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Prevalence of SARS-CoV-2 antibodies from national serosurveillance of Kenyan blood donors

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High SARS-CoV-2 antibody levels have been achieved in most of the population in high-income countries through vaccination.1 However, global inequity exists in COVID-19 vaccine distribution, as highlighted at the June 2021 G7 Summit, which committed to providing 1 billion doses to low-income countries. This focus on doses overlooks the pace of transmission in low-income settings.

To monitor seroprevalence in Kenya, we began surveillance of blood donors (aged 16-64 years) in April 2020.2,3 The national prevalence of SARS-CoV-2 antibodies was estimated at 4.3% in April to June 20202 and 9.1% in August to September 2020.3 In this article we estimate seroprevalence for January to March 2021.

Methods

The sampling, laboratory, and analytic methods were identical to those in the studies conducted in 2020.2,3 Plasma samples with complete donor demographic data and sufficient volume collected from all 6 regional transfusion centers were included and assayed for anti–spike IgG using an enzyme-linked immunosorbent assay. Validated among 910 prepandemic serum samples from coastal Kenya and 174 patients with positive polymerase chain reaction test results from Nairobi, specificity was 99.0% and sensitivity was 92.7%.2 Seropositive results were tabulated by age, sex, and region of residence. Donors were stratified into 8 regions by place of residence; these regions are unrelated to the 6 regional transfusion centers, which collate donations across different geographic catchment areas. Bayesian multilevel regression with poststratification using the rjags package in R, version 3.6.1 (R Foundation), was used to obtain seroprevalence estimates and 95% CIs adjusted for the age, sex, and regional distribution of blood donors compared with national data for individuals aged 16 to 64 years based on 2019 census data.2,3 Adjustment was also done for test performance. The surveillance was approved by the Scientific and Ethics Review Unit of the Kenya Medical Research Institute; written informed consent for use of the data for research was obtained from all donors.

Results

Between January 3, 2021, and March 15, 2021, a total of 3062 samples (median sample date, February 14, 2021) were collected. There were 1145 samples (37.4%) collected from the transfusion center in Nairobi, 879 (28.7%) from Mombasa, 431 (14.1%) from Kisumu, 250 (8.2%) from Embu, 200 (6.5%) from Nakuru, and 157 (5.1%) from Eldoret. Forty-four samples were excluded because of missing information, age-ineligible donors, or collection date before 2021. Of 3018 remaining samples, 1333 were seropositive; crude seroprevalence was 44.2% (95% CI 42.4%-46.0%) (Table).

The blood donor sample differed from the general population of individuals aged 16 to 64 years (n = 25 954 858) regarding age (8.0% of blood donors were aged 45-64 years vs 19.5% of the population), sex (78.1% of donors were male vs 49.2% of the population), and region (Table). Using bayesian poststratification, the adjusted estimate of seroprevalence among those aged 16 to 64 years in Kenya was 48.5% (95% CI, 45.2%-52.1%). This estimate varied little by age or sex, but was higher in Nairobi (61.8% [95% CI, 53.2%-70.6%]), the country’s capital city, and lower in 2 rural regions, Nyanza and Western, adjacent to Uganda.

Discussion

The prevalence of SARS-CoV-2 antibodies in blood donors in Kenya increased from 4.3%2 to 48.5% over 1 year. This is consistent with estimates in other Kenyan populations: 11% among antenatal clinic attendees in rural Kilifi and 50% among clinic attendees in urban Nairobi in August to September 20203; 42% among truckers in August to November 20203; and 12% to 13% among health care workers in rural counties and 44% among health care workers in Nairobi in July to December 2020.4

Study limitations include that the blood donors were a convenience sample and may not represent the population as a whole, although they have provided useful epidemic intelligence in high-income countries.1 Although the antibody assay was highly specific in Kenyan prepandemic samples and demonstrated consistent discrimination in a World Health Organization multilaboratory standardization exercise,5 sensitivity was estimated in individuals sampled a median of 21 (minimum of 7) days after a positive polymerase chain reaction test result. Antibody concentration and test sensitivity may decline with longer times after infection, implying that seroprevalence may be underestimated by the test-performance adjustment.

These data suggest SARS-CoV-2 had progressed rapidly across Kenya by February 2021, before a large third wave of infections began in March 2021. Kenya’s COVID-19 vaccine program also began in March 2021 and had reached 2% of the population by July. Natural infection is outpacing vaccine delivery in Kenya, and this reality needs to be considered as objectives of the vaccine program are set.

Section Editors: Jody W. Zylke, MD, Deputy Editor; Kristin Walter, MD, Associate Editor.

Source: Prevalence of SARS-CoV-2 antibodies from national serosurveillance of Kenyan blood donors

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