During the pandemic regular serosurveys, testing for antibodies to the virus (SARS-CoV-2) that causes COVID-19, have been used to track the proportion of people infected. Today, results from two complementary national serosurveys, one in children and adolescents, and the other in adults, have been released. Taken together, they provide a unique snapshot of the extent of SARS-CoV-2 infection across Australia to the end of August 2022.
The children and adolescents survey found at least 64% of 0–19 year olds in Australia had been infected with SARS-CoV-2. In unvaccinated children aged 1–4 years, approximately 8 out of 10 had evidence of past infection. The adults survey, the third in a series of surveys in blood donors, found at least two thirds of the adult population had been infected with SARS-CoV-2, with approximately 20% infected within 3 months of the previous survey in June 2022.
National paediatric serosurvey
The child and adolescent study, led by the Paediatric Active Enhanced Diseases Surveillance network (PAEDS) and the National Centre for Immunisation Research and Surveillance (NCIRS), in collaboration with Victorian Infectious Diseases Reference Laboratory, tested antibodies to SARS-CoV-2 in 2,046 blood samples collected between June and August 2022. Two types of antibodies were tested: antibody to the nucleocapsid protein of the virus, which indicates past infection, and antibody to the spike protein, which can indicate past infection and/or vaccination. In children aged 1–4 years, all of whom were unvaccinated, the proportion with spike antibody was 79%, indicating past infection in approximately 8 out of 10 pre-schoolers.
For children aged 5 years and over, many of whom were vaccinated, nucleocapsid antibody was a marker indicating past infection. At least 67% of children aged 5–11 years and 70% of 12–19 year olds had nucleocapsid antibodies. Antibody positivity was similar between females and males, and between Aboriginal and Torres Strait Islander and non-Indigenous children. The proportion of children with an underlying medical condition who had antibodies suggesting infection (59%) was lower than those with no medical conditions (67%).
Dr Archana Koirala, infectious disease specialist at NCIRS and paediatric study lead, said “Overall, this study tells us that at least two thirds of children in Australia have had COVID-19. This is more than double the number of cases reported based on nose and throat swab testing for the virus.”
“This is expected, since many children have either mild or no symptoms and are therefore not tested for the virus. By looking at both vaccination status and history of infection in our study we found that 4 out of 10 children with no reported history of COVID-19 in fact were positive for antibodies, indicating infection.”
Dr Koirala also noted that “vaccination reduces the risk of developing severe disease if infected with the virus and should continue to be taken up according to current national recommendations that have been informed by these data.”
This important study used novel opportunistic, consent-based blood sampling from children and adolescents undergoing an anaesthetic procedure at 1 of 8 paediatric hospitals across Australia that form part of the PAEDS sentinel surveillance network. The generous participation from parents/carers and their children underpinned the success of this study. One participant commented: "It was great to take part, my child wanted to support any COVID research to help get an understanding of the spread of the virus in kids in Australia.”
National adult survey
The third round of the national survey of SARS-CoV-2 antibodies in adult blood donors found that at least 65% of adults in Australia had been infected by the virus that causes COVID-19 by the end of August 2022, similar to the proportion found in children and adolescents. This proportion represented a jump of about 20% since the previous survey 3 months ago, showing that at least a fifth of the population had been exposed to the virus in that time period.
The adult serosurveys are being conducted by the Kirby Institute and NCIRS in collaboration with Australian Red Cross Lifeblood and other research partners, at 3-monthly intervals using residual blood donations. Two previous rounds of the survey have been completed to date in 2022. The latest report presents results from round 3, in which 5,005 de-identified residual blood donation specimens from all Australian states and territories received between 23 August and 2 September were tested for the presence of antibodies to SARS-CoV-2. Overall, the highest prevalence of antibodies was in the 18–29 year old age group, at 80%, and decreased steadily with age to 42% in those over 70 years. Prevalence of antibodies was similar across all states and territories.
“These results provide further quantification of the size of the BA4/5 Omicron wave during the winter months in Australia,” said Professor John Kaldor from the Kirby Institute. “Serosurveys continue to provide crucial information to support our efforts to understand the spread of SARS-CoV-2, and the impact of vaccination and infection on antibody levels in the community,” he added.
Image by NSW Health Pathology.