The Relationship Between the Time of Start of Isolation at Facilities and Prevention of Household Transmission in COVID-19 Patients

The Japanese government recommends isolation at dedicated facilities for prevention of household transmission of COVID-19, however, the relation between the time of start of isolation at facilities and the household secondary attack incidence rate is still unknown. Since COVID-19 is contagious from 2 days prior to symptom onset and the viral load is the maximal just after the onset of symptoms, we thought that household infection could have already occurred even if the patient is isolated at a facility soon after the diagnosis. Therefore, we examined the relationship between the period from 2 days before symptom onset and the start of isolation at a facility (the infection exposure period) and the household secondary attack incidence rate among 1,075 index cases in households who were under isolation at facilities. We defined transmissions in households as cases in which household secondary transmission occurred to more than one person, and the household secondary attack incidence rate as the value obtained by dividing the number of households in which secondary transmission occurred by the total number of households. Evaluation of the household secondary attack incidence rate for each infection exposure period revealed that household secondary transmission did not occur in cases with an infection exposure period of 3 days, and the household secondary incidence rate was 22.6% (95% CI, 16.9%-29.5%) for an infection exposure period of 7 days, and 45.5% (95% CI, 21.3%-72.0%) for infection exposure periods of 12 days or more (regression analysis: y=0.037x-0.047;R2=0.863;p < 0.01). These findings suggest that the household secondary attack incidence rate increased as the infection exposure period increased, and that the household secondary attack incidence rate could be reduced to half (from 45.5% for an infection exposure period of 12 days to 22.6% for a period of 7 days) if we could start isolation so as to keep the infection exposure period at 7 days or less (5 days or less after the onset).