COVID-19 vaccine efficacy summary

To project future COVID-19 trends, IHME centralizes and updates all available data on vaccine efficacy. This document summarizes the available data and key underlying assumptions of IHME’s projections. 

Review and summary of available data

Using peer-reviewed publications, reports, and news articles, we review available data to find how effective each of the available vaccines is at preventing various outcomes. “Vaccine efficacy” is not a single number – we capture:

1. Prevention of infection: a vaccine’s efficacy at stopping transmission of the virus from one person to another. An exposed person will not contract the virus, and by definition they will also not develop symptoms or disease.
2. Prevention of asymptomatic disease: a vaccine’s efficacy at preventing an exposed individual who contracted the virus from developing symptoms.
3. Prevention of symptomatic disease: a vaccine’s efficacy at preventing an exposed individual from suffering the symptoms of COVID-19 infection.
4. Prevention of severe disease and death: a vaccine’s efficacy at preventing an exposed person from developing serious symptoms that often require hospitalization and lead to death.

For each of the outcomes above, we also capture the differences by:

1. First dose versus complete regimen (not applicable for 1-dose vaccines, like Johnson & Johnson)
2. Variant of COVID-19: D614G (ancestral type), B.1.1.7 (Alpha), B.1.351 (Beta), P.1 (Gamma), B.1.617.2 (Delta)

Where genomic sequencing of cases did not occur, we use the location of the study as a proxy for the predominant variant type. For example, studies in South Africa were assumed to represent efficacy against Beta.

Data available today consist of clinical trials and several quasi-observational studies. Table 1 summarizes the available data to date:

Table 1: Available vaccine efficacy data (last updated August 9, 2021)

Estimating vaccine efficacy for COVID-19 projections

Currently, the IHME model uses the following inputs of vaccine efficacy, separated by variant:

1. Efficacy at preventing symptomatic disease; this is operationalized as a reduction in hospitalization and death
2. Efficacy at preventing infection; this is operationalized as a reduction in susceptibility

At present, our model only distinguishes between ancestral and Alpha combined and the three main variants of concern combined (Beta, Gamma, and Delta). Due to this current limitation and given that Delta is the predominant variant of concern, we preferentially use data on Delta where this is available.

Wherever data are available (see Table 1) we use available data, and where we do not yet have data, we take different approaches based on variant, vaccine, and outcome. Estimates are generated for each vaccine and outcome as described below.

To estimate efficacy at preventing disease for ancestral and Alpha

To estimate efficacy at preventing disease, we use the result from the corresponding clinical trials as documented in Table 1.

In addition, we modify the effectiveness of the mRNA vaccines (Pfizer-BioNTech and Moderna) and AstraZeneca based on studies that show higher vaccine efficacy for preventing severe disease, hospitalization, and death compared to all symptomatic disease. This adjustment more accurately reflects how these estimates of vaccine effectiveness are used in our model; that is, to reduce the infection fatality ratio (IFR) and the infection hospitalization ratio (IHR). We used the average ratio of vaccine effectiveness for hospitalization compared to symptomatic disease combined from studies in the United Kingdom and Canada to modify the estimated effectiveness from the clinical trials for these vaccines. We also used the vaccine effectiveness against severe disease for the Johnson & Johnson vaccine instead of the efficacy against all symptomatic disease using results from the clinical trial.

For other mRNA vaccines, we assume the same efficacy as Pfizer-BioNTech.

For all vaccines, we assume 75% efficacy.

To estimate efficacy at preventing infection for ancestral and Alpha

For AstraZeneca and Johnson & Johnson, we use the efficacy as reported in the clinical trials as documented in Table 1.

For Pfizer-BioNTech and Moderna, we use the infection-to-disease efficacy ratio from the Israel study of the Pfizer-BioNTech vaccine.  

For all other vaccines, we use the average infection-to-disease efficacy ratio from Pfizer-BioNTech, Johnson & Johnson, and Oxford-AstraZeneca .

To estimate efficacy and preventing disease for variants Beta, Gamma, and Delta

For Pfizer-BioNTech and Moderna, we use the ratio of the efficacy for Delta:Alpha from the Public Health England and Scotland studies of the Pfizer-BioNTech vaccine as described in Table 1.

For AstraZeneca, we use the ratio of the efficacy for Delta:Alpha from the Public Health England and Scotland studies for AstraZeneca.

For all other vaccines, we use the average ratio of the efficacy for Delta:Alpha from the Public Health England and Scotland studies for AstraZeneca and Pfizer-BioNTech.

In addition, we apply the adjustments described for severe disease for the mRNA vaccines, AstraZeneca, and the Johnson & Johnson vaccine.  

To estimate efficacy at preventing infection for variants Beta, Gamma, and Delta

We use the infection-to-disease efficacy ratio for ancestral and Alpha applied to the effect on symptomatic disease for Beta, Gamma, and Delta to estimate the efficacy at preventing infection for these variants of concern.

Table 2 shows the final model inputs for vaccine efficacy at preventing disease and infection, by vaccine and variant type.

Table 2: Vaccine efficacy by coronavirus variant, available data, and modeled estimates

Vaccine	Efficacy at preventing disease: ancestral & Alpha	Efficacy at preventing infection: ancestral & Alpha	Efficacy at preventing disease: Beta, Gamma, Delta	Efficacy at preventing infection: Beta, Gamma, Delta
Pfizer/BioNTech	94%	86%	85%	78%
Moderna	94%	89%	94%	80%
AstraZeneca	90%	52%	85%	49%
Johnson & Johnson	86%	72%	60%	56%
Sputnik-V	92%	81%	80%	70%
Novavax	89%	79%	79%	69%
CoronaVac	50%	44%	43%	38%
Sinopharm	73%	65%	63%	56%
Tianjin CanSino	66%	58%	57%	50%
Covaxin	78%	69%	68%	60%
Other mRNA vaccines	91%	86%	85%	78%
All other vaccines	75%	66%	65%	57%

 

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