Prior Infections With Seasonal Influenza A/H1N1 Virus Reduced the Illness Severity and Epidemic Intensity of Pandemic H1N1 Influenza in Healthy Adults

• Published in: Clinical Infectious Diseases 2012-02, Vol.54 (3), p.311-317
• Main Authors: Couch, Robert B., Atmar, Robert L., Franco, Luis M., Quarles, John M., Niño, Diane, Wells, Janet M., Arden, Nancy, Cheung, Sheree, Belmont, John W.
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Adults; and Commentaries; Antibodies; Antibodies, Viral - blood; ARTICLES AND COMMENTARIES; Biological and medical sciences; Diseases; Epidemics; Etiology; Female; H1N1 subtype influenza A virus; Human viral diseases; Humans; Immunity; Infection; Infections; Infectious diseases; Influenza; Influenza A Virus, H1N1 Subtype - isolation & purification; Influenza, Human - epidemiology; Influenza, Human - pathology; Influenza, Human - virology; Male; Medical sciences; Orthomyxoviridae; Pandemics; Rhinovirus; Seasonal variations; Seasons; Sulfur dioxide; Swine flu; Texas - epidemiology; Time Factors; Vaccination; Vaccines; Viral diseases; Viral diseases of the respiratory system and ent viral diseases; Viruses; Young Adult
• ISSN: 1058-4838; 1537-6591
• DOI: 10.1093/cid/cir809

Background. A new influenza A/H1N1 (pH1N1) virus emerged in April 2009, proceeded to spread worldwide, and was designated as an influenza pandemic. A/H1N1 viruses had circulated in 1918-1957 and 1977-2009 and were in the annual vaccine during 1977-2009. Methods. Serum antibody to the pH1N1 and seasonal A/H1N1 viruses was measured in 579 healthy adults at enrollment (fall 2009) and after surveillance for illness (spring 2010). Subjects reporting with moderate to severe acute respiratory illness had illness and virus quantitation for 1 week; evaluations for missed illnesses were conducted over holiday periods and at the spring 2010 visit. Results. After excluding 66 subjects who received pHlNl vaccine, 513 remained. Seventy-seven had reported with moderate to severe illnesses; 31 were infected with pHlNl virus, and 30 with a rhinovirus. Determining etiology from clinical findings was not possible, but fever and prominent myalgias favored influenza and prominent rhinorrhea favored rhinovirus. Tests of fall and spring antibody indicated pHlNl infection of 23% had occurred, with the rate decreasing with increasing anti-pH1N1 antibody; a similar pattern was seen for influenza-associated illness. A reducing frequency of pHlNl infections was also seen with increasing antibody to the recent seasonal A/H1N1 virus (A/Brisbane/59/07). Preexisting antibody to pHlNl virus, responses to a single vaccine dose, a low infection-to-illness ratio, and a short duration of illness and virus shedding among those with influenza indicated presence of considerable preexisting immunity to pH1N1 in the population. Conclusions. The 2009 A/H1N1 epidemic among healthy adults was relatively mild, most likely because of immunity from prior infections with A/H1N1 viruses.