Is SARS-CoV-2 transmitted by asymptomatic and pre-symptomatic infected individuals?
Asymptomatic and pre-symptomatic transmission of SARS-CoV-2 may occur.
• Manifestations of COVID-19 are highly varied and may include asymptomatic cases, who do not manifest with any
signs and symptoms despite testing positive for COVID-19 by viral nucleic acid tests. Pre-symptomatic cases are
infected individuals who are still in their incubation period, hence do not exhibit any symptoms yet but eventually
• As of June 2020, only 586 (2.8%) of the 20,990 active cases in the Philippines were classified as asymptomatic,
but it is unclear whether cases are pre-symptomatic or carriers (true asymptomatic).
• Based on 36 observational studies (case reports, case series, cross-sectional and cohort studies) and 9 statistical
modeling analysis, asymptomatic and pre-symptomatic transmission of SARS-CoV-2 may occur. However, 3
studies reported no transmission from pre-symptomatic and asymptomatic cases.
• Studies on viral load comparing symptomatic cases with pre-symptomatic and asymptomatic cases reported
contradicting results. The duration of viral shedding was significantly longer for symptomatic patients compared
to asymptomatic patients but similar for asymptomatic and pre-symptomatic patients.
• Therewas no difference in the transmission rates of symptomatic and asymptomatic cases. However,the estimated
infectivity and probability of transmission was higherfor symptomatic cases compared to asymptomatic cases, but
results were imprecise due to a wide confidence interval.
• The World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) recognize the
possibility of pre-symptomatic and asymptomatic transmission. According to WHO, current evidence suggests
asymptomatic cases are less likely to transmit the virus than symptomatic cases.