Working with data gathered from the ZOE COVID Symptom Study app, the TwinsUK cohort and two hospital cohorts, the team found that basal body temperature was lower in older people and those with a lower body mass index (BMI). Older people with COVID-19 were also less likely to have a fever reaching 37.8°C or more, with the chances dropping by 1% with every additional year of age. The research suggests that COVID cases in elderly people, the most at risk group, might be missed as fever temperatures in older people are often lower than the 37.8°C threshold.

The average human body temperature is around 36.5-37.5°C, although this is known to vary with the time of day and how it’s measured. This paper shows that temperature regulation also changes with age and body mass, with older and thinner people usually having a slightly lower temperature and finding it harder to stay warm.

Fever (a raised temperature) is a common symptom of COVID-19, affecting more than two thirds of people with the disease. Current health guidelines advise that anyone with a temperature of 37.8°C or more should be considered to be potentially infected with COVID-19 and should self-isolate.

To find out more about the relationship between temperature and age, the researchers analysed data from four different groups: 1,089 adult twins without COVID-19, 1,284 people hospitalised with COVID-19 in London (520) and Birmingham (764), 3,967 users of the ZOE COVID Symptom Study app with confirmed COVID-19.

Based on their analysis, the researchers found that using a threshold of 37.4°C for over 65s is as sensitive and specific a sign of infection as 37.8°C in adults under 65.

The findings support a call for the National Early Warning Score* guidelines to be revised, reducing the temperature at which COVID-19 is suspected for adults over 65 to this lower value.