Doctor’s Note: Why do we get fevers?

One of the most prominent symptoms of COVID-19 is fever. So, if you are running a temperature, you should stay at home and self-isolate. When we get a fever, we usually respond by trying to bring our temperature down – usually by taking paracetamol.

But evidence suggests that a fever may be part of your body’s strategy for fighting an infection rather than just a by-product of the infection. 

What is a fever?

It is normal for your body’s temperature to fluctuate throughout the day, and it is often highest in the evenings and lowest in the mornings.

A fever is usually defined as a temperature above 37.5 degrees Celcius (99.5 degrees Fahrenheit), but the temperature you read can vary depending on where you take it from.

When taken in the mouth, the average body temperature is usually between 36.5C (97.7F) and 37.2C (98.96F) but, taken in the armpit, it can be 0.2C to 0.3C (0.36F-0.54F) lower.

During the pandemic, there has been a shortage of thermometers, and without one, it can be difficult to determine whether you have a fever.

To assess whether you might have a fever without the use of a thermometer you can use the back of your hand to feel your forehead, check in the mirror for a flushed face, observe for chills, muscular aches and weakness and assess for dehydration from a fever by checking for darker, more concentrated urine.

How do fevers occur?

Fevers are caused by chemicals called pyrogens which occur when proteins and other toxins are released from the cell walls of bacteria and viruses, or by our own bodies when they come into contact with certain bacteria and viruses.

The pyrogens are released into the bloodstream and then reach the hypothalamus in the brain, our temperature regulation centre. When the pyrogens bind to the receptors in the hypothalamus, this causes shivering and the constriction of blood vessels to help the body temperature reach the new setpoint, resulting in a fever which is part of the body’s attempt to kill the pathogen – in this case, the virus.

What are the benefits of a fever?

Fevers are usually self-limiting and are rarely serious in nature. Studies have shown that when your core body temperature increases by between 1C (1.8F) and 4C (7.2F), infections clear up quicker.

It was previously thought that a fever simply created a less hospitable environment for pathogens to survive and replicate, but mounting evidence also shows that a fever optimises our immune system.

It has been found that certain immune cells (known as T-cells) which are capable of destroying infected cells are enhanced by fevers. In a 2011 laboratory study involving mice, half the mice were warmed by 2C (3.6F) and the other half remained at normal body temperature. It was found that the warmed mice had an increased number of T-cells.

It has also been suggested that excess body heat triggers our T-cells to produce heat-shock proteins (HSPs), which bind to another set of proteins called integrins.

These immune complexes all pull together and stick to blood vessel walls, allowing the immune cells to crawl along the blood vessel walls and migrate to the site of the invading pathogen to attack it.

In a study of mice conducted at the Shanghai Institute of Biochemistry and Cell Biology in 2019, researchers mutated the integrin so it could no longer bind to the HSPs, resulting in the mice dying quickly from the infection.

What are the downsides of a fever?

A fever is not universally beneficial, however. High temperatures are damaging to our cells and cell death in humans occurs at a temperature of about 41C (105.8F), with the rate of cell death increasing as the temperature rises.

A raised temperature can also cause increased inflammation and cause the function of our organs to become impaired.

Most patients will fully recover after a period of fever. However, if someone runs a high temperature for a long period of time, there will be a greater risk of complications. In extreme cases, that can include organ failure and death.

The definition of “a long period of time” varies depending on the cause of the fever. For example, a fever caused by heatstroke needs to be cooled down within 60 minutes to improve survival. However, studies of other illnesses – with both infectious and non-infectious causes – have defined “prolonged fever” as one that lasts longer than five days.

UK guidelines say a fever from the coronavirus should improve within seven days. If it goes on longer than this, then you should discuss it with a healthcare professional.

It has also been shown that an uncontrolled fever in patients with extreme inflammation can lead to worse outcomes, including sepsis or neurological injuries. In these cases, treatments that lower body temperature can actually be protective and have a clinical benefit.

In children – usually between the age of six months and three years – a fever can trigger a febrile seizure. These are usually harmless and last for less than five minutes, with the child fully recovering afterwards.

We do not know why some children have these seizures, but we suspect there is a genetic element to it. Research has shown that giving children medicine to bring down high temperatures is unlikely to prevent a further febrile seizure. 

Should we treat fevers at all?

There are a number of ways in which we can help treat the symptoms associated with fever without using medication. For example, by wearing loose, cool clothing, opening windows and drinking plenty of fluids.

However, fevers can also be treated with medications we call antipyretics – the most commonly used over-the-counter forms are paracetamol and ibuprofen. There had been concerns about the use of ibuprofen by those with coronavirus, but the Commission of Human Medicines (CHM) has since reviewed this and found insufficient evidence to establish a link between ibuprofen and the worsening of COVID-19 symptoms. It has therefore been deemed safe to use ibuprofen to self-medicate for the symptoms of COVID-19.

Controversy remains over whether it is best to treat a fever in the critically ill. On the one hand, we know that fevers are a protective response in fighting infections, but on the other, the payoff of a fever puts a lot of stress on the body.

We are aware from studies of patients infected with rhinovirus – one of the causes of a common cold – that those given medicines to reduce their temperature were found to have a suppressed immune response, and they had increased symptoms and signs.

Furthermore, research into seasonal flu virus in 2014 showed that giving a range of different antipyretic medicines to reduce temperature resulted in a 5 percent increase in death from seasonal flu.

The evidence overall suggests that the beneficial effects of a mild fever may balance the negative factors. However, in fevers higher than 40C (104F), there is higher mortality, in which case earlier treatment should be considered.

For COVID-19, there is no rule about treating fevers if the symptoms are only mild and you are feeling well enough. Most people, who show only mild symptoms, will be able to manage it through rest, fluids and keeping cool.

However, if your temperature rises higher than 40C, or you are feeling particularly unwell with a fever, taking paracetamol or ibuprofen to reduce your temperature and ease any muscle pains may be what you need. 

While the “let it ride” philosophy is being increasingly used when it comes to managing fevers, it is important to discuss your symptoms with your healthcare professional if your fever is worrisome, and more urgently if you have any of the red flag signs like trouble breathing, persistent pain or pressure in the chest, bluish lips or face, or new confusion or fainting.