Details of SARS-CoV-2 seroprevalence surveys supported by the US CDC

In a recent study posted to the medRxiv* preprint server, researchers reviewed and analyzed data from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serosurveys supported by the United States (US) Centers for Disease Prevention and Control (CDC) in the low- and middle-income (LMI) countries.

Background

Seroprevalence surveys assess the prevalence of pathogen-specific antibodies in the serum. Accordingly, they have been used to assess the prevalence of the human immunodeficiency virus (HIV), hepatitis B, and C viruses to inform the public health models for disease control and eradication.

SARS-CoV-2 seroprevalence survey data could complement case-surveillance data and also provide critical information to guide public health strategies. It could indicate the actual burden of coronavirus disease 2019 (COVID-19), showcase the population immunity for a region, identify transmission hotspots and at-risk groups, and help assess the vaccination coverage.

Only 23% of seroprevalence studies conducted during 2020 were from LMI countries. Therefore, the CDC and other international players, including the World Health Organization (WHO), offered support for seroprevalence studies in the LMI countries in the mid-2020.

About the study

The US CDC supported 72 international serosurveys between May 12, 2020, and February 28, 2022, in 35 LMI countries. Via its offices in the LMI countries, the CDC provided technical assistance (TA) on statistical, epidemiologic, laboratory methods, and financial assistance (FA) for these serosurveys. They also provided training for sampling and laboratory techniques, local staff, protocol development, data analysis, interpretation, and dissemination of the results. Additionally, the CDC encouraged countries to use the WHO UNITY protocol to ensure some degree of standardization wherever possible.

The US CDC activated its emergency operations center in response to the COVID-19 pandemic on January 20, 2020, and created an international task force to handle its incident management system. Comprised mainly of the CDC staff, this workforce monitored the CDC’s role in the global response to implementation and monitoring of SARS-CoV-2 international seroprevalence studies and received funds from the Coronavirus Aid, Relief, and Economic Security (CARES) Act.

A virtual monthly group call facilitated communication between the CDC headquarters and country office staff engaged in seroprevalence work. A Microsoft Excel-based tracker for CDC-supported international seroprevalence surveys helped the project staff update the status of protocol development and implementation for each survey. The authors used this data to review and analyze information for the current paper.

The current review describes all the CDC-supported serosurveys with their goals and characteristics. The analysis covers the WHO region of the survey, study design and population surveyed, type of assistance provided by the CDC, viz., TA, FA, or both, laboratory testing used, survey implementation, and result dissemination status. The researchers also discuss the challenges faced during the survey implementation and suggested measures to use serosurvey data to inform public health action.

Study findings

Of the 72 serosurveys supported by the CDC, 61% received both TA and FA, while 30% and 3% received only TA and only FA, respectively. The remaining six percent were part of the informal reviews.

Regarding the target audience, 54% of these surveys targeted the general population within the WHO African region, whereas 13%, 7%, and 7% targeted sample pregnant women, healthcare worker cohorts, and other specific population segments, such as patients, and students, etc., respectively. The remaining 18% targeted other populations, e.g., truck drivers. 

Interestingly, around 55% of studies used an enzyme-linked immunosorbent assay (ELISA) to detect SARS-CoV-2 antibodies in serum, while only 11% used a rapid antigen test. While the researchers assessed 63% of survey rounds against WHO UNITY criteria, only 79% used methods aligned with its criteria. The CDC-supported studies were in different stages of implementation. Notably, they were conducted by the local governments, which had ownership over the data. However, CDC could obtain limited and temporary access to seroprevalence data for analysis.

A meager 34 serosurvey rounds had completed data collection, while only five studies (6%) conducted in Africa were published. Among the published data, SARS-CoV-2 seroprevalence estimates ranged from 2.1% in Zambia to 34.7% in Kenya. Likewise, infection to reported cases ratios ranged from 21:1 in Ethiopia to 295:1 in Senegal.

Conclusions

The assistance provided by the CDC strengthened the local capacity to conduct SARS-CoV-2 epidemiologic surveillance in several LMI countries. However, the challenges in the study implementation and the interpretation of seroprevalence results constantly evolved.

Therefore, it was crucial to identify and forecast epidemic patterns over time, resolve the long-lingering question of correlates of protection (CoP) for SARS-CoV-2, and integrate COVID-19 serosurveillance with surveillance of other pathogens, including Plasmodium species, Dengue viruses, and other human coronaviruses. Notably, antigenic cross-reactivity between SARS-CoV-2 and these pathogens fetch false-positive results for SARS-CoV-2 serologic testing. To address this, CDC recommended conducting independent in-country validations of results, especially in the WHO African region.

The introduction of COVID-19 vaccines also challenged the design and implementation of SARS-CoV-2 serosurveys and the interpretation of their results. For instance, in the countries where a SARS-CoV-2 serosurvey design could not distinguish between natural and vaccine-induced immunity, it could not identify potentially at-risk populations and allocate vaccines efficiently. Likewise, the phenomenon of the waning of immunity hampered the assessment of both the actual burden of COVID-19 and vaccination coverage.

It is noteworthy that the type of antibody assay, test timing, and severity of infection impact the efficacy of serosurveys. Therefore, the CDC introduced new seroprevalence protocols. They also adapted the existent ones to address the epidemiologic, laboratory, and other considerations of vaccine introduction. Overall, the CDC's support for international seroprevalence surveys greatly helped guide public health interventions to control the spread of the COVID-19 pandemic at the global level.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.