Fifty to 70 per cent "children" below 19 years globally were susceptible to the SARS-CoV-2 infection by the end of 2021 and before the Omicron wave, showing a need for more effective vaccines and better vaccination coverage, according to a study.

It also found that worldwide seroprevalence estimates varied from 7.3 per cent in the first wave of the COVID-19 pandemic to 37.6 per cent in the fifth wave and 56.6 per cent in the sixth wave.

The highest seroprevalences in different pandemic waves were estimated for South-East Asia -- 17.9 to 81.8 per cent -- and the African region -- 17.2 to 66.1 per cent -- while the lowest seroprevalence was estimated for the Western Pacific region between 0.01 and 1.01 per cent, the study said.

Seroprevalence estimates were higher in children at older ages, in those living in underprivileged countries or regions, and in those of minority ethnic backgrounds, the study published in eClinicalMedicine said.

Seroprevalence was defined as the number of children who tested positive for specific anti-SARS-CoV-2 serum antibodies, using a well-designated threshold in the assay, divided by the total number of children tested, it said.

Scientists conducted a systematic review and meta-analysis in which they searched international and pre-printed scientific databases from December 1, 2019, to July 10, 2022, said the study, conducted to assess SARS-CoV-2 seroprevalence and determinants in children worldwide.

They searched databases such as PubMed, Embase, Web of Science Core Collection, and Scopus and preprint repositories such as MedRxiv, BioRxiv and SSRN for articles. The higher hospitalisation rates of those aged zero to 19 years, referred to as "children" by the study, observed since the emergence of the immune-evasive Omicron variant and sub-variants along with the persisting vaccination disparities, highlighted a need for in-depth knowledge of SARS-CoV-2 sero-epidemiology in children, scientists involved in the study said.

Pooled seroprevalences were estimated according to World Health Organization (WHO) regions using random-effects meta-analyses, the study said.

"Findings indicate that, by the end of 2021 and before the Omicron wave, around 50–70 per cent of children globally were still susceptible to SARS-CoV-2 infection, clearly emphasising the need for more effective vaccines and better vaccination coverage among children and adolescents, particularly in developing countries and minority ethnic groups," it said.

The study said associations with SARS-CoV-2 seroprevalence and sources of heterogeneity were investigated using sub-group and meta-regression analyses. The protocol used in this study has been registered in PROSPERO (CRD42022350833).

The study included 247 publications containing 302 datasets involving 7,57,075 children from 70 countries in six WHO-defined regions. Most of the datasets, 144, were from Europe, and the least, 14, were from the Western Pacific region, the study said.

The rapid transmission of the SARS-CoV-2 Omicron variant has led to unprecedented COVID-19 cases and hospitalisation, especially among children.

The lower risk of disease in children "pre-Omicron", combined with concerns about vaccine safety and parental hesitancy, led to limited vaccine uptake and slow implementation of vaccination in children globally, particularly for children under 12 years of age.

Vaccines for children were also largely inaccessible in many parts of the world that were lagging behind in the vaccination of adults, particularly in low and middle-income countries. To date, children remain the largest unvaccinated group worldwide, the study noted.

With the emergence of Omicron, an increasing number of children requiring critical care have been observed in developed countries such as the US, England and Germany, with sufficient vaccine supplies, it said.

There is evidence that available vaccines do not protect children adequately against SARS-CoV-2 Omicron infection and the waning of both natural infection and vaccine protection are well documented.

The scientists said against this background and the race for the development of a new generation of vaccines that protect against a wide range of variants, including new ones, "we aimed to provide estimates for the proportion of children with detectable antibodies and to identify sources of heterogeneity in SARS-CoV-2 seroprevalence in children for various regions of the world".