Evidence supports the concept of transmission of SARS-CoV-2 while presymptomatic and asymptomatic, according to a rapid literature review conducted by the Centers for Disease Control and Prevention (CDC), published in Emerging Infectious Diseases.
Current diagnosis by reverse transcription polymerase chain reaction (RT-PCR) can infer infectiousness by cycle thresholds (Ct), lower values indicating higher viral loads and imply higher infectiousness. Four reports document low Ct values from patients who never developed symptoms, providing plausible evidence for transmission in asymptomatic patients.
Inherent confounders in early pre- or asymptomatic transmission reports from China was the possibility of community exposure, when transmission in the community may have been undetected.
Two studies that used models to estimate time between symptom onset in a primary and secondary patient (serial interval) estimated the serial interval to be four days, shorter than the estimated median incubation period of five days. One study suggested up to 13 per cent may be transmitted during the presymptomatic stage. Models attempting to estimate the number of infections caused by asymptomatic or presymptomatic individuals range from half to 80 per cent. Thus a large number of these patients contribute to ongoing community transmission.
Critical knowledge gaps exist on the relative incidence of asymptomatic carriers and whether asymptomatic SARS-CoV-2 infection confers protective immunity.