The impact of age structure and vaccine prioritization on COVID-19 in West Africa

The ongoing COVID-19 pandemic has been a major global health challenge since its emergence in 2019. Contrary to early predictions that sub-Saharan Africa (SSA) would bear a disproportionate share of the burden of COVID-19 due to the region's vulnerability to other infectious diseases, weak healthcare systems, and socioeconomic conditions, the pandemic's effects in SSA have been very mild in comparison to other regions. Interest- ingly, the number of cases, hospitalizations, and disease-induced deaths in SSA remain low, despite the loose implementation of non-pharmaceutical interventions (NPIs) and the low availability and administration of vaccines. Possible explanations for this low burden include epidemiological disparities, under-reporting (due to limited testing), climatic factors, population structure, and government policy initiatives. In this study, we formulate a model framework consisting of a basic model (in which only susceptible individuals are vaccinated), a vaccine-structured model, and a hybrid vaccine-age-structured model to assess the dynamics of COVID-19 in West Africa (WA). The framework is trained with a portion of the con ﬁ rmed daily COVID-19 case data for 16 West African countries, validated with the remaining portion of the data, and used to (i) assess the effect of age structure on the incidence of COVID-19 in WA, (ii) evaluate the impact of vaccination and vaccine prioritization based on age brackets on the burden of COVID-19 in the sub-region, and (iii) explore plausible reasons for the low burden of COVID-19 in WA compared to other parts of the world. Calibration of the model parameters and global sensitivity analysis show that asymptomatic youths are the primary drivers of the pandemic in WA. Also, the basic and control reproduction numbers of the hybrid vaccine-age-structured model are smaller than those of the other two models indicating that the disease burden is overestimated in the models which do not account for age-structure. This result is con ﬁ rmed through the vaccine-derived herd immunity thresholds. In particular, a comprehensive analysis of the basic (vaccine-structured) model reveals that if 84%(73%) of the West African populace is fully immunized with the vaccines authorized for use in WA, vaccine-derived herd immunity can be achieved. This herd immunity threshold is lower (68%) for the hybrid model. Also, all three thresholds are lower (60% for the basic model, 51% for the vaccine-structured model, and 48% for the hybrid model) if vaccines of higher ef ﬁ cacies (e.g., the P ﬁ zer or Moderna vaccine) are prioritized, and higher if vaccines of lower ef ﬁ cacy are prioritized. Simulations of the models show that controlling the COVID-19 pandemic in WA (by reducing transmission) requires a proactive approach, including prioritizing vaccination of more youths or vaccination of more youths and elderly simultaneously. Moreover, com- plementing vaccination with a higher level of mask compliance will improve the prospects of containing the pandemic. Additionally, simulations of the model predict another COVID- 19 wave (with a smaller peak size compared to the Omicron wave) by mid-July 2022. Furthermore, the emergence of a more transmissible variant or easing the existing mea- sures that are effective in reducing transmission will result in more devastating COVID-19 waves in the future. To conclude, accounting for age-structure is important in under- standing why the burden of COVID-19 has been low in WA and sustaining the current vaccination level, complemented with the WHO recommended NPIs is critical in curbing the spread of the disease in WA. © 2022 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license cacies of used cacy vaccine prioritization based on age brackets on the burden of COVID-19 in WA. The results of the study indicate that individuals below 65 years old are the major drivers of the COVID-19 pandemic in WA. This provides one reason for which the reported number of COVID-19 cases in WA is low in comparison to other regions. Therefore, incorporating age-structure in COVID-19 models is important in understanding why the burden of COVID-19 has been low in WA. This result calls for COVID-19 control measures which target individuals below 65 years old as opposed to the elderly as was the case in many high-income countries when the availability of vaccines was limited or during the initial stages of booster vaccine dose administration. Furthermore, the study shows that prioritizing vaccines with higher ef ﬁ cacies will enhance prospects of containing the disease and that the pandemic can be contained in West Africa if 68% of the population is fully vaccinated. However, a drastic change in the current vaccination rate is required for the sub-region to achieve the vaccine-derived herd immunity threshold of 68%. Additionally, the study shows that the emergence of a new more transmissible variant of concern, or easing current control measures in WA could lead to a more devastating wave depending on the easing level. Hence, to curtail the spread of the disease in WA, it is imperative to improve on existing measures (e.g., the vaccination level in conjunction with the WHO recommended NPIs).