To our knowledge, this is the only national seroprevalence study conducted in the context of the COVID-19 pandemic in Togo. The present study was conducted between the first and second epidemic waves and shows that nearly two thirds of subjects aged 5 years and older (65.5%) acquired SARS-CoV-2 antibodies between March 2020 and June 2021 in Togo, with a preponderance in urban areas and among adult subjects (30 to 49 years). This prevalence confirms a high circulation of SARS-CoV-2 in the general population of Togo in 2021.
Comparison of SARS-CoV-2 seroprevalence studies is very difficult and should be done with caution. Indeed, the results of these studies are influenced by the study population, the period (before, after an epidemic wave) and the characteristics of the tests. In Togo, a first study conducted in May and June 2020 in Lomé among populations at high risk of infection reported a seroprevalence of 0.9% [10]. Although these results may be underestimated, the observed difference reflects the spread of the virus in the population 1 year and after the first epidemic wave in Togo.
Population-based seroprevalence studies similar to ours, following the WHO generic protocol and using the same ELISA tests, have been performed in the West African sub-region. The reported seroprevalence were 67.9% in Ghana [17] and 78.9% in Nigeria [18]. In both countries, youth (15–19 years) and young adults (30–39 years) were most affected by the infection [17, 18]. These proportions corroborate those observed in Togo.
A meta-analysis performed on data published globally between January 1st and December 31st, 2020, also confirms the high circulation of the virus in sub-Saharan Africa. Median seroprevalence varied by continent, from 0.6% in Southeast Asia, East Asia and Oceania to 19.5% in sub-Saharan Africa (p < 0.001) [5]. A second meta-analysis [19], taking into account data from Africa covering the period from January 2020 to December 2021 reported 151 seroprevalence studies on the continent. SARS-CoV-2 seroprevalence ranged between 3.0% in the second quarter of 2020 to 65.1% in the third quarter of 2021 [19]. Seroprevalence was highly heterogeneous within countries. It was lower in rural geographic areas as demonstrated in our study [19]. A final meta-analysis of individual data from 27,735 participants collected between April 2020 and April 2021 in Africa reported a seroprevalence of anti-SARS-CoV-2 antibodies of 22% with very high heterogeneity [20]. In summary, studies in Africa report a high level of SARS-CoV-2 circulation based on the presence of IgG. However, a cross reaction was reported. Indeed, several studies suggest that Individuals with acute malaria infection or living in malaria endemic settings generated high levels of antibodies that could substantially cross-react with the SARS-CoV-2 serological assays [21, 22]. In a study conducted by the team of Dorkenoo et al., among a population of febrile subjects during the pandemic in Lomé, a higher presence of anti-SARS-CoV-2 antibodies (IgG) was demonstrated in patients diagnosed with malaria [23].
In order to have better idea of the circulation of the virus during this long-lasting pandemic, it would have been more relevant to conduct seroprevalence studies at regular intervals. This approach has been adopted in some countries in the sub-region, notably in Guinea, where a study reports three successive seroprevalence surveys, 3 months apart, using multistage cluster sampling to measure the extent and dynamics of the SARS-CoV-2 epidemic in Conakry, the capital city [24]. Seroprevalence increased from 17.3% in December 2020 in the first survey to 28.9% in March/April 2021 and to 42.4% in June 2021. This significant overall trend of increasing seroprevalence (p < 0.0001) was also significant in each age group. Such an increase could be explained by a high circulation of the virus in the community or by the introduction of the SARS-CoV-2 vaccine in the second quarter of 2021. Indeed, in this study, it has been reported that among seropositive participants in the last study 18% were vaccinated vs 0% in the first study [24] illustrating sustained transmission within the community at large. These data may contribute to the development of efficient response strategies.
