cdc.gov/coronavirus

Matthew Biggerstaff, ScD, MPH 

for the Data, Analytics, and Modeling Task Force 

Advisory Committee on Immunization 

Practices Meeting

10/30/2020

Modeling Strategies for the Initial Allocation of SARS-CoV-2 Vaccines
Question

▪ What is the potential impact, in terms of preventing COVID-19 infections 

and deaths, of initially allocating vaccine to one of the following groups after 

vaccinating healthcare personnel in Phase 1A?

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Adults aged 65+

Adults with high-risk medical conditions

Essential workers
Methods: Population
Population Stratification

▪ 5 Age Groups: 

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0-4, 5-17, 18-49, 50-64, 65+ (~55 M nationally)

▪ Risk Status (within each adult age strata):

Low-risk

High-risk (having 1 or more select high-risk medical conditions)1

• COPD, heart disease, diabetes, kidney disease, or obesity

• Prevalence of having 1+ condition estimated from BRFSS

~40% of adults (100M nationally), increasing with age 

• Assume 3x higher risk of mortality upon infection relative to the 

low-risk group 2

1 https://www.cdc.gov/coronavirus/2019-ncov/need-extra-precautions/people-with-medical-conditions.html 

2 https://www.cdc.gov/coronavirus/2019-ncov/need-extra-precautions/evidence-table.html
Population Stratification, continued

▪ Occupational Status 

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~40% of adults aged 18-64 (80M nationally) classified as “essential 

workers”

• Healthcare Personnel: 25% of essential workers (20M nationally)

Assume essential workers are only able to reduce their workplace 

contact rates 35% as much as other adults of the same age.

• Varied from: 20% to 50% in sensitivity 

▪ Baseline contact patterns:

Social contacts and mixing study, adjusted for the US (Prem 2017)

Stratified by age and setting (home, work, school, and other)
Methods: Vaccination
Vaccine Product Assumptions

▪ Full course: 2 doses given 28 days apart

▪ Vaccine Efficacy (VE)

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Both doses: VE = 70%

First dose: VE = 17.5% (1/4 of full protection)

Sensitivity analysis: reduced immunogenicity in adults aged 65+ (half of 

above) 

• VE{age 65+} = 8.75% (first dose) and 35% (both doses) 

▪ Protection from the first or second dose achieved 14 days following the 

vaccination
Vaccine: Completeness of Protection

▪ Every vaccinated person is partially protected

▪ Infection-Blocking 

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Protection against infection and onward transmission

If breakthrough infection, no attenuation of severity or transmission

▪ Disease-Blocking (Sensitivity Analysis)

No protection against infection or onward transmission

Reduced risk of severe disease if infected

▪ Assume no waning of immunity (naturally or vaccine-induced)
Vaccine Allocation Assumptions: Phase 1

Phase 1: Initial Vaccine Supply

200M Courses* Nationally

Phase 1A: Healthcare Personnel (HCP) 

Phase 1B:

Adults Aged 65+

Adults with High-Risk Medical Condition

Essential Workers

20M Courses

180M Courses

* 1 course = 

2 doses
Vaccine Allocation Assumptions: Phase 1B

Phase 1B: Non-Healthcare Personnel Target

180M Courses* Nationally

Allocated exclusively to one of:

Adults Aged 65+

Adults with High-Risk Medical Condition

Essential Workers

Remaining unvaccinated Phase 1B groups (see above) 

First 20M Courses

Wider Availability: 160M Courses

* 1 course = 

2 doses
Methods: Epidemic Dynamics
Epidemic Scenarios

▪ Percentage of the population infected 2 months prior to vaccine 

introduction= 15% (sensitivity analysis 5% & 20%)

▪ Future epidemic trajectories simulated using compartmental models with 

time-varying mitigation

▪ Outcomes (infections and deaths averted) compared 6 months following 

vaccine introduction
Administration Assumptions

▪ Assumed 100% of the individuals either vaccinated or not yet eligible for the 

second dose before moving to subsequent phases

▪ Vaccine administered regardless of infection history

▪ 10 million people can be vaccinated each week

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Phase 1A and Phase 1B fully vaccinated in ~9 months 

▪ Administration of second doses prioritized over first doses 

▪ Timing of vaccine introduction (first administration) varied

Before rise in incidence

As incidence rises

As incidence falls
Vaccine Introduced Before Rise in Incidence Incidence of SARS-CoV-2 Infections

