It is unknown what proportion of children have been exposed to SARS-CoV-2 and how many have antibodies among children seeking medical care for non-Covid-19 related conditions.

The aim of this study is to identify children with IgG antibodies to SARS-CoV-2 who have not been previously diagnosed and are presumed/confirmed Covid-19 negative, then determine the level of immunity in this population which could inform further decisions about covid-19 vaccine strategies for children

Enrolled participants will have a survey about demographic information, history of previous suspected/confirmed Covid-19 infection or exposed to SARS-CoV-2, Covid-19 vaccination status. For participants aged less than 12 months, participants' mothers' status of Covid-19 vaccination and/or previous infection will be recorded. Participants will then complete serologic testing for SARS-CoV-2 IgG antibodies using residual plasma specimens collected/stored for routine laboratory tests on the date of enrollment. The study will characterize the prevalence of antibody positivity in this population and assess whether there is a correlation between demographic characteristics and antibodies levels.

Inclusion Criteria:

• Age ≤ 18 years old.
• Seeking medical care at Vietnam National Children's Hospital for any condition not related to acute Covid-19
• Have not been previously diagnosed and are presumed/confirmed COVID-19 negative
• Have blood specimens collected for any routine biochemistry test on the date enrollment

Exclusion Criteria:

• Refusal to participate in this study.

In-patient or out-patient children aged from 0 to 18 years old, who have not have not been previously diagnosed and are presumed/confirmed COVID-19 negative, have blood specimens collected for routine biochemistry tests.