A high temperature is one of the key symptoms of flu – both seasonal and coronavirus. For normal flu, the NHS’s top is symptom is a “sudden fever” – which it defines as a temperature of 38.0 °C or above. But just how quickly does your temperature rise? And can you monitor your temperature to get an early warning of infection? Before the lockdown (i.e. before leaving home was only permitted for essential travel) I would check my temperature before heading off to visit my mum – would that have reduced the risk of me passing on coronavirus? To find out, let’s have a look at how body temperature varies naturally, and how quickly it rises with a viral infection.

I’ve been tracking my temperature for a few weeks to see how much it varies, and therefore how easy it might be to spot a sudden increase in temperature.

It looks a bit of a mess, so let’s try to make some sense of it. Firstly, what’s normal?

What is normal body temperature?

Normal body temperature for humans is 36.5 – 37.5 degrees Celsius. As with everything regarding physiology it will vary between people based on their age, sex and health condition – and even throughout the day and on different days. It is quite normal for your temperature to vary by half a degree, which is exactly what mine does – most readings are between 36.5 and 37.0 °C, though don’t forget yours might vary within a higher or lower band.

Although the graph above looks quite random, I quickly noticed a slight fluctuation in my daily temperature. I’m am warmest in the afternoon (average 36.8), coolest in the morning (36.6) and in-between in the evening (36.7).

This fits nicely with scientific research that has found body temperatures following, on average, a regular pattern throughout the day. Your lowest temperature occurs whilst sleeping - about two hours before waking up. It then steadily rises to peak around lunchtime then slowly falls for the rest of the day. So, I started taking my temperature a bit more frequently and averaging readings by the hour to see if I followed the general pattern.

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Basically, yes. I wake up coolest, peak with daytime activity, and then steadily cool down during the evening. But these are just averages. I’ve also hit 36.9 first thing in the morning; I panicked thinking it was the start of a fever, but although my temperature was a bit higher than (my) average all day, nothing came of it and it was back to (my) normal the next day.

There is a lot of natural variation that can change your readings by half a degree. As well being affected by the daily circadian rhythm, body temperature also varies throughout the menstrual cycle (circamensal rhythm) and across the seasons (circannual rhythm). Also, how much the temperature of an individual varies throughout the day (its amplitude) has been found to be dependent on physical fitness and age. Fitter and/or younger people tend to have a larger range of temperature.

How fast does your temperature rise when you get coronavirus?

I searched hard for studies showing this, but didn’t find any in humans (for normal flu or covid-19) – trials are probably limited by ethics of infecting someone to see how long it takes to affect their temperature, rather than tracking their temperatures after someone is known to already be infected. However, I did find a few studies on monkeys and ferrets, plus general experiences of healthcare workers who deal with seasonal flu outbreaks.

Fever, generally, is a symptom of another underlying condition, rather than an illness itself. Long-lasting fevers may be due to other diseases or health problems, but short-lived fevers are typically in response to an infection. When the body’s immune system detects an infection, one of its defences is for the hypothalamus (part of the brain that also controls hormones, appetite and emotions) to turn up the body’s thermostat. It raises the temperature which stimulates white blood cells and increases the production of antibodies, making it harder for germs to multiply.

Fevers are categorised into low and high grades:

• > 38.0 °C is low grade fever (aka pyrexia)
• > 39.4 °C is high grade fever (aka hyperpyrexia)

NOTE: a temperature above 41.2 °C needs urgent medical attention to prevent serious consequences (such as permanent brain damage) and anything over 40.0 °C is potentially dangerous.

Fever tends to happen rapidly, with temperature changing over a few hours rather than days. Though don’t forget, this isn’t how long it takes for your body temperature to jump after being infected, just how quickly symptoms appear. The incubation period of COVID-19 (i.e. time between infection and symptoms) is up to a fortnight, most commonly five days, so you will already have been infectious by the time you get a fever.

Strangely, the first symptom of a fever may be shivering. This is because one of the body’s methods for increasing its temperature is by rapidly contracting muscles (creating the shiver) – we usually relate this to being cold where the body is trying to bring its temperature back up to normal; just with an infection it is endeavouring to raise your temperature above normal.

