Reviewed by Dr. James “Brad” Cutrell, MD, UT Southwestern Medical Center
While early data from the COVID-19 pandemic demonstrated increased risk of household transmission compared to non-household settings, the estimated secondary attack rate was about 30% based on standard contact tracing methods. Grijalva and colleagues conducted detailed assessments in two US cities of 101 index COVID-19 positive patients and 191 of their household contacts to determine the rate of household transmissions, using daily symptom diaries and self-collected nasal (+/- saliva) samples for PCR testing. Within the first 7 days of testing, 102 of the 191 household contacts tested positive by SARS-CoV-2 PCR, for an overall secondary infection rate of 53% (95% CI, 46%-60%), with similar rates seen in both children and adult contacts. Approximately 75% of secondary infections occurred within the first 5 days of illness for the index patient, but 60% of infected household contacts were asymptomatic at the time that their infection was first detected by PCR. These data highlight the high rates of household secondary transmissions and emphasize the importance of immediate self-isolation, use of masking and other interventions early after symptom onset or high-risk exposures in order to prevent secondary spread throughout a household.
Grijalva CG, et al. Transmission of SARS-CoV-2 Infections in Households—Tennessee and Wisconsin, April -September 2020. MMWR 2020 Nov 6; 69 (44):1631-1634.