Differential COVID-19 infection rates in children, adults, and elderly: evidence from 38 pre-vaccination national seroprevalence studies

ABSTRACT Background COVID-19 exhibits a steep age gradient of infection fatality rate. There has been debate about whether extra protection of elderly and other vulnerable individuals (precision shielding) is feasible, and, if so, to what extent. Methods We used systematically retrieved data from national seroprevalence studies conducted in the pre-vaccination era. Studies were identified through SeroTracker and PubMed searches (last update May 17, 2022). Studies were eligible if they targeted representative general populations without high risk of bias. Seroprevalence estimates were noted for children, non-elderly adults, and elderly adults, using cut-offs of 20, and 60 years (or as close to these ages, if they were not available). Results Thirty-eight national seroprevalence studies from 36 different countries were included in the analysis. 26/38 also included pediatric populations. 25/38 studies were from high-income countries. The median ratio of seroprevalence in the elderly versus non-elderly adults (or non-elderly in general, if pediatric and adult population data were not offered separately) was 0.90-0.95 in different analyses with large variability across studies. In 5 studies (all of them in high-income countries), there was significant protection of the elderly with ratio <0.40. The median was 0.83 in high-income countries and 1.02 in other countries. The median ratio of seroprevalence in children versus adults was 0.89 and only one study showed a significant ratio of <0.40. Conclusion Precision shielding of elderly community-dwelling populations before the availability of vaccines was feasible in some high-income countries, but most countries failed to achieve any substantial focused protection of this age group. summary 38 COVID-19 nationally representative seroprevalence studies conducted before vaccination campaigns were systematically identified. Median seroprevalence ratio in elderly versus non-elderly adults was 0.90-0.95, indicating no generally achieved precision shielding of elderly. In 5 studies, substantial protection (ratio <0.40) was observed.