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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. Interestingly, 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 reflect the dynamics of COVID-19 in West Africa (WA). The framework is trained with a portion of the confirmed 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 also confirmed 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 efficacies (e.g., the Pfizer or Moderna vaccine) are prioritized, and higher if vaccines of lower efficacy 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, complementing 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 measures 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 understanding 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.

The 2019 coronavirus (COVID-19) pandemic continues to have a devastating impact on health systems and economies across the globe, with the United States (U.S.) among the worse impacted nations. Implementing public health measures in tandem with effective vaccination strategies is instrumental in halting the transmission of the virus and curtailing the burden of the pandemic. Currently, the U.S. Food and Drug Administration has authorized the use of the PfizerBioNTech, Moderna, and the Johnson & Johnson vaccines to prevent COVID-19 in the U.S. However, these vaccines have varying efficacies (≈ 95% for the Pfizer-BioNTech and Moderna vaccines and ≈ 70% for the Johnson & Johnson vaccine) and waning effects against major COVID-19 strains, hence, understanding their impact on the incidence of COVID-19 in the U.S. is critical. Here, we formulate and use mathematical models 1) to investigate the impact of each vaccine type and booster doses (single/double) on the incidence of COVID-19 in the U.S., and 2) to predict future trends of the disease in the U.S., if existing control measures are reinforced or relaxed. The models are fitted to part of the new daily confirmed case data from the U.S., and validated using the remaining part of the daily data, as well as the full cumulative case data. The fitting and numerical simulations of the models show a 44% (71%) reduction in the reproduction number (number of new daily confirmed cases) at the peak during the wave in which vaccination peaked compared to the preceding wave. Additionally, the estimated disease transmission rate is ≈ 3 times higher for the Omicron variant. Simulations of the model show that in the absence of booster shots, the time to elimination of community transmission in the U.S. would have increased by at least two months compared to the baseline case. However, had more people (i.e., 70% of the fully vaccinated population) been boosted by mid-August 2021, ≈ 78% of the daily incidence could have been prevented as at the time the first case of Omicron was reported in the U.S. Our findings suggest that booster shots with the Pfizer-BioNTech or Moderna vaccines conferred superior protection than those with the Johnson & Johnson vaccine. Furthermore, the simulations show that the baseline value of the new daily cases at the peak of the Omicron variant in January 2022 would have dropped significantly (by ≈ 20%) if a fourth dose of the Pfizer-BioNTech or Moderna vaccine was administered at the start of the Omicron wave. Specifically, three million cumulative cases in the U.S. could have been averted between late November 2021 and March 2022. The study proves that early administration of vaccines and booster doses could have significantly reduced the surge in cases and the observed peak size. In particular, we showed that, while late boosting will result in an increase in the number of cases (compared to the baseline value), early boosting will lead to a decrease in the number of cases. Additionally, we showed that a second booster dose using the Pfizer-BioNTech or Moderna vaccine is important in curtailing the burden of the pandemic in the U.S. Particularly if this second dose is administered soon after the first dose. Furthermore, the study shows that early relaxation of existing control measures can lead to a more devastating wave, especially if both vaccination and transmission rate reducing measures such as mask-use are relaxed simultaneous. Keywords: COVID-19 pandemic; Vaccine efficacy; Booster doses; Delta and Omicron variants; Waning vaccine-derived and natural immunity; Infectious disease models.