Non-pharmaceutical interventions and COVID-19 vaccination strategies in Senegal: a modelling study

Abstract

Background: When vaccines against the novel COVID-19 were available in Senegal, many questions were raised. How long should non-pharmaceutical interventions (NPIs) be maintained during vaccination roll-out? What are the best vaccination strategies?

Methods: In this study, we used an age-structured dynamic mathematical model. This model uses parameters based on SARS-CoV-2 virus, information on different types of NPIs, epidemiological and demographic data, some parameters relating to hospitalisations and vaccination in Senegal.

Results: In all scenarios explored, the model predicts a larger third epidemic wave of COVID-19 in terms of new cases and deaths than the previous waves. In a context of limited vaccine supply, vaccination alone will not be sufficient to control the epidemic, and the continuation of NPIs is necessary to flatten the epidemic curve. Assuming 20% of the population have been vaccinated, the optimal period to relax NPIs would be a few days from the last peak. Regarding the prioritisation of age groups to be vaccinated, the model shows that it is better to vaccinate individuals aged 5-60 years and not just the elderly (over 60 years) and those in high-risk groups. This strategy could be more cost-effective for the government, as it would reduce the high costs associated with hospitalisation. In terms of vaccine distribution, the optimal strategy would be to allocate full dose to the elderly. If vaccine doses are limited, half dose followed by full dose would be sufficient for people under 40 years because whether they receive half or full dose, the reduction in hospitalisations would be similar and their death-to-case ratio is very low.

Conclusions: This study could be presented as a decision support tool to help devise strategies to control the COVID-19 pandemic and help the Ministry of Health to better manage and allocate the available vaccine doses.

Keywords: COVID-19; control strategies; mathematical modelling; prevention strategies; vaccines.

Figure 1

Epidemic curve for daily cases and deaths in Senegal. The daily cases and deaths correspond to smoothed raw data. The red curve represents daily cases, and the blue curve represents daily deaths. Vertical text indicates the official dates intervention measures were taken by the Senegalese government.

Figure 2

Mathematical model of COVID-19 in Senegal. (A) Blue dots represent confirmed cases. Blue lines and regions represent model estimates of daily cases (median and 95% CIs). (B) Orange lines represent the cumulative number of deaths. Blue lines and regions represent model estimates of cumulative numbers of deaths (median and 95% CIs).

Figure 3

Modelling scenarios of daily cases and cumulative deaths according to different end dates for non-pharmaceutical interventions (NPIs). (A) Lines represent model scenarios for daily cases for each date for the relaxation of NPIs. The green (respectively, red, cyan, purple and yellow) line is for the scenario where NPIs are relaxed on 1 July 2021 (respectively, 15 July, 1 August, 1 September and 1 October 2021). (B) Lines represent model scenarios for the cumulative number of deaths for each date for the relaxation of NPIs. Tthe green (respectively, red, cyan, purple and yellow) line is for the scenario where NPIs are relaxed on 1 July 2021 (respectively, 15 July, 1 August, 1 September and 1 October 2021).

Figure 4

Modelling scenarios of daily cases and cumulative deaths according to different vaccine coverage rates and age groups. (A) Lines represent model scenarios for daily cases for different vaccine coverage rates and age groups. Blue represents the model’s output for the baseline scenario. The green (respectively, red and cyan) line is for the scenario where 70% of 30–60 years (respectively, 32% of 5–30 years and 22% of 5–60 years) are vaccinated. (B) Lines represent model scenarios for the cumulative number of deaths for different vaccine coverage rates and age groups. Blue represents the model’s output for the baseline scenario. The green (respectively, red and cyan) line is for the scenario where 70% of 30–60 years (respectively, 32% of 5–30 years and 22% of 5–60 years) are vaccinated.

Figure 5

Cost-effectiveness analysis and effects on deaths and hospital costs. (A1 and A2) The x-axis represents the hospitalisation cost and the y-axis represents the gain in YLS. (A1) shows a zoomed-in area of the best strategies and (A2) represents all strategies. (B) represents the regression coefficients according to age group and number of vaccine doses. On the x-axis, the factors represent the number of doses allocated according to age group and the y-axis shows the regression coefficient. blue bars represent deaths and orange bars represent hospitalisation costs. YLS, years of life saved.