Prevalence of Asymptomatic SARS-CoV-2 Infection in Japan

Area of Science:

• Epidemiology
• Infectious Diseases
• Public Health

Background:

• Understanding the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in Japan is crucial for public health strategies.
• Real-world evidence is needed to assess transmission dynamics within the Japanese population.

Purpose of the Study:

• To determine the prevalence of SARS-CoV-2 infection in asymptomatic individuals in Japan through sentinel screening.
• To complement findings with data from symptomatic patients via active epidemiologic surveillance.

Main Methods:

• A cross-sectional study involving polymerase chain reaction (PCR) testing of approximately 1 million asymptomatic individuals across 14 Japanese prefectures from February to December 2021.
• Saliva samples were used with a cycle threshold (Ct) value of approximately 40, consistent with Japanese standards.
• Data on symptomatic patients were obtained from the Ministry of Health, Labour, and Welfare for complementary analysis.

Main Results:

• The positive rate for SARS-CoV-2 in asymptomatic individuals was 0.03% during non-surge periods and peaked at 0.33% during surges, using a Ct value of approximately 40.
• This positive rate would be significantly lower (10-fold less) if a Ct value of 25, common internationally, were applied.
• Increased PCR positivity with Ct values of 25 or 30 preceded infection surges and identified hotspots of asymptomatic infections.

Conclusions:

• Sentinel surveillance in Japan revealed a low prevalence of asymptomatic SARS-CoV-2 infection in the general population in 2021 compared to global reports.
• The study provides valuable real-world data on the infection status in Japan.
• The findings highlight the importance of considering testing methodologies, such as Ct value thresholds, when comparing prevalence data across different regions.