Dr. Sudhir Mehta refers to a recent study from JAMA highlighting COVID-19 patients may be most contagious soon after infection but a week later is a relatively safe period for contact.
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In a study of 100 index cases, no secondary transmission was observed among close contacts whose exposure started 6 days or more after cases developed symptoms.
Prior reports have described transmission of SARS-CoV-2 from asymptomatic or presymptomatic individuals (NEJM JW Infect Dis Mar 2020 and N Engl J Med 2020; 382:970). Now, to determine how soon after SARS-CoV-2 infection the risk for transmission declines, investigators studied 100 COVID-19 index patients in Taiwan and 2761 of their close contacts.
Close contacts were defined as persons having 15 minutes of face-to-face contact with a case (in nonhealthcare settings) or persons having exposure within 2 meters of a case without appropriate personal protection equipment (in healthcare settings). Among close contacts, 5% were household members, 3% were nonhousehold family members, 25% were healthcare providers, and 67% were other contacts.
A total of 0.8% of close contacts developed COVID-19. The rate of secondary transmission was 4.6% for household members, 5.3% for nonhousehold family members, 0.9% for healthcare providers, and 0.1% for other contacts. The secondary clinical attack rate was 1.0% among 735 contacts whose initial exposure occurred before symptom onset in the index cases. None of the 852 contacts exposed 6 days or more from symptom onset developed COVID-19. No cases were documented from nine asymptomatic index cases.
Dr. Sudhir Mehta's comment
This study is a symptom-based assessment for all contacts except high-risk ones. The household contact occurred early and was more intense than contact in other settings. However, the attack rate among household contacts was surprisingly low. The finding of no cases of transmission occurring at 6 days or more from symptom onset is consistent with data showing decreasing viral loads as the illness progresses (NEJM JW Infect Dis Jun 2020 and Nat Med 2020; 26:672).
The authors concluded that merely isolating symptomatic patients will be insufficient to control COVID-19 without other interventions such as social distancing, mask-wearing etc. Indirectly, this study also suggests that isolation & quarantine period may be shortened and period of 10 days from symptom onset may be good enough.
Reference: Daniel Kaul, MD reviewing Cheng H-Y et al. COVID-19 Patients May Be Most Contagious Soon After Infection. JAMA Intern Med 2020 May 1
Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.
The author, Dr. Sudhir Mehta is a Haematology expert.