Preexisting Immunity to Pandemic (H1N1) 2009

DOI: 10.3201/eid1511.090685
Suggested citation for this article: Xing Z, Cardona CJ. Preexisting immunity to pandemic (H1N1) 2009 [letter]. Emerg Infect Dis. 2009 Nov; [Epub ahead of print]
Preexisting Immunity to Pandemic (H1N1) 2009
To the Editor: The influenza A pandemic (H1N1) 2009 virus contains a combination of 8 gene segments (1–3) antigenically similar to North American influenza A virus (H1N1) but different from seasonal human influenza A viruses (H1N1) (3). Despite the initial high number of deaths among patients in Mexico and among patients with specific preexisting conditions, pandemic (H1N1) 2009 virus in general has caused mild symptoms, and the overall death rate remains around 0.0045% (www.who.int/csr/don/2009_07_06/en). Low virulence of the virus and preexisting immune status are among the main factors that account for lower death rates in influenza outbreaks. The Centers for Disease Control and Prevention (Atlanta, GA, USA) reported that among persons >60 years old, 33% have preexisting, cross-reactive neutralizing antibodies against the new virus, but seasonal flu vaccines do not elicit cross-reactive neutralizing antibodies against pandemic (H1N1) 2009 virus in either younger or older populations (1). However, current data cannot be used to evaluate the full immune capacities of human populations because cell-mediated immunity (CMI) has not been characterized in humans infected with pandemic (H1N1) 2009 virus.
We performed a survey (4) for known human immune epitopes present in the various proteins of seasonal influenza A virus strains and known to be efficient in stimulating lymphocytes. We found that multiple major histocompatibility complex (MHC)–restricted epitopes are conserved in nucleoprotein (NP) and matrix protein (MP), and even a few in the more variable hemagglutinin (HA) protein, in A/California/04/2009, A/Texas/04/2009, and A/New York/18/2009.

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http://www.cdc.gov/eid/content/15/11/pdfs/09-0685.pdf