Epidemiology and Infection

Original Papers

Influenza

Investigating the effect of high spring incidence of pandemic influenza A(H1N1) on early autumn incidence

H. BURKOMa1 c1, K. KNISSa2, M. MELTZERa3, L. BRAMMERa2, Y. ELBERTa1, L. FINELLIa2 and D. SWERDLOWa4

a1 Johns Hopkins Applied Physics Laboratory, National Security Technology Department, Laurel, MD, USA

a2 Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Infectious Diseases, Influenza Division, Atlanta, GA, USA

a3 Centers for Disease Control and Prevention, National Center for Zoonotic and Infectious Diseases, Division of Preparedness and Emerging Infections, Atlanta, GA, USA

a4 Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Infectious Diseases, Office of the Director, Atlanta, GA, USA

SUMMARY

A pandemic H1N1 infection wave in the USA occurred during spring 2009. Some hypothesized that for regions affected by the spring wave, an autumn outbreak would be less likely or delayed compared to unaffected regions because of herd immunity. We investigated this hypothesis using the Outpatient Influenza-like Illness (ILI) Network, a collaboration among the Centers for Disease Control and Prevention, health departments, and care providers. We evaluated the likelihood of high early autumn incidence given high spring incidence in core-based statistical areas (CBSAs). Using a surrogate incidence measure based on influenza-related illness ratios, we calculated the odds of high early autumn incidence given high spring incidence. CBSAs with high spring ILI ratios proved more likely than unaffected CBSAs to have high early autumn ratios, suggesting that elevated spring illness did not protect against early autumn increases. These novel methods are applicable to planning and studies involving other infectious diseases.

(Received June 14 2011)

(Revised January 04 2012)

(Accepted January 04 2012)

(Online publication February 07 2012)

Key words

  • Epidemics;
  • infectious disease epidemiology;
  • influenza;
  • pandemic

Correspondence:

c1 Author for correspondence: Dr H. Burkom, Johns Hopkins Applied Physics Laboratory, National Security Technology Department, MS 8-224, 11100 Johns Hopkins Road, Laurel, MD 20723, USA. (Email: howard.burkom@jhuapl.edu)

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