A study published in The Lancet highlights the high contagiousness of people infected with Covid-19 during the first week after the onset of symptoms, and argues for the isolation of cases at an early stage.
- Unlike SARS-CoV and MERS-CoV, cases of viral transmission of SARS-CoV-2 occur very early: as soon as symptoms appear and within 5 days.
- Asymptomatic people are also contagious during this period, even if their period of contagiousness is shorter.
- These new data, from a meta-analysis of 98 studies, therefore suggest the need to very quickly isolate people infected with Covid-19 to prevent the spread of the virus.
So far, the data available on SARS-CoV-2 indicated a period of infectivity of nine days.
A new study published in the scientific journal The Lancet Microbebased on the systematic review and meta-analysis of three human coronaviruses, gives more details: people infected with Covid-19 are likely to be highly contagious within five days of the onset of symptoms.
“This is the first systematic review and meta-analysis that has comprehensively examined and compared the viral load and shedding of these three human coronaviruses”, explains Dr. Muge Cevik of the University of St Andrews (England), main author of the work. According to him, SARS-CoV-2 spreads more efficiently than SARS-CoV and MERS-CoV (Middle East Respiratory Syndrome Coronavirus). It is also more difficult to contain.
The majority of cases of viral transmission occurring very early, and in particular within five days of the onset of symptoms, it is essential to isolate patients as quickly as possible, insist the researchers.
Highly contagious patients as soon as symptoms appear
To understand when patients are most likely to be infectious, the researchers sifted through the various factors involved: viral load (how the amount of virus in the body changes throughout infection), viral RNA shedding (how long a person exceeds viral genetic material, RNA, which does not does not necessarily indicate that a person is infectious, because it is not necessarily able to replicate), and isolation of live virus (a stronger indicator of a person’s infectivity, because the virus is isolated and tested to see if it can replicate successfully in the laboratory).
A total of 98 studies with at least five participants, cohort studies and randomized controlled trials were included in the meta-analysis. 79 focused on SARS-CoV-2, 73 of which involved hospitalized patients only. Eight concerned SARS-Cov and eleven concerned MERS-CoV.
From these studies, the authors calculated the average duration of viral RNA shedding and examined changes in viral load and the success of isolation of live virus from different samples collected throughout. of an infection.
Analysis of the results of SARS-CoV-2 studies showed that the average duration of viral RNA shedding in the upper respiratory tract, lower respiratory tract, feces and serum was 17 days, respectively. , 14.6 days, 17.2 days and 16.6 days. The longest duration of RNA shedding was also 83, 59, 35 and 60 days respectively.
Of the eleven studies that attempted to isolate live virus, eight succeeded in growing viable virus within the first week of illness. Of the studies that have also measured RNA viral load, these have demonstrated a link between the success of live virus isolation and viral load levels. In contrast, no study has successfully isolated live virus beyond the ninth day of symptoms in any specimen type, despite persistently high RNA viral loads.
“These results suggest that in clinical practice it may not be necessary to repeat PCR testing to consider a patient no longer infectious, as this could remain positive for much longer and does not necessarily indicate that he could transmit the virus to other people”insists Dr. Cevik, who believes that in patients who have developed mild symptoms, “the period of infectiousness could instead be counted as 10 days from the onset of symptoms”.
Asymptomatic people are also contagious
The highest viral load of SARS-CoV-2 RNA was detected at the time of symptom onset, or before the fifth day of symptoms. In contrast, viral loads of SARS-CoV and MERS-CoV peaked at 10-14 days and 7-10 days after symptom onset, respectively. This explains why the transmission of these viruses can be effectively reduced by the immediate identification, isolation and quarantine of people who show symptoms of the disease.
The review of the twelve studies on asymptomatic people infected with SARS-CoV-2 shows that the latter are also infectious at the start of the infection, even if they do not show symptoms. On the other hand, this period of contagiousness is shorter.
Despite the limitations of the study, the results obtained suggest the need to quickly detect and isolate people positive for Covid-19 in order to prevent them from contaminating those around them. “The majority of studies included in our review were carried out on patients who were admitted to hospital. Therefore, our findings may not apply to people with less severe infection, although these results suggest that people with less severe cases can clear the virus from their bodies more quickly”says Dr. Antonia Ho, lead author. “Furthermore, the increasing deployment of treatments, such as dexamethasone, remdesivir as well as other antivirals and immunomodulators in clinical trials are likely to influence viral shedding in hospitalized patients. Additional studies on shedding viral in this context are necessary”she concludes.
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