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Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Kenyan blood donors

Our take —

Compared to the number of laboratory-confirmed COVID-19 illnesses and deaths in other parts of the world, fewer than expected cases and deaths have been reported from sub-Saharan African countries. Using samples from blood donors and Bayesian models, this study provides evidence from Kenya to suggest that transmission in this population has been similarly high to other countries, particularly in urban areas (population-weighted seroprevalence overall was 4.3% and varied from 5.5 – 8.0% in urban areas). Given this finding, the most likely reasons for the lower than expected COVID-19 case reports from Kenya are under-ascertainment from surveillance, milder disease due to younger average age, or hereto unmeasured t-cell mediated immunity from other coronaviruses.

Study design

Cross-Sectional

Study population and setting

3,098 blood transfusion samples from donors aged 15 – 64 years from four Kenya National Blood Transfusion Service regional transfusion centers collected from April 30 – June 16, 2020 were tested for IgG antibodies against SARS-CoV-2. The study used an ELISA test validated by the authors, with a reported sensitivity of 92.7% and specificity of 99.0%. They used Bayesian models to take the crude seroprevalence estimates from this select group of blood donors and adjust to estimate the population-based seroprevalence of Kenya, also taking into account the performance characteristics of the ELISA test used.

Summary of Main Findings

The crude seroprevalence for all participants was 5.6% (174/3098) but was higher among participants from urban areas. The adjusted national seroprevalence estimate was 4.3% (95% CI 2.9 – 5.8%), which remained higher in urban counties of Mombasa (8.0%), Nairobi (7.3%), and Kisumu (5.5%). These seroprevalence estimates are similar to other countries with much higher reported incidence of laboratory-confirmed cases and deaths, and suggest that lower than expected case and death counts in Kenya are unlikely due to differences in number of people infected, unless blood donors included in this study have much higher seroprevalence than others in the population. Other possible contributors to the lower than expected reports of COVID-19 illnesses and deaths in Kenya could be under-ascertainment of these events during surveillance, reduced risk for severe illness and death due to the younger average age in this population, or cell-mediated immunity which is not reflected in seroprevalence estimates.

Study Strengths

Rather than rely on the crude seroprevalence estimates, the authors adjusted for the age, sex and geographic distribution of the underlying population, as well as the performance characteristics of the ELISA test, to estimate the true seroprevalence in this population.

Limitations

The blood samples tested in this study came from blood donors, which were not reflective of the general population; they were more likely to be young adult males. It is possible that their risk for past infection with SARS-CoV-2 may not reflect broader risks in the underlying population.

Value added

The number of laboratory-confirmed COVID-19 cases and deaths has been lower than expected in sub-Saharan Africa, and this study provides evidence to show that this may not be due to a lack of transmission since the proportion of adults tested with antibodies against SARS-CoV-2 is similar to other studies from countries on in North America and Europe.

This review was posted on: 21 December 2020