The decline and resurgence of pertussis in the US

Although the resurgence of pertussis in nations with long-standing vaccination programs has raised serious concerns about the effectiveness of current immunization policy, the epidemiology of resurgence remains poorly understood. We analyzed pertussis notifications in US states obtained from the National Notifiable Disease Surveillance System from 1951 to 2010 to explore the timing, spatial pattern and consistency of resurgence across the country. Here we show that resurgence occurred at different times in different states, spread out over a transition period of roughly three decades. Further, despite this spatial variation, broad patterns in pertussis epidemiology can be described by two dominant phases: (1) a period of decline ending in the mid-1970s, followed by (2) nationwide resurgence. Together, these patterns explain 89.7% of the variation in US case notifications between 1951 and 2005. This resurgence was interrupted, however, by a synchronized downturn in 2005 that continues to the present in many large states. The causes of these two transitions in pertussis epidemiology remain hotly debated, though our findings suggest that evolution of the Bordetella pertussis bacterium, loss of immunity and persistent transmission among adults, and demographic drivers are more probable explanations than changes in reporting or the introduction of acellular vaccines.