In the present SARS-CoV-2 seroprevalence analysis, between 37 and 56% of the population in Bobo-Dioulasso, Ouagadougou, Fianarantsoa, and Kumasi had been exposed to the virus and exhibited antibodies against SARS-CoV-2 in the first half of 2021. Only a small proportion of the study population tested positive for an acute SARS-CoV-2 infection since the onset of the pandemic. Our findings point to an extensive but silent spread of SARS-CoV-2 in the three countries.
Underestimation of acute infections
To date and since March 2020, two epidemic wave have been noted in Burkina Faso, three in Madagascar, and four in Ghana (https://ourworldindata.org/covid-cases [15], accessed on February 14th 2021) [1]. The detection of epidemic waves relies on testing consistency [16]. Extrapolating the seropositivity rate of our study population onto the total population number in the targeted districts of Bobo-Dioulasso and Ouagadougou infers that 352,848 individuals in Bobo-Dioulasso and 579,162 individuals in Ouagadougou had been infected by March 2021. The combined seroprevalence estimate for these two cities is 75 times higher than the reported cumulated case count for the entire country at that time. These results correspond to findings from South Sudan [17] and Zambia [18], where the number of implied infections was found to be 100 and 92 times higher than the number of officially reported cases. Similarly, our extrapolated seroprevalence estimate for Fianarantsoa is twice as high as the official cumulated case number for the entire of Madagascar by June 2021, and nine times higher in Kumasi than the official cumulated case count for the entire of Ghana by May 2021. Reasons for the underestimation of infections may include insufficient testing capacities, limited access to testing, a lower acceptance of testing, and perhaps a low motivation of asymptomatic individuals to get tested, as there were no organized campaigns for large-scale testing [19]. Only individuals with symptoms suggestive of a SARS-CoV-2 infection, and people who travelled outside the country, were targeted for testing.
Limited testing capacities
By October 2021, 4 billion COVID-19 tests for acute infection had been performed since the onset of the pandemic globally but only 1.8% were carried out in Africa [20]. According to the online tool developed by Ritchie and co-workers [15] on the SARS-CoV-2 tests performed worldwide, data for Burkina Faso has only become available since January 2022. Ghana’s testing rate showed a constant high turnover of tested samples since the onset of the pandemic and was among the highest in sub-Saharan Africa due to their applied testing approach of pooled samples [21]. Madagascar showed low but constant activity in tests performed until September 2021.
Impact of SARS-CoV-2
To date, there are more than 5.8 million deaths due to COVID-19 worldwide (https://covid19.who.int/, accessed on February 15th2022) [1]. The three countries with the highest number of deaths are Italy (12,105,675 cases and 151,015 deaths), Spain (10,555,197 cases and 95,606 deaths), and the United States of America (76,983,188 cases and 910,982 deaths). In comparison, 7,942,093 cases and 162,673 deaths were recorded for the entire African continent. The perceived risk of contracting the disease might be lower in populations with a high proportion of asymptomatic cases. Therefore, safety measures, such as physical distancing and wearing masks may be even more difficult to enforce due to the low-risk perception of the population [22, 23]. Health policy makers would need to carefully consider how best to communicate the benefits of protective measures and vaccinations in settings where the majority of people have not experienced severe illness or even know anyone who has.
Affected population groups
In Bobo-Dioulasso and Fianarantsoa, 10–19- and 20–45-year-old males were more exposed to the virus than females of the same age group, while females had higher SARS-CoV-2 seropositivity in the > 45-year-old group. This finding is consistent with previous studies documenting higher infection rates in young human males [24], which might be explained by differences in risk behaviors [25], and responsible attitudes toward the COVID-19 pandemic [26]. The higher level of SARS-CoV-2 seropositivity in older females might be related to having stronger ties to their family members, friends, co-workers, neighbours, and community [27]. However, the age and sex patterns for Ouagadougou and Kumasi were different.
SARS-CoV-2 serosurveys
A recent systematic review and meta-analysis on SARS-CoV-2 serosurveys demonstrated that only 20% of the included 404 studies were of high quality based on a newly developed scoring system that included study design, laboratory assay, and outcome adjustment [28]. 64% of the study populations were convenience samples, and only 12% of the population-based studies achieved the highest score. At the time of writing, no population-based serological surveys for exposure to SARS-CoV-2 had been available for Burkina Faso, Madagascar, or Ghana. One study based on blood donations was available from Madagascar [29], and one study targeting different types of public locations and healthcare/research institutes was available from Ghana [30]. Both studies targeted specific sub-groups of the population so extrapolation of the prevalence to the urban community or general population is not valid.
Policy implications
This study makes a strong case for the need of routine community-level seroprevalence studies as part of COVID-19 surveillance activities in order to inform vaccination schedules and details thereof, including planning for age, geographic location and socioeconomic groupings. Our findings further show that a more coordinated approach to vaccination strategies and SARS-COV-2 seroprevalence estimates across Africa is needed, as uneven vaccination rates (in particular with respect to low rates and waning seroprevalence) risk diluting the positive gains made by other countries which are able to achieve herd immunity thresholds via vaccination within reasonable time frames.
Strengths of the study
This is the first study in Burkina Faso, Madagascar, and Ghana to assess the SARS-CoV-2 seroprevalence in a random sample of residents in urban settings. The age- and sex-stratified, two-stage cluster sampling approach increases the representativeness of the target population not only in terms of age and sex but possibly also for unmeasured confounders and minimizes the risk of selection bias. A highly sensitive and specific ELISA test, validated on local pre-pandemic serum samples, was used. Furthermore, a Bayesian hierarchical logistic regression model with post-stratification was applied for more accurate estimates of previous infections, and has reduced limitations linked to the sampling.
Limitations of the study
A 15% refusal rate to participate was seen in Kumasi, which may have induced a selection bias. Further, underestimation of seroprevalence could be caused by post-infection antibody waning. Both Madagascar and Ghana experienced the first wave of SARS-CoV-2 infections between July and August 2020, while sampling was performed six to eleven months later. Antibodies against the nucleoprotein have been observed to drop significantly within 12 months post infection [31], so this may be another source for an underestimation.