To the Editor: The infl uenza A pandemic (H1N1) 2009 virus contains a combination of 8 gene segments (1-3) antigenically similar to North American infl uenza A virus (H1N1) but different from seasonal human infl uenza A viruses (H1N1) (3). Despite the initial high number of deaths among patients in Mexico and among patients with specifi c preexisting conditions, pandemic (H1N1) 2009 virus in general has caused mild symptoms, and the overall death rate remains around 0.45% (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 infl uenza 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 infl uenza 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.
No takes yet. Share an insight, caveat, or question.
Xing et al. (2009) studied this question.