Effects of Vaccine Program against Pandemic Influenza A(H1N1) Virus, United States, 2009–2010

Rebekah H. Borse¹, Sundar S. Shrestha, Anthony E. Fiore, Charisma Y. Atkins, James A. Singleton, Carolyn Furlow, and Martin I. Meltzer

Author affiliations: Author affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA

Abstract

In April 2009, the United States began a response to the emergence of a pandemic influenza virus strain: A(H1N1)pdm09. Vaccination began in October 2009. By using US surveillance data (April 12, 2009–April 10, 2010) and vaccine coverage estimates (October 3, 2009–April 18, 2010), we estimated that the A(H1N1)pdm09 virus vaccination program prevented 700,000–1,500,000 clinical cases, 4,000–10,000 hospitalizations, and 200–500 deaths. We found that the national health effects were greatly influenced by the timing of vaccine administration and the effectiveness of the vaccine. We estimated that recommendations for priority vaccination of targeted priority groups were not inferior to other vaccination prioritization strategies. These results emphasize the need for relevant surveillance data to facilitate a rapid evaluation of vaccine recommendations and effects.

On April 26, 2009, the United States declared a public health emergency in response to the 2009 pandemic influenza A(H1N1)pdm09 virus (1). The Centers for Disease Control and Prevention (CDC) estimated that in the United States during April 12, 2009–April 10, 2010, there were 61 million clinical cases of influenza and that 274,000 persons were hospitalized and 12,500 died (2). For the purpose of this study, we considered clinical cases as influenza-like illness in persons who did or did not seek medical care (2).
The US Food and Drug Administration approved multiple formulations of monovalent inactivated, unadjuvanted influenza vaccine, and a monovalent live attenuated vaccine against A(H1N1)pdm09 virus in mid-September 2009 (3); a national vaccination program was initiated in October (4). In July 2009, estimating that initial vaccine supplies could be insufficient to meet demand, the Advisory Committee on Immunization Practices (ACIP) recommended priority groups for the vaccination program. These priority groups included pregnant women, household contacts and caregivers of children <6 25="" 6="" a="" age="" all="" and="" aspirin="" associated="" asthma="" blood="" care="" chronic="" complications="" conditions="" disease="" disorders="" emergency="" endocrine="" for="" from="" had="" health="" heart="" higher="" href="http://wwwnc.cdc.gov/eid/article/19/3/12-0394_article.htm?s_cid=eid-gDev-email#r5" immune="" include="" influenza.="" kidney="" liver="" long-term="" lung="" medical="" metabolic="" months="" neurodevelopmental="" of="" personnel="" persons="" receiving="" risk="" services="" such="" system.="" therapy="" title="5" weakened="" were="" who="" with="" years="">5
,6). When a vaccine against the pandemic strain was released for initial use, the supply was only 25%–50% of the amount that had been projected because vaccine production yields were lower than expected (7,8). By January 2010, when 100 million doses had been delivered and an estimated 57 million doses had been administered (9), most states were offering vaccination to anyone >6 months of age. By February 2010, 125 million doses, most of which were inactivated, had been made available and ≈69 million persons had been vaccinated (4,9,10). Final estimates indicated that by the end of May 2010, ≈81 million persons had been vaccinated and 90 million doses had been administered (11). We estimated the number of clinical cases, hospitalizations, and deaths prevented in the United States that were directly attributable to the 2009–2010 A(H1N1)pdm09 virus vaccination program. These results can be used by public health officials, policy makers, and the public to evaluate this program and plan for the management of future pandemics.