Covid-19 cases could be missed because temperatures are more common in the evening than the morning

Covid-19 cases could be missed if people are screened for a fever in the morning, scientists have warned.  

US experts found flu patients were 44 per cent less likely to have a temperature in the early hours of the day, compared with the afternoon to evening.

Researchers analysed a decade’s worth of data from almost 300,000 people who visited emergency departments across the US. 

They concluded that morning checks could result in up to half of people who have a fever being missed.

And they warned this would apply to other infectious diseases like Covid-19, even though the study only looked at patients with a fever from flu. 

A fever is one of the tell-tale symptoms of Covid-19. If it is not spotted, it could lead to someone going undiagnosed and potentially spreading the virus. 

Hospitals regularly take patients’ temperatures, and now screening is set to become commonplace in work places, schools and airports.

The study researchers urged for a twice-a-day approach, and for checks both before and after a long-haul flight. 

Researchers looked at patients representative of the US (right) and at one Boston hospital (left). Both showed that patients’ temperatures were lowest in the morning and highest in the evening, either during a flu outbreak or at a regular time of year

The authors of the study, from various parts of the US, said: ‘The results suggest that morning temperature measurements could miss many febrile disease cases.’

They added it ‘raises concerns’ because workplace and school fever screens often occur during mornings.

The team wrote: ‘Temperature screening is usually recommended once daily at morning arrival to workplaces and schools, yet our results suggest the morning could be the worst time. 


Putting a thermometer into an armpit, mouth, ear or other body cavity is known to be the most accurate way to measure temperature. 

It gives a reading for the body’s core temperature, which may rise in order to help fight illness. A high temperature is regarded as anything within the range of 38°C and 41°C.

Temperature scanners — including thermal imaging and temperature ‘guns’ pointed at the forehead — do not measure the body’s core temperature. 

Instead, they measure skin temperature which, although tends to correlate with spikes in core body temperature, can vary depending on the environment and activity.

The devices can only make an estimation of core body temperature by measuring heat radiating from the skin using infrared technology. 

Therefore the devices are not as accurate as a medical device which takes a patient’s’ temperature. 

Derek Hill, a professor of medical imaging science from University College London, told MailOnline: ‘They certainly shouldn’t be used to say if someone has a fever. But they might be useful for detecting people who need a real temperature check.’ 

If a person is flagged as potentially having a high temperature, they may be denied entry to a venue. But this would depend on the policy of each place.

They are controversial because a temperature above the normal range does not necessarily mean someone has the coronavirus – they may be unwell with something else. And people have variations in their temperature daily and women see fluctuations through their menstrual cycle.

They can also miss Covid-19 patients who do not have the symptom of a high temperature, or not symptoms at all. 

The World Health Organization says thermal camera says temperature screening ‘may not be very effective’ as a singular tool for detecting Covid-19. 

While the medicine regulator in the UK recently stressed that temperature screening products cannot be used to diagnose Covid-19 because there is not evidence to support their use — and reminded suppliers of they should not make such claims. 

‘Our results raise concerns that morning measurements could miss many (perhaps even half) of the individuals with fevers detectable during evenings, potentially allowing them to go to work, attend school, and travel.’

The researchers recommend that temperatures are taken both in the morning and then later in the day.

‘Similarly, departure and arrival screens might both be worthwhile for long flights’, the researchers said.

But the team admitted that their findings are only applicable to the use of traditional thermometers, and not screening devices which have lower accuracy.

Temperature screening devices include thermal imaging cameras in airports and thermal ‘guns’ pointed at the head to give a reading.

These tools are not designed to measure the body’s core temperature. They can only give a guess based on the temperature of the skin.

The research, which was posted as a pre-print on the site MedRxiv, was a ‘retrospective design’, meaning it looked back on data for patients in the past.

Researchers led by Charles Harding, an independent statistical analyst from Seattle, looked at 295,406 American patients.

Some 202,181 came from a nationally representative study of US emergency department visits between December 2002 to December 2010.

The other 93,225 came from a Boston adult emergency department between September 2009 to March 2012.

Therefore the data covers several years (2002 to 2012) in which there were flu outbreaks, including the 2009 swine flu epidemic.

In every outbreak, temperatures – classed as above 38°C (100.4°F) – were less common in the morning. But it is not clear why.  

In the Boston study, fevers were the least common at around 8am, before rising during the early afternoon and peaking at around 10pm. 

The pattern was similar in the national cohort, but not as drastic, meaning it was more likely patients were spotted in the morning than in the Boston group.  

The researchers explained that the body’s temperature naturally drops during the mornings due to the circadian rhythm.  

A circadian rhythm – the body’s internal clock – helps to regulate the body over the day.    

Flu and Covid-19 are caused by different viruses – influenza or SARS-CoV-2, but are both contagious, respiratory illnesses with similar symptoms.

The NHS lists a high temperature as the main symptom of Covid-19 and the CDC lists a fever. 

While some Covid-19 patients may never get a fever, instead suffering with other symptoms such as a cough or loss of taste and smell, others may only have a fever.