Individual factors underlie temperature variation in sickness and in health: influence of age, BMI and genetic factors in a multi-cohort study

Introduction

Ageing affects immune function resulting in aberrant fever response to infection. We assess the effects of biological variables on basal temperature and temperature in COVID-19 infection, proposing an updated temperature threshold for older adults.

Methods

Participants:

• <underline>Unaffected twin volunteers</underline> : 1089 adult TwinsUK participants.

• <underline>London hospitalised COVID-19+</underline> : 520 adults with emergency admission.

• <underline>Birmingham hospitalised COVID-19+</underline> : 757 adults with emergency admission.

• <underline>Community-based COVID-19+</underline> : 3972 adults self-reporting a positive test using the COVID Symptom Study mobile application.

Analysis

Heritability assessed using saturated and univariate ACE models; Linear mixed-effect and multivariable linear regression analysing associations between temperature, age, sex and BMI; multivariable logistic regression analysing associations between fever (≥37.8°C) and age; receiver operating characteristic (ROC) analysis to identify temperature threshold for adults ≥ 65 years.

Results

Among unaffected volunteers, lower BMI (p=0.001), and older age (p<0.001) associated with lower basal temperature. Basal temperature showed a heritability of 47% (95% Confidence Interval 18-57%).

In COVID-19+ participants, increasing age associated with lower temperatures in cohorts (c) and (d) (p<0.001). For each additional year of age, participants were 1% less likely to demonstrate a fever (OR 0.99; p<0.001).

Combining healthy and COVID-19+ participants, a temperature of 37.4°C in adults ≥65 years had similar sensitivity and specificity to 37.8°C in adults <65 years for discriminating fever in COVID-19.

Conclusions

Ageing affects temperature in health and acute infection. Significant heritability indicates biological factors contribute to temperature regulation.

Our observations indicate a lower threshold (37.4°C) should be considered for assessing fever in older adults.

Key Points

• Older adults, particularly those with lower BMI, have a lower basal temperature and a lower temperature in response to infection

• Basal temperature is heritable, suggesting biological factors underlying temperature regulation

• Our findings support a lower temperature threshold of 37.4°C for identifying possible COVID-19 infection in older adults

• This has implications for case detection, surveillance and isolation and could be incorporated into observation assessment