Given that for most people temperature can vary throughout the day, we want to be on the lookout for a sudden rise above your normal range, so the first thing to do is create a baseline. Take your temperature at least 3 times a day (at different times – e.g. morning, afternoon, evening) for 3 consecutive days. Then, monitoring your temperature 3 times a day, and comparing it to your baseline, should give an early indication of trouble. Or even just checking your temperature daily should enable you to spot a sudden jump.

How to take your temperature

Just as body temperature varies slightly between individual, so does how you take your temperature influence the readings. To baseline and monitor your temperature, there are a few things to remember.

Always use the same thermometer and technique (e.g. oral or forehead) as different equipment will produce slightly different results. This should give you a good idea of your normal range. For me, that turned out to be 36.4 to 37.0 °C – so I now know that any reading above 37.0 might be an early indication of flu.

I used an iProven DMT-489 thermometer which is CE and FDA approved. It does forehead and ear measurement, though I always use the forehead mode as I find that easier. It’s out of stock at the moment (with coronavirus there’s been a surge in demand for thermometers) but there are plenty of digital forehead thermometers on Amazon* still.

As well as the natural rhythms mentioned above, there are some things that can also have a short-term effect on temperature.

Activities that raise body temperature:

• eating or drinking anything with calories (though see alcohol below) - so water or black tea won’t affect a forehead reading, but if taking your temperature orally (i.e. with a mouth thermometer) then you should wait 15 minutes after your last food or drink as this can have a big effect on the reading
• exercise
• mental excitement
• electric blanket

Activities that lower body temperature:

• drinking alcohol
• over-the-counter medication – antipyretics such as paracetamol, ibuprofen and aspirin reduce a fever (but don’t treat the underlying illness)

Activities that affect body temperature up or down:

• environment - particularly when using a forehead thermometer, the room temperature can affect your temperature. I took my temperature after popping outside for a few minutes (sunny, but only 7 °C) and even after being back inside for 5 minutes my reading had plummeted to 36.1! Likewise, sitting in the sun will temporarily increase your skin temperature.

How to treat a fever

A fever will go away once the underlying condition has been rectified – which for most ailments, including most sufferers of coronavirus, will happen naturally after a few days. However, assuming your fever doesn’t have a more serious cause (such as malaria, cancer, or chronic infection) then you can treat the symptoms of a fever with over-the-counter drugs.

Fever-reducing medication (antipyretics) that you can purchase from a local chemist include paracetamol (also called acetaminophen), ibuprofen and aspirin. Make sure to read the label and only take as recommended. Remember, these medications only temporarily reduce the fever – so if the underlying condition hasn’t improved by the time they wear off 4 to 8 hours later then your fever may come back.

All well as pharmaceutical drugs, other ways to reduce the effects of a fever are to get plenty or rest and liquids. These are good prescriptions for the fundamental illness too.

Coronavirus self-medication: unless you’re in a vulnerable group, its not currently necessary to even inform the NHS – and definitely don’t visit your GP surgery – stay at home unless symptoms worsen.

In Conclusion

So, is it possible to get an early warning of coronavirus (or any other viral infection) by monitoring your temperature? Well, at a population level it’s definitely possible. Kinsa has reported that by aggregating data from its smart thermometers they can predict spikes in infections, down to the county level, days before the uptick is noticed by healthcare authorities. Whether this happens before other symptoms (including aching muscles and, in the case of coronavirus, a persistent cough) I’m not sure – but I’ll report back if and when I come down with it.

I’m certainly going to continue to monitor my temperature. And even more so when lockdown restrictions are eased, to make sure I minimise the risk that I might pass on covid-19 to others before symptoms are showing.

If you want to monitor your own temperature, you can download the spreadsheet I created to monitor mine. It includes formulas to calculate am/pm/evening and hourly averages and the graphs included in this blog.

More information

Human Body Temperature – Wikipedia
Fever Facts – WebMD
How to Tell When a Fever in Adults Is Serious – HealthLine
How Does COVID-19 Differ from the Flu? – HealthLine
Fever treatment: Quick guide to treating a fever – Mayo Clinic
Kinsa's smart thermometer, an early warning device for COVID-19  - Live Forever Club