Polio by the Numbers-A Global Perspective

Abstract

Background: Investments in national immunization programs and the Global Polio Eradication Initiative (GPEI) have resulted in substantial reductions in paralytic polio worldwide. However, cases prevented because of investments in immunization programs and GPEI remain incompletely characterized.

Methods: Using a global model that integrates polio transmission, immunity, and vaccine dynamics, we provide estimates of polio incidence and numbers of paralytic cases prevented. We compare the results with reported cases and estimates historically published by the World Health Organization.

Results: We estimate that the existence and use of polio vaccines prevented 5 million cases of paralytic polio for 1960-1987 and 24 million cases worldwide for 1988-2021 compared to a counterfactual world with no polio vaccines. Since the 1988 resolution to eradicate polio, our estimates suggest GPEI prevented 2.5-6 million cases of paralytic polio compared to counterfactual worlds without GPEI that assume different levels of intensity of polio vaccine use in routine immunization programs.

Conclusions: Analysis of historical cases provides important context for understanding and communicating the benefits of investments made in polio eradication. Prospective studies will need to explore the expected benefits of future investments, the outcomes of which will depend on whether and when polio is globally eradicated.

Keywords: disease eradication; modeling; polio; prevention; vaccines.

Figure 1.

The total number of globally reported cases from 1950 through 2021. Blue bars show reported cases published by the World Health Organization (WHO) from 1980 through 29 December 2021 [12, 13]. Solid black circles are the most up-to-date annual reported cases in WHO published reports as of mid-1989 [23–30], representing the best available data at the time of 1988 World Health Assembly resolution to eradicate polio. The solid horizontal bars covering the periods 1954–1955 and 1961–1965 are published averages for these periods [24]. Inset highlights the period between 2000 and 2021 at a magnified vertical axis scale.

Figure 2.

The total number of globally reported cases from 1980 through 2021. Blue bars show reported cases by WHO from 1980 through 29 December 2021 [12, 13]. Red diamonds show published point estimates [32–36]. Solid horizonal red line represents published average point estimate for the years 1980 to 1985 [29]. Solid red vertical bars represent published range estimate for the years 1986 to 1988 [31]. Solid black curve shows annual paralytic cases of the integrated global model reference case (global RC). Dashed and dotted curves show annual global paralytic cases estimated using simple models for parameters indicated in the figure legend. Abbreviations: aPIR, average paralysis-to-infection ratio; elim, elimination; RC, reference case; sPIR, serotype-specific paralysis-to-infection ratio; WHO, World Health Organization.

Figure 3.

Estimated total annual paralytic polio cases since 1950 with GPEI (solid black curve) and without poliovirus vaccines (dotted red curve). For reference, blue bars show reported cases of paralytic polio through 29 December 2021 [12, 13]. The inset shows the period 2000 to 2021 with a magnified vertical axis to make visible the reported and modeled cases of paralytic polio in the past 2 decades. Abbreviations: GPEI, Global Polio Eradication Initiative; RC, reference case; WHO, World Health Organization.

Figure 4.

Estimated total annual paralytic polio cases prevented since 1950 using the no vaccine scenario compared to either the global RC (solid black curve) or the reported cases (dotted purple curve). Black diamonds show WHO published point estimates of the paralytic polio cases prevented for the years 1988 to 1995 [, , –38–42]. Abbreviations: RC, reference case; WHO, World Health Organization.

Figure 5.

Estimated cases prevented by GPEI based on the global RC compared to a world with (1) no GPEI but with flat RI coverage at the 1988 levels (top two red curves), (2) no GPEI but with increasing RI coverage based on WHO annual estimates (bottom two blue curves), and (3) no GPEI but with moderately increasing coverage characterized by a linear increase to 50% of 2019 RI coverage (middle two green curves). For each scenario, the solid lines show the results for all 200 countries, and the accompanying dashed lines show the results for the 105 GPEI countries only. Abbreviations: GPEI, Global Polio Eradication Initiative; RC, reference case; RI, routine immunization; WHO, World Health Organization.