Substantial coronavirus spread seen before symptoms show up 

#GS2 #Health #Pandemic 

Within 77 infector-infectee pairs, 44% of secondary cases had been infected when the person was yet to show symptoms 

  • 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 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. 
  • 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 

  • 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. 
  • 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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