Surveillance and notification programs implemented during the past decade at the request of the World Health Organization permitted the prompt identification of a novel pandemic H1N1 influenza virus in 2009, a retrospective analysis showed.
"The surveillance system capabilities that were most essential were the availability of laboratory networks capable of identifying a novel pathogen, notification systems that made health officials aware of the epidemiologic facts emerging from numerous locations in at least two countries, and the intelligence necessary to 'connect the dots' and understand their implications," wrote Michael A. Stoto, PhD, of Georgetown University in Washington, D.C., and colleagues.
Three specific events triggered the investigations into the pandemic.
These were the identification of H1N1 infection in two children in California, the recognition that an assortment of serious respiratory infections in Mexico were all caused by a single type of virus, and the realization that illness in a group of New York City students was related to the California and Mexico cases, the researchers wrote online in PLoS ONE.
The 2007 implementation of the WHO's International Health Regulations mandated cooperation among almost 200 countries in the containment of public health emergencies, and led to the development of tools for tracking diseases, including the Global Public Health Intelligence Network, ProMED, and HealthMap.
To explore the efficacy of this worldwide effort to curb potentially lethal outbreaks, Stoto's team reviewed the international events associated with the H1N1 outbreak.
The first confirmed human case was identified on February 24 in central Mexico.
Influenza-like disease began to be seen in large numbers of young Mexican children in early March, and by March 15, a major outbreak of atypical pneumonia was reported in Veracruz.
As the number of cases in Mexico mounted in early April, officials from the Mexican National Surveillance System, the Institute for Epidemiologic Diagnosis and Reference, and the Directorate General of Epidemiology realized that the clusters of disease throughout the country could be related and might not be isolated cases of seasonal influenza.
They also determined that the illness was disproportionately affecting young people, and was associated with high morbidity and mortality.
Meanwhile, in late March two children in California developed a flu-like illness, which experimental surveillance being done by the Naval Health Research Center in San Diego quickly identified as an unknown subtype of influenza A.
The San Diego researchers sent samples to several reference laboratories and ultimately to the CDC, where in mid-April the the pathogen was determined to be H1N1 influenza, likely of swine origin.
When the Mexican authorities learned of the CDC's findings, they alerted the American regional branch of WHO about their similar, possibly related cases.
Then, during the week of April 19, a dozen New York City students developed symptoms of influenza after returning from a trip to Mexico during spring break; within 4 days, 100 symptomatic students had been sent home.
The city's Department of Health and Mental Hygiene immediately notified the CDC, and confirmed with PCR testing that the cause was influenza A without any recognized human H1 or H3 subtypes.
By April 26, the U.S. Department of Health and Human Services had realized that the New York, California, and Mexican cases represented a pandemic and declared a public health emergency. The next day, the CDC advised that U.S. travelers avoid visiting Mexico.
As of April 30, there had been 100 reported U.S. cases, and by the end of the summer, as the outbreak waned, there had been cases in all 48 continental states.
Throughout April, public health authorities in Mexico, the U.S., and Canada, had joined forces through the Security and Prosperity Partnership of North America, which was established several years before, to assess the extent of the outbreak.
Their findings prompted the WHO to declare a "public health emergency of international concern" on April 25 and by early May there had been almost 2,000 cases reported in 23 countries.
Stoto and colleagues noted that recognition of the pandemic might have been hastened, perhaps by about a week, but the early epidemiologic signals had been unclear.
"To find two children with unsubtypable influenza at the end of the flu season is not remarkable, and indeed it is only because of the research being conducted at the Naval Health Research Center that these cases came to light at all," they observed.
"Because pH1N1 emerged during the normal flu season, there were too few cases to have been detected by standard alerting algorithms," they added.
Nonetheless, the broader implementation of surveillance systems, increased laboratory capabilities such as the wider use of PCR, and international investments in public health awareness limited the extent of the pandemic.
"The early events in the 2009 H1N1 pandemic thus illustrate the important contribution of the 2005 International Health Regulations and the paradigm shift that accompanied it," Stoto and colleagues stated.
"The picture that emerges from this analysis is a global public health system, and particularly public health agencies in Mexico, Canada, and the United States, that worked together effectively to solve a challenging epidemiologic puzzle in a reasonably timely fashion," they concluded.
A limitation of the study was its retrospective design with the possibility of recall bias.
The authors reported no competing interests.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner