People can suffer different Covid symptoms depending on age and gender, with even members of the same family experiencing separate conditions.
New research suggests older people are less likely to lose their smell but more likely to develop diarrhoea.
The study, showed over-80s are most likely to experience a sore throat, chest pain, unusual muscle pain, eye soreness, and chills or shivers in the early stages of illness, the Mirror reports.
Patients aged 16 to 39 tend to suffer loss of smell, chest pain, shortness of breath, eye soreness and abdominal pain, experts found, according to the research published in The Lancet.
It also reported differences between men and women. Males are more likely to suffer shortness of breath, fatigue and chills and fever in the first three days of symptoms, while most common symptoms in females were loss of smell, chest pain and a persistent cough.
The research was carried out based on data from the Zoe Symptom Tracker app, which was analysed by experts at King's College London.
A statement from the Zoe app states: "The clearest symptoms for early detection of Covid-19 overall included loss of smell, chest pain, persistent cough, abdominal pain, blisters on the feet, eye soreness and unusual muscle pain.
"However, for older groups, loss of smell was not any early sign, whereas other early symptoms such as diarrhoea were key. Fever, while a known symptom of disease, was not an early feature of the disease in any age group.
"Men were more likely to report shortness of breath, fatigue, chills and fever, whereas women were more likely to report loss of smell, chest pain and a persistent cough."
Dr Claire Steves, lead scientist at Zoe Covid Study and reader at King’s College London, said: "It's important people know the earliest symptoms are wide-ranging and may look different for each member of a family or household.
"Testing guidance could be updated to enable cases to be picked up earlier, especially in the face of new variants which are highly transmissible.
This could include using widely available lateral flow tests for people with any of these non-core symptoms."
Overall, the research team examined 18 different symptoms associated with Covid-19, and the early signs included loss of smell, chest pain, persistent cough, abdominal pain, blisters on the feet, eye soreness and unusual muscle pain.
The academics also found fever was not an early feature of the disease in any age group - despite being a known Covid-19 symptom.
Dr Marc Modat, Senior Lecturer at KCL, said: "As part of our study, we have been able to identify that the profile of symptoms due to Covid-19 differs from one group to another.
"This suggests that the criteria to encourage people to get tested should be personalised using individuals' information such as age. Alternatively, a larger set of symptoms could be considered, so the different manifestations of the disease across different groups are taken into account."
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