Estimating the potential effects of a vaccine program against an emerging influenza pandemic--United States

Abstract

Background: Human illness from influenza A(H7N9) was identified in March 2013, and candidate vaccine viruses were soon developed. To understand factors that may impact influenza vaccination programs, we developed a model to evaluate hospitalizations and deaths averted considering various scenarios.

Methods: We utilized a model incorporating epidemic curves with clinical attack rates of 20% or 30% in a single wave of illness, case hospitalization ratios of 0.5% or 4.2%, and case fatality ratios of 0.08% or 0.53%. We considered scenarios that achieved 80% vaccination coverage, various starts of vaccination programs (16 or 8 weeks before, the same week of, or 8 or 16 weeks after start of pandemic), an administration rate of 10 or 30 million doses per week (the latter rate is an untested assumption), and 2 levels of vaccine effectiveness (2 doses of vaccine required; either 62% or 80% effective for persons aged <60 years, and either 43% or 60% effective for persons aged ≥ 60 years).

Results: The start date of vaccination campaigns most influenced impact; 141,000-2,200,000 hospitalizations and 11,000-281,000 deaths were averted when campaigns started before a pandemic, and <100-1 300,000 hospitalizations and 0-165,000 deaths were averted for programs beginning the same time as or after the introduction of the pandemic virus. The rate of vaccine administration and vaccine effectiveness did not influence campaign impact as much as timing of the start of campaign.

Conclusions: Our findings suggest that efforts to improve the timeliness of vaccine production will provide the greatest impacts for future pandemic vaccination programs.

Keywords: influenza; influenza A(H7N9); influenza vaccine; mathematical modeling; pandemic.

Figure 1.

The estimated epidemic curve without vaccination and the cumulative number of persons protected by an influenza vaccination program with the following assumptions: an overall clinical attack rate of the influenza pandemic of 20% or 30%; administered 10 million (left) or 30 million (right) vaccine doses; vaccination programs that begin 8 or 16 weeks before, the same week, or 8 or 16 weeks after the first cases of a novel influenza virus occur in the United States (US); and the vaccine effectiveness (VE) equivalent to the H1N1pmd09 monovalent vaccine. 2009 H1N1-like VE: 2 doses of vaccine administered 3 weeks apart required to be fully effective (62% for persons aged <60 years and 43% for persons ≥60 years) in protecting against subclinical and clinical cases, hospitalizations, and deaths. We assumed 1 dose of vaccine to be 0% effective for all age groups.

Figure 2.

Number of hospitalizations (top) and deaths (bottom) if the overall clinical attack rate of the influenza pandemic is 20% and the overall case fatality ratio is 0.53% (high-severity scenario); 10 million doses (left) or 30 million doses (right) of vaccine are administered each week; the vaccination program begins 16 weeks after, 8 weeks after, the same week as, 8 weeks before, and 16 weeks before the first cases of a novel influenza virus occur in the United States; and the efficacy is “H1N1pmd09 monovalent vaccine–like.” 2009 H1N1–like vaccine effectiveness: 2 doses of vaccine administered 3 weeks apart required to be fully effective (62% for persons aged <60 years and 43% for persons ≥60 years) in protecting against subclinical and clinical cases, hospitalizations, and deaths. We assumed 1 dose of vaccine to be 0% effective for all age groups.

Figure 3.

Number of hospitalizations (top) and deaths (bottom) if the overall clinical attack rate of the influenza pandemic is 20% and the overall case fatality ratio is 0.084% (low-severity scenario); 10 million doses (left) or 30 million doses (right) of vaccine are administered each week; the vaccination program begins 16 weeks after, 8 weeks after, the same week as, 8 weeks before, and 16 weeks before the first cases of a novel influenza virus occur in the United States; and the efficacy is “H1N1pmd09 monovalent vaccine–like.” 2009 H1N1-like vaccine effectiveness: 2 doses of vaccine administered 3 weeks apart required to be fully effective (62% for persons aged <60 years and 43% for persons ≥60 years) in protecting against subclinical and clinical cases, hospitalizations, and deaths. We assumed 1 dose of vaccine to be 0% effective for all age groups.