This is a summary, written by members of the CITF Secretariat, of:

Bergeri I, Whelan M, Ware H, Subissi L, Nardone A, Lewis HC, Li Z, Ma X, Valenciano M, Cheng B, Al Ariqi L, Rashidian A, Okeibunor J, Azim T, Wijesinghe P, Le LV, Vaughan A, Pebody R, Vicari A, Yan T, Yanes-Lane M, Cao C, Clifton DA, Cheng MP, Papenburg J, Buckeridge D, Bobrovitz N, Arora RK, Van Kerkhove MD, Unity Studies Collaborator Group. Global SARS-CoV-2 seroprevalence from January 2020 to April 2022: A systematic review and meta-analysis of standardized population-based studies. PLOS Medicine. 2022. DOI: 10.1371/journal.pmed.1004107

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A study published in PLOS Medicine, carried out by CITF-funded SeroTracker, in partnership with the World Health Organization Unity Studies Team, found global SARS-CoV-2 seroprevalence (due to infection and vaccination) was 59.2% by September 2021. Overall in 2021, the global seroprevalence of SARS-CoV-2 increased markedly due to infection in some regions and/or mass vaccination campaigns in others. After the emergence of the Omicron variant in March 2022, infection-acquired seroprevalence reached 47.9% and 33.7% in Europe and the Americas’ high-income countries, respectively.

In the third quarter of 2021, global seroprevalence estimates suggested there were 10 times more infections than the number reported, meaning many infections were still going undetected by national surveillance systems in a period when vaccines were not yet widely available.

Key findings:

  • In September 2021, combined seroprevalence from infection and vaccination globally was 59.2%; global seroprevalence from infections alone (excluding vaccination) was 35.9%.
  • Global combined seroprevalence increased in 2021 due to infection, in Africa, for example, from 26.6% to 86.7% in December 2021. It also increased due to vaccination, as in European high-income countries (from 9.6% in June 2020 to 95.9% in December 2021).
  • The ratio of combined seroprevalence to reported cases, globally, was 10.5 in July-September 2021. By region, this ranged from twice as many infections as reported in the Americas and European high-income countries, to over 100 times as many infections as reported in African low- and middle-income countries.
  • The median asymptomatic combined seroprevalence was similar between males and females (64.6% vs 58.6%).
  • Children under 9 and adults over 60 were at lower risk of being seropositive than adults aged 20 to 29.
  • Rigorous public health and social measures were found to be associated with lower seroprevalence, based on a modelling using pre-vaccination data.

A total of 52% of WHO member states (100/194) and four countries, areas and territories, across all six WHO regions, were represented in this study.

To determine the global seroprevalence of SARS-CoV-2, a systematic review and meta-analysis was conducted. A total of 513 full texts, comprising 965 unique studies identified in a search from January 1, 2020 to May 20, 2022, were included in the analysis for this manuscript. This included published studies, pre-prints, and other publications from WHO Unity study collaborators.

These findings provide a detailed picture of global seroprevalence, demonstrating stark differences between regions, as well as an in-depth breakdown by age group. Seroprevalence data that is of high quality and standardized (such as the WHO Unity studies) continue to be essential to inform health policy decision-making around COVID-19 control measures, particularly in resource limited regions with low vaccination rates.

See summaries of earlier preprinted data reported by this research team, covering January 2020 to October 2021, here, and January 2020 to December 2021, here.

Explore the SeroTracker dashboard here.