Covid-19 patients are at their most infectious one day before they develop symptoms, a mathematical study reveals.
Researchers used a computer model to process data on viral load — the amount of coronavirus a person is infected with — and how it decreases throughout infection.
Previous studies have found viral load aligns with infectivity and also increases the likelihood of death, meaning an infected person with a high amount of the virus in their system is more infectious and also at greater risk of dying from Covid-19.
The researchers found that the amount of virus in a person’s system takes longer to diminish in older patients, leading to a protracted fight against Covid-19.
Pictured, graphs showing the amount of virus at various points following the development of symptoms for someone who is under 65 without risk factors (left) and with a risk factor (right). The middle dotted line is the average, with the others showing upper and lower confidence level
These graphs show the viral load in over-65s with (right) and without (left) additional risk factors at various points following the development of symptoms. The middle dotted line is the average, with the others showing upper and lower confidence levels
Data from 655 patients admitted to French hospital in March 2020 was analysed by scientists, led by the University of Paris.
Of the patients in the study, 40 per cent received oxygen therapy, 144 were admitted to ICU and 78 died.
Complex mathematical models allowed researchers to retrospectively track a patient’s course of infection to understand how the virus behaves in the body.
Dynamics such as viral load, duration of viral shedding and mortality were all factored in to the equations.
The average time for symptoms to develop was calculated as seven days after infection.
‘We predict that peak viral load occurs one day before symptom onset, on average,’ the researchers write in their study, published in PNAS.
A graph showing predictions of how long it takes people under 65 (blue) and over 65 (red) to remove the virus from their system. The average (shown by the dotted lines) for younger people is 13 days and 16 days for people who are over 65
However, they add the time between catching the virus and first symptoms emerging is shorter for people with higher viral loads.
This suggests patients admitted early after symptom onset had higher viral load compared to patients arriving later on in the course of their infection.
The study also found that in patients aged 65 or older, 59 per cent of the cohort, their cells were less able to eradicate the virus compared to younger people.
As a result, it took their bodies 16 days after symptoms first began to flush the virus out of their system, on average. For younger patients this occurs in just 13 days.
The study also looked at how effective antiviral treatments would be in patients with high viral loads.
In the hypothetical situation that there was a drug which can stop either 90 per cent of the virus replication or 90 per cent of cell infection, the researchers say this would reduce the time it takes to clear out all the virus by two days for under-65s and by three days in over-65s.
‘Considering a treatment blocking 99 per cent of viral production, the effects would be further improved in patients aged under 65, with a median reduction in time to viral clearance of 5.4 days,’ the researchers write.
‘The effects of treatment would be more sensible in patients above the age of 65, with a mortality reduced from 6.4 to 5.0 per cent for those without additional risk factor and from 19 to 14 per cent in those having at least one other risk factor.’
