True extent of SARS-CoV-2 Infection through seroprevalence studies

The WHO UNITY Studies Collaborator Group has published the largest ever systematic review and meta-analysis of standardized SARS-CoV-2 seroprevalence studies, showing that true infections of COVID-19 around the world far exceed reported cases.

Since the 2009 influenza A(H1N1) pandemic, WHO and partners have supported Member States to implement standardized sero-epidemiological investigations to facilitate international comparisons, address knowledge gaps and inform national public health measures, especially in low-and-middle income countries (LMICs). Key initiatives, including the Consortium for the Standardization of Influenza Seroepidemiology (CONSISE) and WHO’s Influenza Pandemic Special Investigations and Studies (IPSS), have had a crucial role in enhancing global pandemic preparedness against emerging respiratory threats, such as the Middle East Respiratory Syndrome coronavirus.

Most recently, WHO and partners have developed the UNITY Studies, a set of ten standardized protocols for early sero-epidemiological investigations that can be adapted to any resource setting to collect robust data on the COVID-19 pandemic and inform an evidence-based response.

In December 2021, the UNITY Studies and collaborators published a pre-print of the largest ever global meta-analysis of SARS-CoV-2 seroprevalence studies. It summarizes the results of studies aligned with the UNITY Studies‘ standard “Population-based age-stratified sero-epidemiological (seroprevalence) investigation protocol” from January 2020 to October 2021. In total, the meta-analysis included data from 92 countries, including 53 LMICs and nearly half of the countries covered by the Global Humanitarian Response Plan for COVID-19.

Key findings from the meta-analysis include:

  • About 1 in 4 (26%) people around the world had SARS-CoV-2 antibodies in April 2021 (due to infection or vaccination), suggesting that most of the global population remained susceptible to infection at that time.
  • Seroprevalence differed by region: the proportion of those with SARS-CoV-2 antibodies varied from as low as 0.3% in WHO’s Western Pacific Region to as high as 57% in high-income countries in the Americas.
  • Seroprevalence differed by age: children aged under 9 years and adults aged over 60 years were less likely to be seropositive than adults aged 20–29 years.
  • COVID-19 surveillance and reporting largely under-estimates the true extent of infection and immunity, especially in LMICs, with an estimated seroprevalence to confirmed case ratio of 34:1 in the third quarter of 2020.

    Seroprevalence data that are robust, representative and standardized are essential to inform an evidence-based COVID-19 response. The UNITY Studies enable the provision of such data. Moreover, because they can be applied in any resource setting, they offer a critical tool for ensuring research equity in pandemic preparedness and response. Seroprevalence studies continue to be essential to inform public health and social measures, policy decisions, and overall pandemic preparedness strategies, particularly in resource limited settings.

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