Substantial coronavirus spread seen before symptoms show up

Caution: Infectiousness begins at least two days before exhibiting symptoms.   | Photo Credit: M_VEDHAN

In a study published recently, researchers found that people infected with novel coronavirus (SARS-CoV-2) start shedding the virus and therefore transmitting it to others before symptoms show up. The infectiousness begins at least two days before the person exhibits symptoms. The study says that substantial proportion of transmission probably occurs even before first symptoms appear in the infected person.


The work was done by a team led by Gabriel M. Leung from the School of Public Health at the University of Hong Kong and the results were published in the journal Nature Medicine.

Studying 77 infector-infectee pairs — pairs of people who passed on the infection and those who got infected — from publicly available sources, the researchers estimate that 44% of secondary cases were infected when the person was yet to show symptoms.

The researchers used information on the timing of symptom onsets in transmission pairs to infer the infectiousness profile of people with COVID-19 and found “substantial transmission potential” before symptom onset. Since most cases were isolated soon after they showed symptoms, the possibility of virus spread after symptoms showed up was prevented.

Tracing contacts

The study suggest that efforts to track down contacts of people with coronavirus infection should include people who have come in contact with the infected person not just after symptoms show up but at least two-three days before the person exhibited symptoms.


In the case of the 2003 SARS, infectiousness increased seven-10 days after symptom onset. Hence, virus spread can be reduced once the infected person is quarantined soon after symptoms show up. In contrast, influenza is characterised by increased infectiousness shortly around or even before symptom onset. This makes it difficult to contain the spread by isolating people who have come in contact with the infected person after the person exhibited symptoms.

Two clusters

According to a preprint posted on MedRxiv server on March 8, the pre-symptomatic transmission was high in two clusters — Tianjin in China which is a shopping mall cluster consisting of 45 cases, and Grace Assembly of God cluster in Singapore consisting of 25 cases. In the case of the Singapore cluster the pre-symptomatic transmission was 48% , while it was 62% for the Tianjin cluster.

Explaining the high prevalence of pre-symptomatic transmission in these two clusters, Prof. Leung writes in Nature Medicine: “Places with active case finding would tend to have a higher proportion of presymptomatic transmission, mainly due to quick quarantine of close contacts and isolation, thus reducing the probability of secondary spread later on in the course of illness.” According to the authors, where immediate contact tracing and quarantine is not possible, a lower proportion of pre-symptomatic transmission will be seen.

Viral loads

The researchers collected samples (nose swabs) from 94 coronavirus patients who were admitted to the Guangzhou Eighth People’s Hospital in China from January 21 to February 14. They detected high viral loads soon after symptoms showed up, which then declined by day 21. They say based on analysis the viral shedding may begin two-three days prior to symptoms appearance and then decrease gradually.

When one person can infect 2.5 people, contact tracing and quarantine alone are “less likely to be successful” if more than 30% of transmission occurred before symptom onset, they write. But contact tracing can be still effective in preventing the spread if people who came in contact with the infected person two-three days prior to symptom onset are also located and quarantined, as was done in Hong Kong and mainland China since late February.

They caution that even when mitigation measures such as lockdown are in place, contact tracing would still has an important role to play, particularly in the case of super-spreading events that may occur in high-risk settings including nursing homes or hospitals.

Since the proportion of pre-symptomatic transmission is substantial, maintaining hand hygiene and physical distancing can play an important role in containing virus-spread in the community.

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Printable version | Jan 23, 2021 8:26:43 AM |

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