Are vegetarians & vegans less likely to get Covid-19?

Introduction

A year after discussing the unconventional "COVID Vaccine Hesitancy and Risk of a Traffic Crash," I came across another attention-grabbing title: "Vegetarians and vegans 'less likely to get Covid.'" I took a look at the study (published in BMJ Nutrition, Prevention & Health) to scrutinise its methodology and findings.

The Study

Conducted in Brazil by lead author Acosta-Navarro, the study comprised 702 participants recruited through social media networks and the internet between March and July 2022. A basic food frequency questionnaire served as the tool for collating self-reported dietary patterns. The study divided participants into omnivorous, flexitarian, and vegetarian/vegan groups.

On first glance, I spotted a number of flaws with the methodology:

1 - Dietary Questionnaire Reliability: Dietary questionnaires are notoriously unreliable. This one was described as "basic", so probably even more unreliable than usual.

2 - All groups consumed animal foods: The definition of the so-called "plant-based" group included flexitarians (who consumed animal products - including meat), and vegetarians (who consume eggs and dairy). There was no pure plant-based group in this study ostensibly about a plant-based diet.

3 - Self-Reported Data: The reliance on self-reported data, including COVID-19 incidence, introduced subjectivity into the findings, raising questions about the accuracy of the reported results.

4 - Healthy Person Confounder: Adjusting for variables like BMI and pre-existing conditions does not eliminate the possibility that individuals with healthier lifestyles self-reported COVID-19 less often, independent of their diet.

5 - Covid-19 risk factor bias. The omnivore group were far more likely to have higher BMI, non-white ethnicity, pre-existing conditions - all known risk factors for Covid-19.

Despite a high vaccination rate amongst participants (98%), the study reported a 47% self-reported COVID-19 incidence. The adjusted results indicated a 39% lower incidence in the plant-based group compared to omnivores. However, the lack of significant differences in severity of illness or preventative measures raised questions about the validity of the claim.

The study needed a plausible mechanism for the claimed findings. It proposed that plant-based diets meant enhanced immunity. However, the data contradicted this hypothesis, not least because there was no consistent relationship between higher plant consumption and lower COVID-19 incidence within diet groups. Additionally, the nutrients that the body needs are found in animal foods, so the immunity claim doesn't make sense.

Conclusion

While the study suggested a link between plant-based diets and reduced COVID-19 incidence, there were a number of methodological flaws, confounding variables, and a lack of a plausible mechanism to support the claim. A more plausible explanation would be that people with more Covid-19 risk factors would be more concerned about Covid-19, more likely to test for it, more likely to think that they have symptoms of it etc. These such people happened to be mainly in the omnivore group. The plants or otherwise had nothing to do with it!

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