BACKGROUND AND PURPOSE:

• Li et al. (Lancet Infectious Diseases, 2021) examined household transmissibility of SARS-CoV-2 as well as risk factors associated with infectivity and susceptibility to infection

METHODS:

• Retrospective cohort study
• Setting
  • Wuhan, China
  • Cases confirmed between Dec 2, 2019, and April 18, 2020
• Participants
  • All individuals positive for SARS-CoV-2 (lab or clinical tests) and their households
  • Asymptomatic and symptomatic
• Study design
  • Household definition: Groups of family members who did not necessarily live at the same address
    • Households were considered epidemiologically linked if they shared common contacts
  •  A statistical transmission model was used to estimate household secondary attack rates and to quantify risk factors associated with infectivity and susceptibility to infection
    • The model accounted for individual-level exposure history
  • The effect of intervention policies on household reproductive number was also assessed
  • Reproductive number was defined as the mean number of household contacts a case can infect

RESULTS:

• 27,101 households
  • Primary cases: 29,578
  • Household contacts: 57,581
• Estimated secondary attack rate: 15.6% (95% CI, 15.2 to 16.0%)
  • This assumed a mean incubation period of 5 days and a max infectious period of 22 days
• Individuals aged ≥60 years were at a higher risk of infection than all other age groups
• Infants (≤1 year) were more likely to be infected than
  • Children aged 2 to 5 years: odds ratio (OR) 2.20 (95% CI, 1.40 to 3.44)
  • Children aged 6 to 12 years: OR 1.53 (95% CI, 1.01 to 2.34)
• Given the same exposure time, children and adolescents <20 years were more likely to infect others than were adults ≥60 years
  • OR 1.58 (95% CI, 1.28 to 1.95)
• Asymptomatic individuals were much less likely to infect others than were symptomatic individuals
  • OR 0.21 (95% CI, 0.14 to 0.31)
• Symptomatic cases were more likely to infect others before their symptoms started than they were after symptom onset
  • OR 1.42 (95% CI, 1.30 to 1.55)
• After mass isolation of cases, quarantine of household contacts, and restriction of movement policies were implemented, household reproductive numbers declined
  • Primary cases: 52% decline
    • From OR 0.25 (95% CI, 0.24 to 0.26) to 0.12 (95% CI, 0.10 to 0.13)
  • Secondary cases: 63% decline
    • From OR 0.17 (95% CI, 0.16 to 0.18) to 0.063 (95% CI, 0.057 to 0.070)

CONCLUSION:

• Within households
  • Children were more likely to infect others than were older individuals, but children were also less susceptible to infection
  • Presymptomatic cases were more infectious
  • Asymptomatic cases were less infectious
• The authors conclude

Although children and adolescents were much less likely to have severe disease, they were as likely as adults to develop symptoms

We confirmed the high infectiousness of cases during the incubation period and found asymptomatically infected individuals were about 80% less infectious than symptomatic cases

Learn More – Primary Sources:

Household transmission of SARS-CoV-2 and risk factors for susceptibility and infectivity in Wuhan: a retrospective observational study

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