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]. Therefore, better understanding of SARS-CoV-2 RNA loads in children, and the prevalence of children among those with SARS-CoV-2 infection remains critical to inform infection control measures. Here, we focused our analysis on SARS-CoV-2 prevalence and RNA loads in children and adults detected in Da Nang city, central Vietnam between July 1, 2021 and September 30, 2021.The present study formed part of the national COVID-19 outbreak response. Accordingly, obtaining inform consent from the study participants were deemed unnecessary.
Central to the COVID-19 outbreak response in Da Nang city has been a cost-effective sample pooling strategy for mass screening of SARS-CoV-2 [
]. Accordingly, SARS-CoV-2 testing was performed on any contacts of a confirmed case, regardless of clinical status, and on all those living in areas of the city with ongoing community transmission or arriving in Da Nang city from other provinces/cities with on-going community transmission.
We extracted available demographic information and data on Ct values of SARS-CoV-2 RT-PCR obtained from the aforementioned community screening. To identify the responsible SARS-CoV-2 variant, a convenient sample of nasopharyngeal swabs with sufficient volume and viral loads were whole-genome sequenced using ARTIC3 protocol as previously described [
During the study period, Da Nang city experienced an outbreak of COVID-19 with 4,590 PCR-confirmed cases detected after testing over 3.3 million people. The majority of cases were detected in August and early September 2021 (Figure 1). Information about demographic and SARS-CoV-2 RT-PCR Ct values was available from 3040 individuals, accounting for 66.2% of the reported cases. The 3040 study participants included 1408 (46.3%) males and 1632 (53.7%) females, and were aged from 1 to 97 years (median: 36 years). 629 were children (median in years: 11, range, 1 to 17 years), accounting for 20.7% of the total PCR-confirmed cases included for analysis.
The SARS-CoV-2 RT-PCR Ct values of the whole group ranged between 10 and 39 (median: 20). Compared with adults, children had higher Ct values (i.e. lower RNA loads): median (range): 20 (13-39) vs. 19 (10-39) (p<0.001) (Figure 1A). Among adults but not children, females had higher RNA loads than males: median Ct value (range): 19 (11-39) vs. 20 (10-39) (p<0.001) (Figure 2B). Among children, lower Ct values (i.e. higher RNA loads) were detected among those between 0-4 and 12-17 years old than those between 5-11 years old: median (range): 20 (13-37), 21 (14-39) and 19.5 (14-37) respectively (p<0.003) (Figure 2C).
A total of 64 SARS-CoV-2 whole-genome sequences were obtained, including 16 (25%) from children and 48 (75%) from adults. All were assigned to the SARS-CoV-2 Delta variant (sub-lineage AY57) by Pangolin, and lineage 21I by Nextstrain (Supplementary Figure 1). The lineage 21I, including sub-lineage AY57, is one of the three reported clades of the Delta variant. Details about the genetic diversity of SARS-CoV-2 Delta variant detected across Vietnam will be described in a separate report.
Here, we showed that children accounted for a substantial proportion of PCR confirmed cases of SARS-CoV-2 infection detected through mass screening in Da Nang city, Vietnam. Consistent with previous reports [
], we showed that children carried lower RNA loads than adults did. Yet, 50% of the infected children across age groups had a Ct value of ≤21 (i.e. high RNA loads). Although we did not perform virus culture to demonstrate the presence of live virus in PCR positive samples, previous studies showed that Ct values <24 were highly predictive of virus culture positivity (i.e. infectiousness) [
]. These data suggested that children might play an important role in the transmission of SAR-CoV-2. Because the current COVID-19 vaccination programme does not cover children below 5 years old, the rapid vaccine deployment worldwide may make the role of young children in the transmission of SARS-CoV-2 more important in the near future. The observed higher RNA loads in female adults than in male adults merits further research, in particular whether high viral burden is associated with more severe disease.
Our sequencing results have expanded the geographic distribution of the SARS-CoV-2 variant 21I, one of the three reported clades of the Delta variant. SARS-CoV-2 Delta variant accounted for 100% of the SARS-CoV-2 whole-genome sequences from Vietnam deposited to GISIAD since June 2021 (gisaid.org). Therefore, the demographic and virological features described here might be extrapolated for SARS-CoV-2 Delta variant infection in Vietnamese people.
The strength of our study includes that we included the majority of cases tested positive through mass screening for analysis regardless of the clinical presentations. As such, our data more accurately reflect the RNA loads of SARS-CoV-2 in children, whether or not they develop symptoms. Indeed, the proportion of children among SARS-CoV-2 PCR confirmed cases in the present study was comparable with that from a recent community-based study in the USA [
]. However, since we did not follow up the participants during quarantine/hospitalization, we were not able to assess the kinetics of RNA loads over the course of the infection. A previous study showed children had a faster SARS-CoV-2 clearance than adults [
]. Additionally, we were not able to compare the RNA loads between asymptomatic and symptomatic infections in children, although a previous report demonstrated that these two groups had comparable RNA loads [
In summary, we report that children accounted for 20.7% of RT-PCR confirmed cases of SARS-CoV-2 infection detected during a community outbreak in Da Nang City, Vietnam. High RNA loads (i.e. Ct values of ≤21) were recorded in half of the infected children across age groups. The contribution of children, especially those not eligible for the current COVID-19 vaccination programme, to the transmission dynamics of SARS-CoV-2 requires further research.
The contact tracing and PCR testing of community cases were funded by Da Nang People's Committee, Da Nang, Vietnam. The genomic surveillance work was funded by Wellcome (222574/Z/21/Z). LVT and GT are supported by the Wellcome Trust of Great Britain (204904/Z/16/Z and 106680/B/14/Z, respectively).