Apple Watch owners have been shocked by the latest update which can predict illness before symptoms even start.

The device has already been hailed for its ability to identify heart problems and detect falls in the elderly. 

The new Vitals app however — available with the latest operating system WatchOS 11 — has been able to predict common illnesses including the flu and Covid.

The software analyses heart rate, respiratory rate, wrist temperature, blood oxygen, and sleep duration — to monitor health day to day.

Users have now reported their Apple Watch spookily picking up on subtle changes in their bodies, and flagging potential health issues.

In a Reddit thread titled 'The vitals app knew I was sick 3 days before I did', a user shared a screenshot of his Vitals data. 

It revealed that a key measurement — their wrist temperature — had been higher than normal for three days. 

The app notifies wearers when two or more of their vitals — heart rate, breathing rate or wrist temperature — change.

It dubs these 'outliers'. An an example, the notification may say something like: 'During sleep, your respiratory rate and wrist temperature were higher than normal.' 

Apple says that 'notifications will include a tailored message for your specific situation with a list of potential causes to give you better context on your health.'

The Reddit user got notifications on two days and on the third day came down with the flu. 

The user noted: 'Woke up today feeling off. Now, as evening approaches, I'm starting to feel that old familiar ache in my head. I'm sick.'

They weren't the only Apple Watch users to notice the astonishing new feature. 

'I started using Vitals when it first came out and since then I've gotten sick about twice,' wrote one user on Reddit.

'Both times it knew a couple days in advance and I hadn't felt anything wrong.'

On X, formerly known as Twitter, Rory Evans said: 'My Apple Watch detected a few things with my vitals days before I tested positive for Covid! 

'So clever and very useful tech to have - especially in such a fast-paced world.'

Users were shocked when they shared the news, as one commented: 'I didn't even realise the Apple Watch measures wrist temperature until seeing this post…'

A different user said that they had the same experience: 'Mine too, I'm still sick but watch noticed before I did.'

Users also noted that the app identified unusual readings linked to their vaccinations before they began to feel any side effects.

'My watch knew I wasn't 100% after my COVID and flu jabs as all three vitals (Wrist Temperature, Heart Rate and Respiratory Rate) were outliers and that's how I knew I was not well, amazing tbh,' a Reddit user claimed. 

Apple does not claim it can miraculously diagnose or treat medical conditions.

Instead, the company positions the app as a tool for users to gain better context on their health. 

Users can then opt to turn on notifications if their watch takes measurements outside of their normal range.

'When two or more metrics are out of their typical range, users can receive a notification, along with a message detailing how the changes in these specific metrics may be linked to other aspects of their life, such as elevation changes, alcohol consumption, or even illness,' says an Apple spokesperson.

'Vitals uses an algorithm developed using real-world data from the Apple Heart and Movement Study, research that aims to advance the understanding of the links between heart health and physical activity.'