Overall, one fifth of our sample was vaccinated. This proportion is high considering the study period. A social desirability bias cannot be excluded. However, the seroprevalence of anti-SARS-CoV-2 antibodies was higher in vaccinated subjects of all ages. The presence of antibodies remains a qualitative marker that should be coupled with the titration of neutralizing antibodies in order to demonstrate the level of immunization and to guide vaccination policy. This issue should be discussed in a context of strong hesitation with the COVID-19 vaccine coverage remaining below 38% in April 2022 in Togo. Similarly, the issues around vaccine boost doses deserve particular attention.
Given the variation in antibody kinetics over time, it is important to repeat seroprevalence surveys and couple them with antibody titration. This would document the long-term persistence of anti-SARS-CoV-2 antibodies that may protect against reinfection. Indeed, there is evidence that by significantly increasing neutralizing antibody titers, a single dose vaccination enhances protection against variants. Assessment of anti-SARS-CoV-2 antibody kinetics is essential to predict protection against reinfection and durability of vaccine protection [25]. Based on our results, Togo might be on its way to achieving herd immunity, if the virus continues to circulate and if vaccination campaigns are maintained and reinforced. Herd immunity occurs when a large part of a community becomes immune (through natural infection or vaccination) to a disease. As a result, the entire community is protected, not just those who are immune [26].
As of June 30, 2021, 13,917 confirmed cases of the COVID-19 have been reported in Togo. Extrapolating the prevalence of 65.5% to the 8 million Togolese, there would have been approximately 5.2 million cases of the COVID-19, corresponding to almost 375 times more than the number of cases notified in Togo at the same date. At the time of the study, only 3.1% of the population was vaccinated against COVID-19 [27]. This study confirms that cases are under-detected and under-notified. The under identification of cases is due to the particularity of this disease which is asymptomatic in more than 40.5% of the cases [28]. This clinical particularity makes it necessary to identify anti-SARS-CoV-2 antibodies through a serological survey in order to know the extent of the disease. Repeating such serological surveys, especially after the Omicron wave, would help refine the profile of the epidemic in the country. The under-reporting of cases is related to the low capacity of the epidemiological surveillance system to test and to take into consideration data coming from private and public health facilities. Indeed, a study in South Africa reported that more than 95% of the COVID-19 cases were not identified by the national surveillance system [29].
This is the first national seroprevalence study of SARS-CoV-2 in Togo and one of the few available in West Africa. The relatively large size of our sample allowed us to perform analyses according to vaccination status, sex, place of residence and age groups. We have thus partially filled the data gap and documented the actual level of SARS-CoV-2 circulation in the general population in Togo in 2021.
As in most cross-sectional studies, selection bias cannot be ruled out. Indeed, the districts concerned by the study were selected according to a reasoned approach on the basis of the incidence of the COVID-19 on March 1, 2021 at the time of the survey. In each health region, we included the districts with the highest and lowest incidence of the COVID-19. This choice meets WHO recommendations for SARS-CoV-2 sero-surveillance surveys [14]. Random selection of districts would have exposed us to the risk of including only districts with high incidence leading to an overestimation of SARS-CoV-2 seroprevalence or vice versa. In addition, to reduce the collinearity effect and avoid over representation of some households, we limited the number of subjects to be included in the households to 4.
In this study, a classification bias related to the use of ELISA tests was taken into account. No laboratory test is 100% sensitive and specific, and many likely have substantial measurement error. This measurement error could result in biased prevalence estimates [30]. To control for this type of bias, we adjusted the weighted seroprevalences using the sensitivity and specificity adjustment formula recommended by Sempos and Tian [30]. By stratifying on the COVID-19 vaccination status, we controlled for confounding bias in the multivariate analysis.
Finally, the results of the analysis of factors associated with the presence of anti-SARS-CoV-2 antibodies should be interpreted with caution. Indeed, in cross-sectional studies, it is difficult to establish the temporality between the exposure and the main event since both are collected at the same time.
SARS-CoV-2 antibody seroprevalence in Togo: a national cross-sectional household survey, May–June, 2021 - BMC Public Health - BMC Public Health
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