The high prevalence of asymptomatic SARS-CoV-2 infection reveals the silent spread of COVID-19

Abstract

Purpose: The COVID-19 pandemic has led to over 92 million cases and 1.9 million deaths worldwide since its outbreak. Public health responses have focused on identifying symptomatic individuals to halt spread. However, evidence is accruing that asymptomatic individuals are infectious and contributing to this global pandemic.

Methods: Observational data of 320 index cases and their 1289 positive contacts from the National COVID-19 Database in Bahrain were used to analyze symptoms, infectivity rate and PCR Cycle threshold (Ct) values.

Results: No significant difference (p = 1.0) in proportions of symptomatic (n = 160; 50.0%) and asymptomatic index cases (n = 160; 50.0%) were seen; however, SARS-CoV-2 positive contact cases were predominantly asymptomatic (n = 1127, 87.4%). Individuals aged 0-19 years constituted a larger proportion of positive contact cases (20.8%) than index cases (4.7%; p < 0.001). A total of 22% of the positive contacts were infected by symptomatic male index cases aged between 30-39 years. The total numbers of exposed contacts (p = 0.33), infected contacts (p = 0.81) and hence infectivity rate (p = 0.72) were not different between symptomatic and asymptomatic index cases. PCR Ct values were higher in asymptomatic compared to symptomatic index cases (p < 0.001), and higher in asymptomatic compared to symptomatic positive contacts (p < 0.001). No differences between the infectivity rates of index cases with Ct values <30 and values ≥30 were observed (p = 0.13).

Conclusion: These data reveal that the high asymptomatic incidence of SARS-CoV-2 infection in Bahrain and subsequent positive contacts from an index case were more likely to be asymptomatic, showing the high "silent" risk of transmission and need for comprehensive screening for each positive infection to help halt the ongoing pandemic.

Keywords: Asymptomatic; Bahrain; COVID-19; Cycle threshold; Public health; SARS-CoV-2; Symptomatic; Transmission.

Figure 1

Comparison of the proportions of index cases and positive contacts by age group. Proportions represent the total number of individuals in each sample that belong to each age group, as a proportion of the total sample size, and are plotted in percentages above each bar. The proportions were statistically compared using two-sample z-tests. Statistical significance is indicated above the error bars: p > 0.05 (ns), p < 0.05 (*), p < 0.01 (**), p < 0.001 (***), p < 0.0001 (****).

Figure 2

Number of individuals infected by index case gender, age group and clinical features. The total number of individuals infected represent the positive contacts, with the total count indicated by the number above the bar. The key representing the categories of data plotted is indicated on the top-right corner.

Figure 3

Analysis of infectivity rates by PCR Ct values. (a) PCR Ct values and corresponding infectivity rates for 282 index cases, with a fitted Lowess curve showing the association between these variables; (b) Average infectivity rates for index cases with Ct values <30 and ≥30, plotted as mean±95% confidence limits.