan or pescatarian diets are less likely to develop severe forms of COVID-19 than meat eaters.

The study, published in BMJ Nutrition, Prevention and Health, looked at workers in the United Kingdom, United States, France, Germany, Spain and Italy, who were likely to have had "substantial exposure" to COVID-19 patients, and found that there was small statistical increase of moderate to serious illness among those whose food intake was higher in protein and lower in carbohydrates.

More than 2,300 people who took part reported that they did not have COVID-19 and 568 did, with 138 of those respondents saying they had experienced moderate to severe symptoms. All those who took part in the study were asked to choose from a list of 11 different dietary lifestyles which they had followed in the year before falling ill.

The authors of the study, based at Stamford University in Connecticut, USA, estimated that based on the data supplied, people with plant-based diets were 73 percent less likely to have reported moderate to severe disease, with the figure for pescatarians 59 percent, compared to those with other lifestyles.

The vegan diet was not found to alter the risk of contracting the virus in the first place, or helping with a faster recovery, but it did seem to ward off complications.

Cambridge-based nutrition educational organization the NNEdPro Global Centre for Nutrition and Health said the study's limited scope - only speaking to people working in a specific job sector, and with the majority being male - meant it should be treated with caution.

"The trends in this study are limited by study size and design (self-reporting on diet and symptoms) so caution is needed in the interpretation of the findings," said nutritional scientist Shane McAuliffe.

Gunter Kuhnle, a professor of nutrition and food science at the University of Reading, agreed that the study was subject to limitations, and also highlighted the variability of some of the data.

"There are a number of limitations that need to be considered: The study relied entirely on self-reporting, and a lot of data have shown that self-reported dietary intake is unreliable," he said. "In this study, participants were asked about their diet after they were diagnosed with COVID-19, and this might lead to further misreporting, especially among participants who are interested in a potential link between diet and disease.

"Finally, the study has been conducted in different countries with widely different diets - a plant-based diet in Spain or Italy is likely to be different from a mainly plant-based diet in Germany or the UK."

Francois Balloux from the Genetics Institute at University College London was slightly more enthusiastic, saying the sample size was "decent "and the analyses "look competently performed", but pointed out that diet alone may not be enough to explain the findings. "Indeed, unaccounted lifestyle variables correlated with diet might influence general health of the subjects of the study, and hence how well they coped with COVID-19 infection," he added.