A new study in the Journal of Environmental Research claims that “People who reported walking their pets had a higher estimated prevalence of COVID-19 compared to those who did not take their pet for a walk.” How much more? The study authors claim that dog walking increases your risk of getting COVID-19 by 78%. There’s only one problem: it is not true. But that hasn’t stopped it from being repeated by media outlets with alarming headlines:

Despite the claims, even the study authors admitted that the risk is “extremely unlikely.” But that admonition doesn’t even go far enough; it is actually impossible. Why? The mechanism for transmission cited by the study: that dogs are acquiring and then transmitting SARS-CoV-2, the virus that causes COVID-19, to humans has already been scientifically ruled out.

According to the American Veterinary Medical Association, there is no evidence that dogs “spread SARS-CoV-2 to other pets or people.” To the contrary, dogs have repeatedly been shown to be dead-end hosts. Even if they were to come down with the virus — which is very, very rare and unlikely — they do not shed it and thus cannot transmit it to others.

So what evidence was the study based on? In a survey of 2,000 people conducted in Spain following that nation’s first coronavirus lockdown, 6.9% of respondents who walked their dog said they were positive for the virus, compared to 4.2% for those without a dog. Thus, they concluded that dog walking “increases the risk of contagion of COVID-19 by 78%.”

There are several problems with these statistics. First, surveys like this suffer from self-selection bias and response bias. The survey respondents were not a representative sample of the population so the study can’t tell us much about that population. Second, and more importantly, the authors admit that half the people who reported acquiring COVID-19 in their survey did so based on a self-diagnosis. They did not test positive. In fact, they were not tested at all. The data is just not there.

But even if we ignore this and accept, for the sake of argument, that the numbers are accurate, the conclusion does not follow. It is a basic rule of statistics that observational studies like this cannot prove causation:

[T]he results of observational studies are, by their nature, open to dispute. They run the risk of containing confounding biases. Example: A cohort study might find that people who meditated regularly were less prone to heart disease than those who didn’t. But the link may be explained by the fact that people who meditate also exercise more and follow healthier diets. In other words, although a cohort is defined by one common characteristic or exposure, they may also share other characteristics that affect the outcome.

At best, this study tells us we should wear a mask and keep our distance from others when going outside. But we already knew that. So walk your dog. And while you are at it, hug and kiss him, too.

The study, The spread of SARS-CoV-2 in Spain: Hygiene habits, sociodemographic profile, mobility patterns and comorbidities, is here.


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