When we look at the numbers, our brains can play tricks on us, writes Siouxsie Wiles.

Did you hear about the testing of the general population in Iceland for Covid-19 that found that half of the people who tested positive didn’t have any symptoms? When I heard that, the first thing I thought was: shit, there are heaps of people walking around spreading Covid-19! And they don’t even know it, aaargh!

I had a similar thought when I heard that the entire 4,800 crew on the US aircraft carrier Theodore Roosevelt had been tested and 60% of those that were positive didn’t have symptoms. I wasn’t alone. I did a quick poll on Twitter and almost a third of the 4,721 people who voted thought there were thousands of asymptomatic crew members. More than half picked the right answer, though, which is that there wasn’t enough information provided to say anything about how many crew members had Covid-19 and no symptoms.

(As an aside, the ship’s commanding officer, Captain Brett Crozier, asked for his crew to be able to disembark because there wasn’t space to quarantine those who were sick. He was relieved of his command at the beginning of April for sending a plea for help using a non-secure email to a “broad array of people” rather than up the chain of command.)

If we go back to the Iceland example, the first bit of mental arithmetic my brain did when it heard “the general population” was to jump to the conclusion they’d tested tens of thousands of people. Again, I ran a quick Twitter poll to see if anyone else thought the same. Of the 2,167 people who voted, three quarters thought that either more than 10,000 people or more than 100,000 had been tested. It was pretty much neck and neck between those options.

I don’t know about you, but for me it was then just a short hop to assuming that hundreds or maybe even thousands of Icelanders had tested positive for Covid-19. And if half of those were asymptomatic, then that must be hundreds or thousands too, right? Wrong.

At that point my brain engaged properly, and I realised that’s probably not what the testing showed at all. So, to find out what was really going on I looked for more details of the study being reported. All I could find were some articles and interviews with Dr Kári Stefánsson, founder of a company called deCODE Genetics. He’s reported as saying that half of those who tested positive in Iceland didn’t have symptoms. Then I found a news article on the Icelandic Ministry of Health’s website from March 25. It reported that 6,163 tests for the Covid-19 virus had been carried out on members of the general population who were generally asymptomatic or showed mild symptoms. Out of those tests, 6,111 were negative and 52 were positive. If you need reminding how the test works, I’ve written about it here.

Now the actual study has been published. It describes the results of the two different groups of people who’ve been tested in Iceland. The first group are people who were at high risk for infection. So that’s people who had the symptoms of Covid-19 and had recently travelled to high-risk countries or had contact with someone with Covid-19. As of April 4, they’d tested 9,199 people who fell into this category and found 1,221 of them to be positive. That’s just under 13.3%.

The second group of people were members of the general population. They were the 10,797 people who responded to an open invitation and the 2,283 who were sent a random invitation to get tested by deCODE Genetics. Of these people, 100 tested positive. That’s about 0.8%. According to the paper, just over half of these people experienced some symptoms. That means that fewer than 50 people didn’t have symptoms at the time they tested positive for Covid-19.

Unfortunately, all the paper says about these people is that symptoms “almost certainly developed later in some of them”. Given we know people are infectious for a few days before symptoms, that’s almost certainly the case here. But it is so frustrating that they didn’t find this information out and report it. They could have helped answer two important questions we still don’t have a firm answer to: do proper asymptomatic people actually exist, and if they do, are they infectious or not? Instead the way this work has been shared on social media and in media headlines can lead everyone, myself included, to leap to worrying and completely unjustified conclusions.

If you’re curious to know how many of the 4,800 navy crew tested positive for Covid-19, it was about 600. So, the 60% who were asymptomatic is roughly 360 sailors. Again, this isn’t a published study but a media report. But it must be followed up and published as a proper study because it is so important to know if these mostly young fit men were really asymptomatic or whether they were pre-symptomatic. Given the crew were all housed together on an aircraft carrier without space to properly quarantine people, if they really never developed symptoms this could also help answer the question of whether asymptomatic people are infectious or not.