Days From Vaccine Introduction
Vaccine Introduced as Incidence Rises Incidence of SARS-CoV-2 Infections

Days From Vaccine Introduction
Vaccine Introduced as Incidence Falls Incidence of SARS-CoV-2 Infections

Days From Vaccine Introduction
Phase 1A

Approximate Timing of Vaccine Rollout in Context Incidence of SARS-CoV-2 Infections

Days From Vaccine Introduction

Phase 1B Adults Aged 65+ OR High-Risk Adults OR Essential Workers

Phase 1B: All 3 groups

Healthcare Personnel
Findings
~1%

Population-Wide Averted Infections:

Infection-Blocking Vaccine, Older Adults Receive Full Protection

~5%

~3%

Initial Phase 1B Target:

Age 65+

High-Risk Adults

Essential Workers

▪ Initially vaccinating high-risk adults or 

essential workers in Phase 1B averts 

approximately 1–5% more infections, 

compared to targeting age 65+

– This difference is greatest in the 

scenario where the vaccine is 

introduced before incidence 

rises

▪ Findings are robust to assumptions of 

reduced VE in older populations
Population-Wide Averted Deaths:

Infection-Blocking Vaccine, Older Adults Receive Full Protection

Initial Phase 1B Target:

Age 65+

High-Risk Adults

Essential Workers

~1%

~4%

~3%

▪ Initially vaccinating age 65+ in Phase 

1B averts approximately 1–4% more 

deaths, compared to targeting highrisk adults or essential workers

– As before, this difference is 

greatest in the scenario where 

the vaccine is introduced before 

incidence rises
Population-Wide Averted Deaths:

Infection-Blocking Vaccine, Older Adults Receive Half Protection

Initial Phase 1B Target:

Age 65+

High-Risk Adults

Essential Workers

▪ The percentage of deaths averted 

changes if VE is reduced in older 

populations

▪ Initially vaccinating high-risk adults, 

age 65+, or essential workers in 

Phase 1B averts a similar percentage 

of deaths across the scenarios
Population-Wide Averted Deaths:

Disease-Blocking Vaccine, Older Adults Receive Full Protection

Initial Phase 1B Target:

Age 65+

High-Risk Adults

Essential Workers

~2%

~11%

~7%

▪ Initially vaccinating age 65+ in 

Phase 1B averts approximately 

2–11% more deaths, compared 

to targeting high-risk adults or 

essential workers

– As before, this difference is 

greatest in the scenario 

where the vaccine is 

introduced before incidence 

rises

▪ Findings robust to assumptions 

of reduced VE in older 

populations but percentage 

averted drops
Conclusions
Limitations

▪ The efficacy and ability of the vaccine candidates to prevent transmission, as well as 

the time vaccine may become available, is currently unknown

▪ Modeled epidemic trajectories are only for illustration and are not forecasts

▪ Overall averted burden should be interpreted cautiously:

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Sensitive to the future trajectory of the epidemic

Findings reflect an idealized rollout, with minimal delays and 100% uptake

The aim of this study was to demonstrate the relative impact of different initial 

vaccine allocation strategies
Limitations

▪ The following inputs were assumed and will require reassessment as more 

information becomes available

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All infections confer protective immunity

Immunity (either naturally- or vaccine-acquired) doesn’t wane significantly 

within a year of infection/immunization

Given exposure, younger age groups are just as likely to become infected as 

older age groups (susceptibility independent of age)

Individuals with comorbidities are just as likely as their peers to practice social 

distancing and other protective behaviors

No reduction in VE among those with high-risk medical conditions
Discussion

▪ Initially vaccinating adults 65+ in Phase 1B generally averts greatest % of deaths

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Approximately 1 to 11% increase in averted deaths across the scenarios

▪ Initially vaccinating essential workers or high-risk adults in Phase 1B generally 

averts greatest % of infections

Approximately 1 to 5% increase in averted infections across the scenarios

▪ Earlier vaccine roll-out relative to increasing transmission, the greater the averted 

percentage and differences between the strategies

Differences not substantial in some scenarios

Emphasizes need to continue efforts to slow the spread

▪ Findings are consistent in sensitivity analyses where the % of the population 

infected prior to vaccine introduction was varied
Consistency with External Literature

▪ Reviewed peer-reviewed and pre-publication studies that model the impact of 

vaccination under different initial allocation strategies

▪ General agreement across the study results with results presented here
Questions