They performed a meta-analysis of 54 studies looking at the household secondary attack rate of SARS-CoV-2.
According to the CDC, the secondary attack rate is the number of new cases among contacts divided by the total number of contacts.
The researchers confirmed that SARS-CoV-2 is more contagious than other coronaviruses, with a secondary attack rate of 16.6% (95% CI 14.0%-19.3%) compared to 7.5% (95%CI 4.8%-10.7%) for SARS-CoV and 4.7% (95%CI, 0.9%-10.7%) for MERS-CoV.
Their findings also confirmed the attack rate is higher to adult contacts compared to child contacts and to spouses compared to other family members.
The secondary attack rate for symptomatic index cases was 18.0% (95% CI 14.2%-22.1%), and the rate of asymptomatic and presymptomatic index cases was 0.7% (95% CI 0%-4.9%), “although there were few studies in the latter group.”
The asymptomatic/ presymptomatic secondary attack rate is not statistically different from zero, and the confidence interval is technically 0.7 ± 4.2, resulting in a range of -3.5%-4.9%, but attack rates cannot be negative, so it is truncated at 0.