What Scientists Really Mean When They Say ‘No Evidence’

Some things are just unknown. And these days, that’s a lot of things. From the moment the coronavirus emerged as a threat, there have been far more unanswered questions than answered ones. When a virus is only five months old, how much can you possibly know?

This uncertainly is why we’ve been seeing a lot of statements circulating about there being “no evidence” of something coronavirus-related. The World Health Organisation famously said in January that there was no evidence of human-to-human transmission; more recently, they tweeted that there is no evidence that people who recover from the coronavirus are immune to it.

But it turned out that humans were transmitting the virus to each other. And many scientists and doctors believe it’s likely that people will turn out to be immune after recovering from it. So was the WHO wrong?

As they say, absence of evidence is not evidence of absence. In this context, “there is no evidence that…” really means “available evidence does not tell us whether…”

Try it out: In January, available evidence did not tell us whether the coronavirus can be transmitted from human to human. (True.) Right now, available evidence does not definitively say whether people are immune to the coronavirus after recovering from it—or, if they are, for how long.

Earlier today we tweeted about a new WHO scientific brief on “immunity passports”. The thread caused some concern & we would like to clarify:

We expect that most people who are infected with #COVID19 will develop an antibody response that will provide some level of protection. pic.twitter.com/AmxvQQLTjM

— World Health Organization (WHO) (@WHO) April 25, 2020

It doesn’t help that this phrase has a second meaning. Sometimes we say there is “no evidence” when we mean “there is no reason to believe” a thing. For example, there is no evidence that wishing on a crystal can cure cancer. One could rigorously test and attempt to disprove this claim, but do we need to? Saying there is “no evidence” is true and sidesteps the question of whether or not anybody has tried to disprove it.

The phrase “no evidence” is handy, but it can be confusing without context. So when you see someone using it, be sure to read more closely to determine what they’re actually trying to say. Some things are just unknown.